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Complete Guide to physics 2

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Table of Contents Table of Contents CHAPTER 7: SOUND ............................................................................................... 1 INTRODUCTION: ........................................................................................................................................... 1 Characteristics of Wave Motion: .................................................................................................. 2 Reflection of Sound Waves: .............................................................................................................. 2 Echo: ................................................................................................................................................. 2 Use of Echoes: .................................................................................................................................. 2 Determination of Speed of Sound by the Method of Echo: ............................................................... 3 Free (or Natural) Vibrations: .............................................................................................................. 3 Damped Vibrations: ........................................................................................................................... 5 Forced Vibrations: ............................................................................................................................. 5 Resonance: ....................................................................................................................................... 6 Some Examples of Resonance: ........................................................................................................ 6 Characteristics of Sound: .................................................................................................................. 7 Music and Noise: ............................................................................................................................. 10 Distinction between light and sound waves ............................................................................... 10 Review Questions: ................................................................................................................. 10 Numericals: ............................................................................................................................ 11 Application Type: ................................................................................................................... 12 Miscellaneous: ....................................................................................................................... 14 Previous Year Board Questions: ........................................................................................... 18 Answers: ................................................................................................................................ 19 CHAPTER 8: CURRENT ELECTRICITY ................................................................ 20 CHAPTER MAP .......................................................................................................................................... 20 INTRODUCTION: ......................................................................................................................................... 20 Electric Current: ............................................................................................................................... 20 Electric Potential:........................................................................................................................ 21 Ohm s Law:...................................................................................................................................... 22 Resistance: ...................................................................................................................................... 22 Electromotive Force (e.m.f.): ........................................................................................................... 24 Factors affecting e.m.f. of a cell: ................................................................................................ 24 Internal resistance: .......................................................................................................................... 25 Resistors in Series: ......................................................................................................................... 25 Resistances in Parallel: ................................................................................................................... 25 Review Questions: ................................................................................................................. 26 Numericals: ............................................................................................................................ 27 Application type: .................................................................................................................... 30 Miscellaneous: ....................................................................................................................... 32 Previous Year Board Questions ............................................................................................ 34 Answers: ................................................................................................................................ 37 CHAPTER 9: ELECTRICAL POWER AND HOUSEHOLD CIRCUITS ................... 38 CHAPTER MAP: ......................................................................................................................................... 38 INTRODUCTION: ......................................................................................................................................... 38 Expression for the Measurement of Electrical Energy: ................................................................... 38 Electrical Power: .............................................................................................................................. 39 Commercial Unit of Electrical Energy: ............................................................................................. 39 Power Rating: .................................................................................................................................. 39 Volume 2 of 2 Universal Tutorials X ICSE Physics Power rating of Some Common Appliances: .................................................................................. 40 Calculation:................................................................................................................................. 40 Household Wiring System: .............................................................................................................. 40 Transmission of Power: .............................................................................................................. 40 Power Distribution: ..................................................................................................................... 41 Supply to a House: ..................................................................................................................... 41 Household Wiring is Done in Parallel: ........................................................................................ 42 Essential Components of Household Wiring: .................................................................................. 42 Fuse: .......................................................................................................................................... 42 Switches: .................................................................................................................................... 43 Dual Switch ................................................................................................................................ 43 Earthing: ..................................................................................................................................... 43 Three Pin Plug: ................................................................................................................................ 44 Colour Coding of Wires in a Cable: ................................................................................................. 45 Review Quesitons: ................................................................................................................. 45 Numericals: ............................................................................................................................ 46 Application type: .................................................................................................................... 47 Miscellaneous: ....................................................................................................................... 48 Previous Year Board Questions: ........................................................................................... 52 CHAPTER 10: ELECTRO MAGNETISM ............................................................... 53 CHAPTER MAP: ......................................................................................................................................... 53 Magnetic Effect of Current:.............................................................................................................. 53 Magnetic Field Due to Current: ....................................................................................................... 53 Rules to Find the Direction of Magnetic Field: ........................................................................... 54 Electromagnet: ................................................................................................................................ 55 Force on a current carrying conductor in a magnetic field .............................................................. 56 D.C. Motor: ...................................................................................................................................... 56 The Main Parts of an Electric Motor are: ................................................................................... 56 Ways of Increasing the Speed of Rotation of Coil: .................................................................... 57 Electromagnetic Induction: .............................................................................................................. 57 Faraday s Laws: ......................................................................................................................... 57 A. C. Generator (or Dynamo): ......................................................................................................... 58 Transformer: .................................................................................................................................... 59 Energy losses in a Transformer: ................................................................................................ 61 Review Questions: ................................................................................................................. 61 Numericals: ............................................................................................................................ 62 Application type: .................................................................................................................... 63 Miscellaneous Exercise: ........................................................................................................ 65 Previous Year Board Questions ............................................................................................ 66 CHAPTER 11: CALORIMETRY .............................................................................. 68 CHAPTER MAP: ......................................................................................................................................... 68 Introduction: ..................................................................................................................................... 68 Heat Capacity: (C ) .......................................................................................................................... 69 Specific Heat Capacity: ................................................................................................................... 69 Relationship between Heat Capacity & Specific Heat Capacity: .................................................... 69 Calorimeter: ..................................................................................................................................... 70 Principle of Mixtures (or Principle of Calorimetry): .......................................................................... 70 Measurement of Specific heat of a solid: ................................................................................... 71 Specific heat capacity of a solid or liquid by electrical method: ................................................. 71 Natural Phenomena and Consequences of High Specific Heat Capacity of Water:....................... 71 Universal Tutorials X ICSE Physics Volume 2 of 2 Table of Contents The Climate Near the Seashore is Moderate:............................................................................ 71 Hot Water Bottles are Used for Fomentation: ............................................................................ 72 Water is Used as an Effective Coolant: ..................................................................................... 72 Farmers fill their fields with water to protect the crops from frost: ............................................. 72 Some Examples of High and Low Thermal Capacity: ..................................................................... 72 Change of Phase (State): ................................................................................................................ 72 Melting and Fusion: ......................................................................................................................... 72 Change in Volume on Melting: ........................................................................................................ 73 Effect of Pressure on the Melting Point: .......................................................................................... 73 Effect of Impurities on the Melting Point:......................................................................................... 73 Vaporisation or Boiling: ................................................................................................................... 73 Latent Heat: ..................................................................................................................................... 74 Definition: ................................................................................................................................... 75 Specific Latent Heat of Vaporisation: ......................................................................................... 75 Change in Phases of Ice: ................................................................................................................ 75 Natural Consequences of High Specific Latent Heat of Fusion of Ice: ...................................... 76 Consequences of High Specific Latent Heat of Steam: ............................................................. 76 Review Questions: ................................................................................................................. 77 Numericals: ............................................................................................................................ 77 Application type: .................................................................................................................... 79 Miscellaneous: ....................................................................................................................... 81 Previous Year Board Questions: ........................................................................................... 83 Answers: ................................................................................................................................ 85 CHAPTER 12: THERMIONIC EMISSION AND RADIOACTIVITY .......................... 86 CHAPTER MAP: ......................................................................................................................................... 86 INTRODUCTION: ......................................................................................................................................... 86 Thermionic Emission: ...................................................................................................................... 87 Factors Affecting the Rate of Thermionic Emission: ....................................................................... 87 Requisites for a Good Electron Emitter: .......................................................................................... 87 Hot Cathode Ray Tube:................................................................................................................... 88 Uses of a cathode ray tube: ....................................................................................................... 90 Use of cathode Ray Tube in Television: .................................................................................... 90 Isotopes: .......................................................................................................................................... 91 Isobars: ............................................................................................................................................ 91 Radioactivity: ................................................................................................................................... 91 Radioactivity as Emission of Alpha ( ), Beta ( ) & Gamma ( ) Radiations: ................................... 92 Distinction Between The Properties of , and Radiations: ........................................................ 92 Changes within the Nucleus in Alpha,Beta & Gamma Emission .................................................... 93 Uses of Radioactivity Radio Isotopes:............................................................................................ 93 Harmful Effects and Safety Precautions: ........................................................................................ 94 Background Radiations: .................................................................................................................. 94 Review questions:.................................................................................................................. 95 Application Type: ................................................................................................................... 96 Miscellaneous Exercise: ........................................................................................................ 98 Previous Year Board Questions: ........................................................................................... 99 Volume 2 of 2 Universal Tutorials X ICSE Physics Chapter 07: Sound 1 Chapter 7: Sound Introduction Characteristics of wave motion Reflection of sound waves Echoes Uses Determination of speed of sound Forced vibration, Natural vibration and Resonance Definitions Examples Characteristics of musical Notes Loudness Factors Subjective Nature Pitch Factors Nature Quality Difference between music and noise Differences between light and sound Introduction: Sound is produced when a body vibrates in a medium. Sound waves are mechanical or elastic waves which require a medium for propagation. Sound waves are longitudinal (The vibration of the particles of the medium is parallel to the propagation of wave) in gases / air longitudinal wave, are propagated as compressions and rarefactions. If the vibrations of the particles of the medium are perpendicular to the direction of wave, a transverse wave is formed. Alternate crests and troughs are formed in transverse waves. Our ears are sensitive only to a range of 20 Hz to 20,000 Hz. This range is called audible range. Frequencies below 20 Hz are called infrasonic and frequencies above 20,000 Hz are called ultrasonic Speed of sound in a medium is given by V= Wave velocity (V) = f and f = Volume 2 of 2 E D [E is elasticity and D is density of the medium] 1 T Universal Tutorials X ICSE Physics 1 2 Frequency or time period depends on the source. Velocity and wavelength depend on properties of the medium. Characteristics of Wave Motion: A wave is produced due to periodic disturbance at a point in a medium. During wave motion, only energy is transferred. The energy transfer takes place with constant speed and depends only on the nature of the medium. When two waves travel in a medium, the total displacement is the algebraic sum of individual displacement (Super position principle) Reflection of Sound Waves: The bouncing of sound waves on striking a surface such as wall, metal sheet, plywood etc. is called the reflection of sound wave. The phenomenon of reflection of sound is used in megaphone, sound boards and ear trumpet. For reflection the size of the reflecting surface must be bigger than the wavelength of sound wave. Echo: The sound heard after the original sound due to reflection from a rigid obstacle (such as a cliff, a hillside, wall of a building, edge of a forest etc.) is called an echo. For an echo to be heard, the reflected sound should reach the person after 0.1s. Condition for hearing an echo: z z z z The minimum distance in air between the source of sound and the reflector must be 17 m. It is different in different medium depending upon the speed of sound in that medium. The size of the reflector must be large enough as compared to the wavelength of sound wave. The intensity of sound should be sufficient so that the reflected sound reaching the ear is audible. d= V t 340 0.1 = 17 m in air. = 2 2 Use of Echoes: Ultrasonic waves are used for both sound ranging and echo depth sounding. The reason is that the ultrasonic waves (frequency above 20,000 Hz) travel undeviated through long distances and they can be confined to a narrow beam. They are not easily absorbed in a medium. Audible sound waves (frequency 20 Hz to 20,000 Hz) do not possess these properties. However, the ultrasonic waves in a medium have the same speed as the speed of audible sound waves in that medium. Use of Echoes: z 2 By Bats: Animals have different range of audible frequency e.g. bats, dolphins and dogs have a much higher upper audible limit than the human beings. Bats can produce and detect the sound of very high frequency up to about 100 kHz. Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 07: Sound z z z z z z z z 3 The bats fly with speed much lower than the speed of sound. The sounds produced by flying bats get reflected back from any obstacle in front of it. By hearing the echoes, bats come to know where the obstacles are, even in the dark. So they can fly safely without colliding with the obstacles. This process of detecting obstacles is called sound ranging. By Dolphins: Dolphins detect their enemy and obstacles by emitting ultrasonic waves and hearing their echo. By Fishermen: The trawler man or fisherman sends a pulse of ultrasonic waves from a source (a very high frequency vibrator) into the sea and receives the waves reflected from the shoal of fish in the detector. The total time t of the to and fro journey of the wave is recorded. The position of fish is then vt where V is nearly 1400 m s 1 (speed of ultrasonic calculated by using the relation d = 2 waves in sea water. SONAR: In sonar (sound navigation and ranging), ultrasonic waves are sent in all directions from the ship and they are then received on their return after reflection. If there is some obstacle such as an enemy submarine, iceberg, a sunken ship etc. its distance from the source can be calculated by measuring the time interval t between the instant when waves are produced and the instant when waves are received after reflection. vt where v is the speed of ultrasonic 2 waves in water. The depth of sea can also be found by this method. This process is called echo depth sounding. The distance of obstacle from the source is then d = In radar (radio detection and ranging), a signal of electromagnetic waves such as radio waves or micro waves is sent in space which after reflection from the object (such as enemy s aeroplane) in its path, returns back to the radar itself. Thus like sonar, the echo method is used in radar to detect the presence of an obstacle and finding its range. Determination of Speed of Sound by the Method of Echo: The echo method can be used for determining the speed of sound in air. For this, sound is produced from a place at a known distance say, d at least 50 m from the reflecting surface. The time interval t in which the echo (i.e. the sound after reflection from the reflecting surface) reaches the place from where the sound was produced, is noted by a stop watch. Then the speed of sound is calculated by using the following relation, v= Total distance travelled 2d = ms 1 Time interval t The experiment is repeated several times and to get the average value of speed of sound is calculated. Free (or Natural) Vibrations: Everybody has a natural frequency with which it vibrates when it is disturbed slightly from its position of equilibrium. The vibrations so produced are called free or natural vibrations of the body. The period (or frequency) of vibration depends on the shape and size (or structure) of the body. The time period of a freely vibrating body is called its free or natural period and the frequency of the freely vibrating body is called its natural frequency. Volume 2 of 2 Universal Tutorials X ICSE Physics 3 4 Diffe erent bodies vibrate with different nat tural frequen ncies. The am mplitude of a freely vibra ating body remains constan nt. The free vibrations v of a body actua ally occur in vacuum bec cause the pre esence of med dium offers some s resistan nce due to which w the amplitude of vib bration decre eases. Def finition: z z The vibrat tions of a bo ody with cons stant amplitu ude and cons stant frequen ncy are called d the free vibrations s. The vibrat tion of a bod dy in the abse ence of any external e forc ce is called free vibrations s. Exa amples of f Free (or Natural) N V Vibrations: : z z z z z z z z If the bob b of a simple e pendulum is displaced slightly from m its mean (or resting) position, p it starts vibr rating with its s natural freq quency. When a tu uning fork is struck again nst a hard rub bber pad, it vibrates v with its natural frequency. When we e strike the keys k of a pia ano, various s strings are set into vibration at the eir natural frequencie es. Frequencies of organ pipe or flute are inversely proportiona al to length o of air column n. When an air column in a flute or organ pipe is made to vibrate, v it vib brates with its natural frequency y. When the e string in th he instrumen nts like sitar, , guitar, violi in etc. is plu ucked, the tr ransverse vibrations s are produce ed in the strin ng with a def finite natural frequency. In organ pipe p open at both ends, the t different modes of vib brations are of frequencie es in ratio 1 : 2 : 3 while w frequencies for pipe closed at on ne end is 1 : 3 : 5. A string of o a given len ngth stretche ed between it ts ends unde er a given ten nsion can be e made to vibrate in different modes by pluck king the string at different t points. If the string s is pluck ked in the middle it vibrat tes with one loop (princip pal note) z If it is plucked p in 1 th length, it vibrates with h two loops (1st subsidiary y 2f) 4 i) f ii) 2f (Principle note) (1st harmon nic) In string in nstruments frequency f is iii) 3f (2nd harmoni ic) 1 Inverse ely proportio onal to length h f l 1 Inverse ely proportio onal to radius s f r Directly proportiona al to tension in the strings (f T) = f = 1 2l T r 2d Nat ture of Fre ee Vibratio ons: z z 4 The amplitude and fre equency are constant. The figure e below show ws the displa acement-time e graph for free vibrations of a body with constant am mplitude and const tant frequenc cy. Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 07: 0 Sound 5 Dam mped Vib brations: : Def finition: The perio odic vibratio ons of a bo ody with decreasing am mplitude are e called the damped vibrations s. The e amplitude of motion decreases du ue to the frictional force e which the surrounding g medium exer rts on the bo ody vibrating g in it. The fr rictional force e at any inst tant is found d to be propo ortional to the velocity of th he vibrating body b and it has the tende ency to resist t the motion. As a result, the vibrating bod dy continuou usly loses energy in doing g work again nst the force of friction and so its ampl litude gradua ally decreas ses. After so ome time, wh hen it has lo ost all its en nergy, the motion finally sto ops. The e energy los st by the vib brating syste em thus gra adually chan nges to heat energy an nd it gets diss sipated in the e surrounding g medium. The e rate at whic ch the energ gy is lost to the surroundings (or the e rate of decrease of am mplitude), depends on the e nature (i.e. . viscosity, density d etc.) of the surro ounding med dium and als so on the shape and size of o the vibrating body. The e figure sho ows the displacement-time graph for f damped. osci illations /vibr rations z Exa amples of f Damped Vibrations s: z z z A body fr ree to vibrat te in a med dium such as a air, when disturbed f from its rest position, executes damped vibrations. For example, wh hen a slim branch of a tr ree is pulled and then released, it makes dam mped vibrations. A tuning fork f vibrating g in air and a simple pen ndulum oscillating in air (o or any other r medium) also execute the damp ped vibration ns. Difference e between th he free and damped d vibra ations: F Free vibratio ons amped vibra ation Da The amplitude of free vibrations rema T ains c constant and they continu ue forever. The am mplitude of damped d vibra ations decrea ases with time and ultimately y the vibrations crease. There is no T o loss of energy e in free f v vibrations. In eac ch vibration, there is som me loss of energy e as heat. No external force acts on N o the vibrat ting b body. The vib brations are only under the re estoring forc ce. Frictional or damping forces act to opp pose the motion n. The frequen T ncy of vibra ations rema ains c constant equa al to the natu ural frequenc cy. equency of vibrations v is less than th he natural The fre freque ency. Forc ced Vibra ations: Def finition: z The vibrations of a bo ody which take place und der the influe ence of an e external perio odic force on it are called c the for rced vibration ns. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 5 6 When an external periodic force is applied on a vibrating body, the body no longer vibrates with its own natural frequency, but it gradually acquires the frequency of the applied periodic force. The external applied force is called the driving force. The amplitude of forced vibrations remains constant with time, but its magnitude depends on the frequency of the external force. If the frequency of the external force is very different from the natural frequency of the body, the amplitude of oscillations is very small. Examples of Forced Vibrations: z z z z When the stem of a vibrating tuning fork is pressed against the top of a table, the tuning fork forces the table top to vibrate with its own frequency. The vibrations produced in the table top are the forced vibrations. Since the table top has large surface area the forced vibrations produce a loud sound. The vibrations produced in the diaphragm of a gramophone sound box with frequencies corresponding to the tones conveyed from the record are forced vibrations. When a guitar is played, the artist forces the strings of the guitar to execute forced vibrations. All stringed instruments are provided with a hollow sound box which contains air. In these instruments, vibrations produced in the air of the sound box when the strings on it are made to vibrate by plucking, are forced vibrations. Since surface area of air in the sound box is large, the forced vibrations of air cause a loud sound. Resonance: Definition: z z When the frequency of an externally applied periodic force is equal to the natural frequency of the body, free to vibrate, the body readily responds to the force and begins to vibrate with increased amplitude. This phenomenon is known as resonance. In many cases Resonance is accompanied by a large sound because intensity of sound is proportional to square of amplitude. Condition for Resonance: z Resonance occurs when the frequency of the applied force is equal to the natural frequency of the vibrating body. Some Examples of Resonance: Sympathetic vibrations of pendulums: If two pendulums of same RUBBER STRING Q lengths are suspended from a rubber string and one pendulum is made to P vibrate, the other pendulum also starts vibrating with the same amplitude and in same phase because of resonance C Resonance in machine parts: When a vehicle is driven, the piston of the A B engine makes in and out motion at a frequency depending upon its speed. D The vibrations caused by the movement of piston are transmitted to all parts of the vehicle. It is just possible that some parts of the vehicle (or its frame), may have natural frequency of vibration equal to the frequency of to and fro movement of piston at a certain speed of the vehicle. When this happens, then at this particular speed of the vehicle that part starts vibrating vigorously due to resonance. If this part is not tightly fixed, then the vibrations may force the part to drop out. To stop these vibrations, the speed of vehicle is changed, so that the condition of resonance will not then hold. It is true, not only for vehicles, but for all types of machines. 6 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 07: 0 Sound 7 Res sonance in stretched string s and sound s box of o musical instruments s and sono ometer: A vibra ating string by itself pro oduces a ve ery weak so ound which cannot be heard at a distance. The erefore all musical m string ged instrume ents and sonometer are e provided w with a sound d box (or sound chamber) ). The box is so constructed that the column c of air r inside it, ha as a natural frequency f whic ch is the same as that of the string gs stretched on it, so that when the e strings are made to vibra ate, the air column c inside e the box is set s into force ed vibrations. Since the s sound box ha as a large area a, it sets a la arge volume of air into vib bration of the e same frequ uency as that t of the string g. So due to re esonance, a loud sound is i produced. Res sonance in air column and tuning g fork: Whe en the freque ency of the air column becomes equal to the freq quency of the e tuning fork vibrating ove er it, a loud sound s is heard due to res sonance. Res sonance in a bridge. When W a troop p crosses a suspension bridge, the soldiers are asked to brea ak steps. The e reason is that when soldiers are in steps, all the e separate fo orces exerted d by them are in same phase and the erefore vibrations of a particular freq quency are produced. Now N if the ural frequenc cy of the brid dge happens s to be equal to the frequ uency of the steps, the bridge b will natu vibra ate with a lar rge amplitude due to reso onance and the suspens sion bridge co ould collapse e. Res sonance in radio and TV T receivers s: Radio and d TV receive ers have ele ectronic circu uits which prod duce electric cal vibrations s, the freque ency of which h can be changed by ch hanging the values of the electronic co omponents of o that circuit. When we want w to tune a radio or TV V receiver, we w merely adju ust the value es of the elec ctronic comp ponents to pr roduce vibrat tions of frequ uency equal to that of the incoming ra adio waves which w we wa ant to receiv ve. When the two freque encies match, due to reso onance, the energy e or sig gnal of that particular p fre equency is re eceived from m the incomin ng waves. This s signal is the en amplified in the receiv ver set. Diffe erence betwe een the force ed and reson nant vibration ns: Forced vibr ration R Resonant vib bration Th he vibration of a body under an external pe eriodic force of frequenc cy different than t the na atural frequency of the body, b are ca alled the for rced vibrations. The T vibration n of a body under an external e periodic force of freque ency equal to the natural freque ency of the b body are calked the re esonant vibra ations. Th he amplitude of vibration is usually sm mall. The T amplitude e of vibration n is very large. Th he vibrations s of the bod dy are not in phase wit th the extern nal periodic fo orce. The T vibrations s of the bod dy are in pha ase with th he external periodic p force e. Th hese vibrations last for a very small ti ime after the e periodic force has ceas sed to act. These T vibratio ons last for a long time after a the periodic force e has ceased d. to act . Char racterist tics of Sound: S Two o sounds can n be distinguished from one o another by b the following three diff ferent charac cteristics: i) lou udness ii) ) pitch or shrillness and iii) quality y or timbre i) Loudness L and intens sity: z z Loudness s is the prope erty by virtue e of which a loud sound can be distinguished fro om a faint one, both having the same s pitch and a quality. Louder so ound corresp ponds to the wave w of larger amplitude e. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 7 8 z z Thus loud dness of soun nd depends on amplitude e (or intensity y) of the wav ve. The inten nsity at any point of the e medium is s measured as the amo ount of soun nd energy passing per p second no ormally throu ugh unit area a around that t point. Note: : Weber Fe echner s rela ation between n loudness (L L) and intens sity (I): L = K log I. U Unit of Inten nsity is micro o watt per me etre2 ( W m 2) 1 Mw m 2 = 10 6 J/ /sm2 Fac ctors Affecting the Loudness s of Sound d: z Loudness s is proportional to the square of amplitude. L A2 z 1 Loudness s varies inver rsely as the square s of dis stance L 2 d z Loudness s depends on n the surface e area of the vibrating bod dy [L a] z Loudness s depends on n the density of medium.[ [L ] Loudness s depends on n the presenc ce of resonant bodies. The unit of loudnes ss is decibe el: The loudness of any y sound in p phone is equ ual to the loudness in decibel (d dB) of an eq qually loud sound s of freq quency 1 kHz. The soun nd level is usually ex xpressed in decibel d (dB). I Sound lev vel in dB = 10 0 log10 Io z 2 (I is intensity of sound and I0 is re eference inte ensity = 10 12 W) Sub bjective Nature N of Loudness L and Objec ctive Natu ure of Intensity: z z z z Loudness s depends on n the energy y conveyed by b the wave near the ea ardrum of the e listener. Loudness s being a sensation also o depends up pon the sens sitivity of the e ears of the e listener. Thus loud dness of soun nd of a given n intensity ma ay differ from m listener to l listener, in pe erception Two soun nds of the sa ame intensity y, but of diffe erent frequen ncies may di iffer in loudn ness even to the sam me listener because the sensitivity s of the ears is different d for d different frequ uencies. Loudness s of sound he eard by peop ple at different distances from the source differs. More the distance less is the lou udness perceived. For norm mal ears, the e sensitivity is maximum m at frequency 1 kHz. Thus loudn ness is a subjective e quantity, while intensity y, being a me easurable qu uantity, is an objective qu uantity for a sound wave. w Noise Polluti ion: z z A constan nt hearing of f sound of le evel above 120 1 dB can cause head dache and permanent damage to o the ears of f the listener. This is calle ed noise pollution. The sound d of level 30 dB to 10 dB B has the soo othing sensation, while th he level 0 dB B of sound represent ts the limit of f hearing. ii) Pitch P (or Shrillness) S ) and Freq quency: z z 8 Pitch is th hat characte eristic of sound d by which h an acute (or shrill) note can be distinguished from a grave or flat f note. Pitch re efers only to musical sounds s and each Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 07: 0 Sound z 9 musical note has a de efinite pitch. Pitch of a note depend ds on its wav velength or fr requency. Note: : In a tape re ecorder bass is low pitch and treble is s high pitch. Sub bjective nature of pitch p and objective o nature n of frequency f y: z z z z z z Pitch is not the same e as frequenc cy. The pitch h refers to th he sensation n as perceive ed by the listener. It may not be b the same e for a soun nd of a par rticular frequ uency to the e different listeners i.e. pitch is su ubjective. On the ot ther hand, frequency f is a measurab ble quantity and it has a definite va alue for a given sound and it has s nothing to do with the listener i.e. fr requency is a an objective quantity. Examples s: Instrumen nts such as piano, violin and guita ar have strin ngs which vibrate v to produce notes. n The fr requency of vibration dep pends on the e tension, length and thic ckness of strings. A note of hi igher pitch from f stringe ed instrumen nt is obtaine ed by increa asing the frequency y of the vibra ating string fo or which eithe er the tensio on on the string is increas sed or the length of the string is s shortened or a thinner r string is us sed. It can a also be increased by changing the place of o plucking th he string (i.e e. by shifting g the place o of plucking the t string more towa ards either of o the fixed en nd). In case of f a flute, a lo ower note is obtained o by closing some e more holes s so that the e length of the vibrati ing air colum mn increases. As the water w level in n a pitcher kept under a water tap p rises the length of ai ir column decreases s, so the frequency of sound produc ced increase es i.e. the so ound become es shriller and shrille er. Thus by hearing h the sound s from a distance, one o can get t the idea of water w level in the pitc cher. The voice e of women is s usually of higher h pitch than t that of men. m iii) Quality (o or Timbre) and Wave e Form: z z z z z z z z Quality or timbre of a sou und is that characteristic which distinguish hes the tw wo sounds of f the same loudness l and same pitc ch, but emitte ed by two diff ferent instrum ments. Figure sh hows wave forms of tw wo sounds of same lou udness and d same fre equency, but emitted by y two differe ent sources. They produc ce different sensations s o the ears because the on ey differ in wave w form. The qualit ty of a music cal sound depends on the e wave form. . The qualit ty of a music cal sound de epends on th he number of f the subsidia ary notes pre esent in it along with h the principa al note and th heir relative amplitudes. Different instruments i emit differen nt subsidiary y notes. A no ote played on n a piano ha as a large number of subsidiary notes, while the same no ote when pla ayed on a flut te contains only o a few subsidiary y notes. Thus, we can easily distinguish d be etween the sounds s of a piano p and a flute by their different waveform ms, though they may be of o exactly the same loudn ness and sam me pitch. Each vibr rating body has its char racteristic wave form. This T makes i it possible fo or one to recognise e the vibrating g body even without seeing it. We gener rally recognis se a person by hearing his h voice on telephone t wi ithout seeing g him. It is because the vibration ns produced by the voca al chord of each e person n has a char racteristic wave form m which is dif fferent for dif fferent perso ons. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 9 10 Music and Noise: Music When two or more notes are sounded together and the combined effect is pleasing to the ear it is music otherwise noise. It is a pleasant, continuous and uniform sound produced by the regular and periodic vibrations. For example, the sound produced by a violin, piano, flute, tuning fork etc., are the musical sounds. Its sound level is usually between 10 dB to 30 dB. Noise Sounds other than the musical sounds are called the noise. It is a sound produced by an irregular succession of disturbances and it is a discontinuous sound. It is discordant and unpleasant to the ear. For example, the sound produced when a stone is thrown on a tin sheet is a noise. Usually the sounds of level above 120 dB are termed as noise. Distinction between light and sound waves Light waves These are electromagnetic waves. They can travel in vacuum Sound waves These are mechanical waves. They require a material medium for propagation. The speed of light waves is very high (= 3 10 m s in air). The speed of sound waves is low (= 330 m s 1 in air). The wavelength of light waves (visible) is very small, of the order of 10 6 m. The wavelength of sound waves is in the range of 10 2 m to 10 m These waves are transverse. These waves are longitudinal (through air). 8 1 REVIEW QUESTIONS: Direct questions: 1) Mention two properties of a wave, one which changes while the other which does not change, when a wave passes from one medium to another. 2) State two differences between light and sound waves. 3) What is an echo? 4) What are the conditions necessary for an echo to be heard distinctly? 5) State two applications of echo. 6) How do bats avoid obstacles in their path while in flight? 7) What is SONAR? State the principle on which it is based. 8) State the medical use of echoes. 9) What do you understand by free vibrations of a body? Give one example. 10) On what factors does the natural time period of vibration of a body depend? 11) What adjustments would you make for tuning a stringed instrument for it to emit a note of a desire frequency? 12) Explain why strings of different thickness are provided on a stringed instrument. 13) What are damped vibrations? How do they differ from free vibrations? Give one example. 14) What are forced vibrations? Give one example to illustrate your answer. 15) Distinguish between the free (or natural) and forced vibrations. 16) State the condition for the occurrence of resonance. 10 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 07: Sound 11 17) When a troop crosses a suspension bridge, the soldiers are asked to break steps. Explain the reason. 18) The rear view mirror of a motor cycle starts vibrating at some particular speed of the motor cycle. Why does this happen? Suggest one way to stop these vibrations. 19) Name three characteristics of a musical sound. 20) Name the unit in which loudness of sound is measured. 21) Comment on the statement loudness of sound is a subjective quantity, while intensity is an objective quantity. 22) What is meant by noise pollution? Name one source of sound causing noise pollution. 23) Name the subjective property of sound related to its frequency. 24) Why do the qualities of sound of the same pitch differ when produced by different instruments? 25) State the factors that determine i) the pitch of a note, ii) the loudness of the sound heard, iii) the quality of the note 26) Differentiate between a musical note and a noise. NUMERICALS: Class Work: 1) The wavelength of waves produced on the surface of water is 20 cm. If the wave velocity is 24ms 1, calculate: (i) the number of waves produced in one second and (ii) the time required to produce one wave. [(i) 120 (ii) 8.3 10 3 s] 2) What should be the minimum distance between source and reflector in water so that echo is heard distinctly? (The speed of sound in water = 1400 m s 1). [70 m] 3) A man standing 48 m away from a wail fires a gun. Calculate the time after which an echo is heard. (The speed of sound in air is 320 m s 1) [0.30 s] 4) A pendulum has a frequency of 5 vibrations per second. An observer starts the pendulum and fires a gun simultaneously. He hears echo from the cliff after 8 vibrations of the pendulum. If the velocity of sound in air is 340 m s 1, what is the distance between the cliff and the observer? [272 m] 5) A man fires a gun and hears its echo after 5 s. The man then moves 310 m towards the hill and fires his gun again. This time he hears the echo after 3 s. Calculate the speed of sound. [310 m s 1] 6) a) An electromagnetic wave has a frequency of 500 MHz and a wavelength 60 cm. Calculate the velocity of the wave. Name the medium through which it is travelling. b) A wave has a wavelength of 0.01 . Name the wave. [3 108 m/s, ray] 7) A sound wave of wavelength 1/3 m has a frequency 996 Hz. Keeping the medium same, if frequency changes to 1328 Hz, calculate a) velocity of sound b) new wavelength. [(a) 332 ms 1(b) 0.25 m] 8) A boy standing in front of a wall at a distance of 85 m produces 2 claps per second. He notices that the sound of his clapping coincides with the echo. The echo is heard only once when clapping is stopped. Calculate the speed of sound. [340 ms 1] 9) In a Sonar, ultrasonic waves are sent into the sea water and the reflected waves from a sunken ship are received after 2.0 s. If the velocity of waves in sea water is 1450 ms 1, find the depth of sunken ship. [1450 m] Home Work: 1) Calculate the minimum distance in air required between the source of sound and the obstacle to hear an echo. Take speed of sound in air = 350 m s 1. [17.5 m] Volume 2 of 2 Universal Tutorials X ICSE Physics 11 12 2) A radar sends a signal to an a aeroplane e at a distanc ce 45 km awa ay with a spe eed of 3 10 08 m s 1 Aft fter how muc ch time is the e signal received back fro om the aerop plane? [3 10 4 s] 3) A ship on the surface s of wa ater sends a signal and receives r it ba ack from a su ubmarine ins side water aft ter 4s. Calcu ulate the dist tance of the submarine fr rom the ship p. (The speed d of sound in n water is [2.9 km] 14 450 m s 1). 4) A person stan nding betwee en two vertic cal cliffs pro oduces a sou und. Two su uccessive ec choes are he eard after 4 s and 6 s. Calculate th he distance between the e cliffs. (Spe eed of sound d in air = [1600 m] 32 20ms 1) [Hint : First ech ho will be heard from the nearest cliff f and the sec cond echo fro om the farthe er cliff] 5) On n sending an a ultrasonic c wave from a ship tow wards the bo ottom of a s sea, the time e interval be etween sendi ing the wave e and receivin ng it back is found to be 15 s. If the v velocity of wa ave in sea [1050 m] wa ater is 1400 m s 1, find th he depth of th he sea. 3 6) A sound wave of waveleng gth 0.332 m has a time period p of 10 3 s. If the time e period is decreased d to 10 4 s, calcu ulate the wav velength and d frequency of o the new wa ave. [0.0332 m, m 104 Hz] 7) A man standin ng in front of a vertical cliff c fires a gun. g He hear rs the echo after 3 s. On moving clo oser to the cl liff by 82.5 m, m he fires again and hear rs the echo after a 2.5 s. Find: a) the distance e of cliff from m the initial po osition of ma an, and b) the e speed of sound. [(a) ) 495 m, (b) 330 3 ms 1] 8) A person stand ding between n the two ver rtical cliffs an nd 640 m aw way from the nearest cliff, shouted. He e heard the first f echo afte er 4 s and the second echo 3 s later. Calculate: [(i) 320 ms 1 (ii) (i) the speed of o sound in ai ir and (ii) the distance between the cliffs. ) 1760 m] APPLICATION TYPE: T 1) Th he diagram below b in Fig shows s three ways in whic ch the string of an instrum ment can vibrate. a) Which of the t diagram m shows the e pri incipal note? ? tha b) Which has the frequenc cy four times at of the first? ? c) What is the e ratio of the frequency of the vibration in (i) and (i ii) ? 2) A vibrating tuning fork, held over an air column of a given length with its one end closed, pro oduces a lo oud audible sound. Nam me the phen nomenon re esponsible fo or it and ex xplain the ob bservation. 3) fig g. shows two o tuning for rks A and B of the sam me frequenc cy mounted on se eparate sound boxes with h their open ends e facing each e other. The T fork A is set int to vibration. (a) Describe e your obser rvation. (b) State S the prin nciple illustra ated by y this experim ment. 4) In Fig. A, B, C and D are four pendulums suspen nded from the same elas stic str ring XY. Len ngths of pend dulum A and d D are equal, while the e length of B is sm maller and C is longer. The T pendulum A is set in nto vibration n. What is yo our ob bservation? Give G reason for f your obse ervation. 5) In the followin ng diagram, A, B, C and d D represent test tubes s each of he eight 20 cm which w are filled with wat ter up to heights of 12 cm, c 14cm, 16 6 cm and 18 8 cm respect tively. If a vibrating tunin ng fork is pla aced over the e mouth of te est tube D, a loud sound is heard. a) Describe the e observations with the tu ubes A, B an nd C. b) Give the rea ason for your r observation n in each cas se. c) State the principle illustr rated by the above a experiment. 6) If the t amplitude e of a wave is doubled, what w will be the t effect on its loudness s? 12 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 07: 0 Sound 13 7) Tw wo waves of f the same pitch p have am mplitudes in the ratio 1: 3. What wil ll be the rati io of their int tensities? 8) A 0.6 m long stretched s wir re is made to o vibrate in tw wo dif fferent mode es as shown it Fig (i) and (ii). 9) a) If the freque ency of the no ote produced d in case (ii) is f, what is the frequenc cy in case (i) ? b) In which cas se (i) or (ii) is s the note lou uder? Give re eason. c) In which cas se is the pitch higher? Give reason. 10) What change do d you expec ct in the char racteristics of o a musical sound s if i) its i frequency y is increased d, ii) its amplitude e is increased d? 11) Na ame the characteristic which enables s one to dist tinguish the sound s of two o musical ins struments ev ven if they are e of the sam me pitch and same s loudne ess. 12) Th he sketches I to IV in Fig. . show sound d waves, all formed f in the e same time interval. Which diagram m shows i) a note from a musical ins strument, ii) a soft (not lo oud) note, iii) a bass (low frequency) note. n 13) A microphone is connected d to the Y input of a C.R R.O. Three dif fferent sound ds are made e in turn in fro ont of the mic crophone. Th heir traces (a a), (b), and (c c) produced on o the scree en are as sho own in fig. i) Which trac ce is due to the loudest t sound? Give reason for your ans swer. ii) Which trace e is due for the sound with the low west pitch? Explain your answer. 14) Give reasons: i) Bats have poor p eyesigh ht but are able to home in n on their pre ey with great accuracy. ii) Dolphins ca an avoid fishing nets and can detect fish f at night. iii) For the con nstruction of huge h auditor riums and ha alls, carpets and a curtains are used. iv) ) Sonar scan nners are use ed by doctors s. 15) What type of waves w are pro oduced when n a bell rings s in air? 16) a) State four differences be etween light waves and sound s waves s. b) If v stands for f the veloc city of a wave e, f is the fre equency of th he wave and d is the wav ve length, an equation n relating v, f and 17) Th he figure alo ongside sho ows a tunin ng fork with one of its s prongs fas stened to on ne end of a spring s whose e other end is i fastened to t a rigid su upport. When n the tuning fork f is made to vibrate, how h does the e pattern of the coils of the t spring ch hange? 18) a) Which of the t diagrams s A or B re epresents a sound of higher A frequency? Give a reas son for your answer. a b) Which of the t diagram ms represent ts a louder sound? Give a B reason for your y answer. Time interval is s same 19) Ex xplain why: a) If you place e your ear close to an iron railing wh hich is tappe ed some dist tance away, you hear the sound twice. t b) Sound can be heard by y putting the ear near the e railway line e although th he train is ve ery far off and is not visible. v c) The intensi ity or loudne ess of the sound s emitte ed by a pluc cked wire is increased when w it is mounted on n a board. d) The velocity y of sound in n hydrogen is s more than that t in oxyge en. e) Echo canno ot be heard in a small roo om. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 13 14 20) A sound wave produces compressions and rarefactions as it passes through air. How would the distance between successive compressions change if the sound were of a higher pitch? MISCELLANEOUS: 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27) 28) 14 Women s voice is shriller than men s voice. Why? Give one example of a phenomenon based on resonance. Distinguish between musical note and noise. Differentiate between resonance and forced vibrations. What is the condition necessary for an echo to be heard distinctly? State two applications of an echo. The echo of a sound made by us is not heard in a small room, but it is heard distinctly in a big hall. Explain why? State the use of echo by a bat, dolphin and fisherman. Name the waves which are used in sonar to find the depth of sea. Which quantity out of the following determines the pitch of a sound wave? i) wavelength ii) frequency and iii) amplitude. How is pitch related to the quantity mentioned above? If the amplitude of a wave is doubled, what will be the effect on its loudness? Name the unit in which loudness of sound is measured. Why is the loudness of the sound heard by a plucked wire increased when it is mounted on a sound box? Define the term intensity of a sound wave. State the unit in which it is measured. Comment on the statement loudness of sound is a subjective quantity, while intensity is an objective quantity What is the safe limit of sound level in dB for our ears? What is the sound level for noise pollution? Name the subjective property of sound related to its frequency. How does the frequency and amplitude affect the musical sound? What change do you expect in the characteristics of a musical sound if i) its frequency is increased ii) its amplitude is increased? How can one distinguish the sound of two musical instruments even if they are of the same pitch and same loudness? How do you account for the fact that two string instruments can be used to give notes of the same pitch and same loudness, but of different quality? How is it possible to recognize a person by his voice without seeing him? State the factors that determine i) the pitch of a note ii) the loudness of the sound heard iii) the quality of the note. In what respect does the frequency pattern of a noise and music differ? A wire stretched between two fixed supports, is plucked exactly in the middle and then executes (neglect the resistance of the medium) a) resonant vibrations with decreasing amplitude b) free vibrations c) damped vibrations d) forced vibrations. What is meant by the natural frequency of vibration of a body? When a body vibrates with its natural frequency, the force acting on the body is: a) zero b) proportional to its velocity c) proportional to its displacement d) a constant force Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 07: Sound 15 29) State one way of increasing the frequency of a note produced by an air column. 30) Name three factors on which the frequency of vibration of a stretched string depends. 31) What adjustments would you make for tuning stringed instrument for it to emit a note of desired frequency? 32) A blade, fixed at one end, is made to vibrate by pressing its other end and then releasing it. State one way in which the frequency of vibration of the blade can be lowered. 33) A string stretched between its ends, is made to vibrate by placing the stem of a vibrating tuning fork at its one end. State three ways how you will increase the frequency of the note produced by the string. 34) How does the medium affect the amplitude of free vibrations of a body? 35) Distinguish between free (or natural) and forced vibrations. 36) Give two examples of forced vibrations. 37) What is meant by resonance? Describe and explain a simple experiment to illustrate the phenomenon of resonance. 38) An observer stands at a distance of 850 m from a cliff and fires a gun. After what time gap will he hear the echo, if sound travels at a speed of 350 m s 1 in air? 39) A bat emits an ultrasonic sound of frequency 0.25 MHz. Calculate the time in which one vibration is completed. 40) The wavelength of waves produced on the surface of water is 20cm. If the wave velocity is 24ms 1, calculate: i) the number of waves produced in one second, and ii) the time required to produce one wave. 41) A radar is able to detect the reflected waves from an energy aero plane after a time interval of 0.02 millisecond. If the velocity of the waves is 3 108 ms 1, calculate the distance of aero plane from the radar. 42) When a wave travels through a medium: a) particles are transferred from one place to another b) energy is transferred in a periodic manner c) energy is transferred at a constant speed d) none of the above statements is applicable 43) The minimum distance between the source and the reflector in air, so that an echo is heard, is approximately equal to: a) 10 m b) 17 m c) 34 m d) 50 m 44) The echo of a sound made by us is not heard in a small room, but it is heard distinctly in a big hall. Explain why? 45) Explain how the speed of sound can be determined by the method of echo. 46) State the use of echo by bats, dolphins and fisherman. 47) Bats detect the obstacles in their path receiving the reflected: a) infrasonic waves b) ultrasonic waves c) electromagnetic waves d) radio waves. 48) Name the waves which are used in SONAR for the depth of a sea. Give one reason for their use 49) A wire stretched between two fixed supports, is plucked exactly in the middle and then released. It executes (neglect the resistance of the medium): a) resonant vibrations with decreasing amplitude b) free vibrations c) damped vibrations d) forced vibrations. 50) What is meant by the natural frequency of vibration of a body? Volume 2 of 2 Universal Tutorials X ICSE Physics 15 16 51) When a body vibrates with its natural frequency, the force acting on the body is a) zero b) proportional to its velocity c) proportional to its displacement d) a constant force. 52) Draw a graph between displacement from mean position and time for a body executing free vibrations in vacuum. 53) Name one factor on which the frequency of sound emitted due to vibration in an air column depends. 54) State one way of increasing the frequency of a note produced by an air column. 55) Name three factors on which the frequency of vibration of a stretched string depends. 56) State any two ways of increasing the frequency of vibration of a stretched string. 57) A blade, fixed at one end, is made to vibrate by pressing its other end and then releasing it. State one way in which the frequency of vibration of the blade can be lowered. 58) A string stretched between its ends, is made to vibrate by placing the stem of a vibrating tuning fork at its one end. State three ways to increase the frequency of the note produced by the string. 59) How does the medium affect the amplitude of free vibrations of a body? 60) Draw a sketch showing the displacement of a body executing damped vibrations, against time. 61) Give two examples of forced vibrations. 62) Differentiate between forced vibrations and resonance. 63) Complete the following sentence: Resonance is a special case of ____________ vibrations, when frequency of the driving force is __________ natural frequency of the body. 64) What is meant by resonance? Describe and explain a simple experiment to illustrate the phenomenon of resonance. 65) Why is a loud sound heard at resonance? 66) Why are stringed instruments provided with a sound box? 67) How do you tune your radio set to a particular station? Name the phenomenon involved in doing so and define it. 68) Which quantity out of the following determines the loudness of a sound wave? i) wavelength, (ii) frequency, and (iii) amplitude. How is loudness related to the quantity mentioned above? 69) How does the wave form of a loud note differ from that of a soft note? Draw a diagram. 70) Why is the loudness of the sound heard by a plucked wire increased when it is mounted on a sound board? 71) Define the term intensity of a sound wave. State the unit in which it is measured. 72) How is loudness of sound related to the intensity of wave producing it? 73) State three factors on which loudness of sound heard by a listener depends. 74) Name the unit used to measure the sound level. 75) What is the safe limit of sound level in dB for our ears? 76) What determines the pitch of a sound? 77) Name and define the characteristic which enables one to distinguish two sounds of same loudness, but of different frequencies. 78) Draw a diagram to show the wave form of high pitch note and a low pitch note, but of the same loudness. 79) How is it possible to detect the filling of a bottle under a water tap by hearing the sound at a distance? 80) How do frequency and amplitude affect a musical sound? 16 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 07: 0 Sound 17 81) Co omplete the following f sen ntences: a) The pitch of f sound incre eases if its fre equency ___ _________ b) If the amplitude of a sou und is halved d, its intensity y becomes _________ __ 82) What do you understand by the term quality of a musical no ote? Illustrate e your answ wer with a dia agram. 83) Tw wo identical guitars g are played by two o persons to give notes of the same p pitch. Will the ey differ in qu uality? Give a reason for your y answer r? 84) Tw wo musical notes n of the same pitch and same loudness are e played on a violin and on a piano. Their wave forms f are as s shown in the diagram be elow. Ex xplain why th he wave patte erns are diffe erent 85) Ho ow do you ac ccount for th he fact that tw wo string ins struments ca an be used to o give notes of o the same pitch and sa ame loudness s, but of diffe erent quality? ? 86) In what respec ct does the fr requency pat ttern of a noise and music differ? 87) Dr raw a diagra am to show the t wave for rms of a pure e note and a musical no ote, of the sa ame pitch an nd same loud dness. 88) State the facto ors on which the velocity of sound dep pends. nge of audio frequencies? 89) What is the ran 90) Na ame the soun nd waves: ii) Above i) Below B the au udio range A the au udio range 91) Give two pract tical applicati ions of echo. 92) Distinguish bet tween the following: i) Free ii) Forced F and forc ced vibration ns F vibrations and res sonance iii) Musical note and noise iv) Damped vib brations and Natural vibra ation 93) Give three exa amples of res sonance. 94) Ex xplain with ex xamples, the e characterist tics of loudne ess, pitch an nd quality. 95) Na ame a music cal instrumen nt that makes s use of the vibration v of stretched s strin ng. 96) a) Name the part of the ins strument that t vibrates to produce p sound in each o of the followin ng: i) a violin ii) an organ pip pe iii) a drum b) If the sound d of the sam me pitch is pr roduced by th hree instrum ments mentioned above, why w does the same sound differ? 97) a) Write down the factors on o which the frequency of a vibrating stretched str ring depends s. b) What adjus stments will you make fo or tuning a stringed s instrument, such h as a violin n, for it to emit a desir red pitch? 98) State the facto ors that deter rmine the lou udness of a sound s wave. . What is the e relationship p between lou udness and amplitude? a 99) In which case is the speed of sound mo ore: in humid d air or dry air? Why? 100) What is the audible range in i (a) Humans (b) bats? 101) What is meant t by reverber ration ? 102) A violin v and a piano sound the same no ote in turn. What W enables s us to identify each instr rument by on nly listening to the note pr roduced? 103) a) If the rate of o vibration of o a soundin ng body is doubled, wha at is the effect on the pit tch of the sounding body? b) A lampshad de vibrates in n response to o a certain note. Explain this. 104) Write down the e factor or fa actors on wh hich the inten nsity of soun nd depends t taking your answer(s) a fro om the four fa actors listed below: Frequency, am mplitude of vibration, v pre esence of ha armonics, dis stance betwe een the obse erver and ource. so 105) State one facto or that determ mines: a) pitch of a no ote b) the intensity y of the sound d heard c) t the quality of f the note Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 17 18 PR REVIOUS YE EAR BOARD D QUESTION NS: 1) Which charac cteristic of sound will ch hange if the ere is a cha ange in (i) its amplitud de, (ii) its wa [2012] aveform? 2) i) Name one factor which h affects the e frequency of sound em mitted due to vibrations in an air column. [2012] ii) Name the unit u used for measuring the sound lev vel. 3) i) What is me eant by reson nance? [2012] ii) State two ways w in which h resonance differs from forced vibrat tions. 4) i) A man stan nding betwee en two cliffs s produces a sound and hears two s successive echoes e at intervals of 3 s and s re espectively. Calculate C the e distance be etween the tw wo cliffs. The e speed of sound in the air is 330 m s 1. ii) Why will an echo not be heard when the dist tance betwe een the sour rce of sound d and the reflecting su [2012] urface is 10 m? 5) Th he adjacent diagram sh hows the di isplacementtim me graph for a vibrating body. b i) Name the type of vibr rations produced by the e vibrating bo ody. ii) Give one example e of a body producing such h vibrations. iii) Why is tile e amplitude of the wav ve gradually y decreasing? iv) ) What will happen h to th he vibrations of the body y after some time? [2012] 6) When acoustic c resonance takes place, a loud soun nd is heard. Why W does thi is happen? Explain. E [2010] 7) i) Three mus sical instrume ents give ou ut notes at the frequenc cies listed b below. Flute: 400 Hz; Guitar: 200 Hz; Trumpe et: 500 Hz. Which W one of these has th he highest pit tch? ii) With which of the follow wing frequen ncies does a tuning fork of o 256 Hz re esonate? 288 8 Hz, 314 Hz, 333 Hz, 512 Hz. [2010] 8) i) Name the ty ype of waves s which are used u for sound ranging. ii) Why are the ese waves mentioned m in (i) above, no ot audible to us? iii) Give one us [2010] se of sound ranging. 9) A man standing 25 m away y from a wall l produces a sound and receives r the reflected sou und. i) Calculate th he time after r which he re eceives the reflected r sou und if the speed of sound in air is 350 ms 1. ii) Will the man be able to hear a distin [2010] nct echo? Giv ve a reason for your answ wer. 10) A stringed musical instrum ment, such as s the sitar, is s provided with w a numbe er of wires of f different ickness. Exp plain the reas son for this. thi (2009) 11) What is meant by noise pollution? p Write the nam me of one so ource of sound that caus ses noise po (2009) ollution: 12) i) What is the e principle on n which sonar is based? ii) Calculate the t minimum m distance at a which a person shou uld stand in front of a reflecting (2009) surface so that t he can hear h a distinc ct echo. (Tak ke speed of sound s in air = 350 m/s) 13) i) Name the characteristic c c of sound which enables s a person to o differentiate e frequencies s. ii) Define the characteristic c c named by you in (i) iii) Name the characterist tic of sound d which ena ables a pers son to differ rentiate betw ween two requency but t produced by b different in nstruments. sounds of the same loudness and fr (2009) 18 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 07: Sound 19 14) a) What is meant by an echo? Mention one important condition that is necessary for an echo to be heard distinctly (2008) b) Mention one important use of echo c) Sometimes when a vehicle is driven at a particular speed, a rattling sound is heard. Explain briefly, why this happens and give the name of the phenomenon taking place. d) Suggest one way by which the rattling sound could be stopped. 15) Explain why musical instruments like the guitar are provided with a hollow box. (2006) 16) When a tuning fork, struck by a rubber pad, is held over a length of air column in a tube, it produces a loud sound for a fixed length of the air column (2006) a) Name the above phenomenon b) How does the frequency of the loud sound compare with that of the tuning fork? c) State the unit for measuring loudness. 17) State two ways by which the frequency of transverse vibrations of a stretched string can be decreased. 18) a) What is Sonar? State the principle on which it is based (2004) b) Differentiate between resonance and forced vibrations. c) The wavelength of waves produced on the surface of water is 20 cm. if the wave velocity is 24 ms 1, calculate i) The number of waves produced in one second and ii) The time required to produce one wave. ANSWERS: Previous Year Board: 1) (i) Loudness (ii) Quality. 1 2) (i) Length of air column. frequency length of air column . (ii) decibel (dB) or bel. 3) i) The phenomenon of vibration of a body with an increased amplitude and with its natural frequency under a periodic force of frequency equal to the natural frequency of the body, is called resonance. ii) 1) The forced vibrations of small amplitude occur when frequency of external periodic force differs from the natural frequency of the body while the resonant vibrations of large amplitude occur when frequency of external periodic force is exactly equal to the natural frequency of the body. 2) The forced vibrations of the body are not in phase with the external periodic force while the resonant vibrations of the body are in phase with the external periodic force. 4) i) Let distance of man from nearer cliff = d1 m and from farther cliff = d2 m Given t1 = 3 s and t2 = 4 s. Speed of sound V = 330 m s 1 Vt1 Vt 2 330 3 330 4 = = 495 m; d2 = = = 660 m d1 = 2 2 2 2 Total distance between the two cliffs = d1 + d2 = 495 + 660 = 1155 m ii) Time taken by sound to travel from source to the reflecting surface at distance 10 m and then back to the source will be t = 2 d/V = 2 10/330 = 0.06 s which is less than 0.1 s for which the original sound persists in the human ear. Therefore, echo will not be heard. 5) i) Damped vibrations. ii) A vibrating tuning fork in air. iii) The vibrating body loses energy continuously in each vibration in doing work against the force of friction of air (medium). iv) The vibrations will cease. Volume 2 of 2 Universal Tutorials X ICSE Physics 19 20 Chapter 8: Current Electricity Chapter Map Introduction Electric Current Definition Units One ampere Flow of current Potential Measurement of potential Definition Units One Volt P.D Ohm s Law Law Concept of Resistance Factors Resistivity factors Emf and internal Resistance of cell Combination of resistors Series Parallel Introduction: Electricity is a form of energy It is of two types Static electricity Current electricity Electric Current: Electric charges in motion cause electric current 20 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 08: Current Electricity 21 Definition: z Electric current is the amount of charge flowing through a cross section of a conductor in Q one second. i.e. I = where I is current, Q is quantity of charge and t is duration of flow of t charge. z S I Unit. of current is ampere (A) Def.: Current is said to be one ampere when a charge of 1 coulomb flows through a cross section of a conductor per second. z 1A= 1C 1s z Smaller units: milliampere (mA) and microampere ( A) z 1 m A = 10 3 A, 1 A = 10 6 A It is a scalar quantity. Flow of current: z In metals, the moving electrons (negative charges) constitute current. z 1 coulomb of charge = z z 1 electron charge = 6.25 1018 e . 1.6 10 19 In electrolytes and ionized gases, positive and negative ions constitute current. If n electrons pass through a cross section in time t, then total charge Q = ne and current = Q ne = t t Electric Potential: Potential is the electrical state of a conductor which determines the direction of flow of charge when the two conductors are either kept in contact or they are joined by a metallic wire. Definition: z The potential at a point is defined as the amount of work done in bringing a unit positive charge from infinity to that point. Measurement of Potential: z If a charge is brought near a like charge some work is done against the repulsive force. Hence potential is measured in terms of work done in moving a test charge Potential V = z z SI unit is volt (V) If 1 joule of work is done in bringing 1 coulomb of charge from infinity to a point, the potential at that point is 1 volt. Potential difference between two points in an electric circuit is the amount of work done to move a unit positive charge from one point to the other. P.D. = z work W = ch arg e Q Work (W ) . Ch arg e Q SI unit is volt (V): Potential difference is said to be 1 volt when 1 Joule of work is done in moving a charge of one coulomb, across two points. Volume 2 of 2 Universal Tutorials X ICSE Physics 21 22 1J 1Q z 1V= z It is a scalar quantity. Ohm s Law: It states that the current flowing through a metallic conductor at constant temperature and similar physical Condition is proportional to the potential difference applied to its ends. V I V = IR Where R is the resistance of the conductor. Ohm s law is obeyed only when the temperature remains constant. V R= Graph of V I for a metallic conductor is a straight line The slope gives resistance (R) Greater the slope greater is the resistance. V I I Conductors which obey ohm s law are called ohmic conductors or linear conductors. Non ohmic conductors are those conductors which do not obey Ohm s law. Then V I graph is a curve. The resistance of non ohmic conductors is obtained by the slope of the V V V tangent drawn at the corresponding point. Since is different for I I different values of V I hence the resistance is called dynamic resistance. I In case of certain substances, the resistance decreases considerably with the decrease in temperature (near absolute zero) such substances are called super conductors. Slope of I-V graph gives conductance. S.I. unit is ohm 1 or siemen. I V Resistance: The opposition to the flow of current is called resistance. S I unit is ohm. One ohm is the resistance of a conductor which requires a p.d. of one volt to be applied to its ends to send a current of one ampere through it. Resistance of a wire depends on the following: z z It is directly proportional to the length of the wire. It is inversely proportional to the area of cross section of the wire. Depends on material. It depends on the temperature. l l , where R = when is specific resistance (resistivity) of the material of the A A conductor. z Resistivity or specific resistance of a material is the resistance of wire of that material of unit length and unit area of cross section z Unit: ohm m. z 22 R Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 08: Current Electricity 23 Note: If the length of the wire is doubled by stretching it, the area of cross section gets halved (because volume remains unchanged). If a wire is doubled on itself, length gets halved and area of cross section is doubled. Factors affecting Resistivity: z z z Material: Resistivity of metals is less and is in the range of 10 8 to 10 10 m. Temperature: Resistivity increases with increase in temperature for metals. Resistivity does not depend on Shape and size of conductor / length and area of cross section Potential difference applied Current in the circuit Note: Connecting wires are made of copper or aluminum as their resistivity is small. Standard resistors are made of Magnanin as alloys have high resistivity. Fuse wire is made of tin and lead because of its high resistivity and low melting point. z z = Reciprocal of resistivity is called conductivity. 1 l = Ra Unit: Ohm 1 m 1 or Siemen metre 1. Resistance and resistivity remain Unchanged for alloys with increasing temperature. For semiconductors, resistance decreases with increase in temperature. Substance Resistivity ( m) Metals Silver 1.63 10 Copper Substance Resistivity ( m) Alloys Brass 6.6 10 8 1.73 10 8 Manganin 44 10 8 Aluminium 2.63 10 8 Constantan 49 10 8 Steel 20 10 8 Nichrome 100 10 8 Semiconductors Carbon 3.5 10 5 Insulators Wood 108 1011 Germanium 0.6 10 5 Glass 1010 1014 Silicon 2.3 10 5 Amber 5 1014 Mica 1011 1015 z 8 Difference between the ohmic conductor and Non ohmic conductor: Ohmic conductor Non ohmic conductor It obeys the Ohm s law. i.e. V/I is constant for all values of V or I. It does not obey the Ohm s law i.e. V/I is not same for all values V/I. The graph for current I versus potential V is a straight line. The graph for current I versus potential difference V is not a straight line. The slope of V-I graph is same at all values of V or I The slope of V-I graph is different at different values of V or I Example: All metallic conductors such as silver, copper etc. Example: Junction electrolyte etc. Volume 2 of 2 Universal Tutorials X ICSE Physics diode, transistor, 23 24 Superconductors: z z z z z It is observed that the resistance of some substances like tin, lead etc., decreases tremendously with the decrease in temperature in the very low temperature range (i.e. near absolute zero). The substances under this condition are called the superconductors. The resistance of a super conductor is almost zero in that very low temperature range. A superconductor is a substance of zero resistance at a very low temperature. The property due to which a conductor has zero resistance at a low temperature is called superconductivity. Difference between resistance and resistivity: Resistance Resistance of a conductor is the obstruction offered by the conductor in the flow of current through it. It is measured by the potential difference needed across the conductor to flow one ampere current through it (R = V/I) The resistance of a conductor depends upon its material; temperature, length and area of cross section. Its S. I. unit is ohm ( ) Resistivity It is the property of the conductor due to which it offers resistance to the flow of current through it. It is measured by the resistance offered by 1 m length of wire of that material of area of cross section 1 m2 (P = Ra / l ) The resistivity of a conductor depends only on its material and temperature. Its S. I. unit is ohm metre ( m). Electromotive Force (e.m.f.): When no current is drawn from a cell, the P. D. between the terminals of the cell is called e.m.f. or it is potential difference between terminals of a cell in open circuit. It is a measure of work done in moving a unit positive charge once round the closed circuit. Factors affecting e.m.f. of a cell: Material of electrodes The electrolyte used Emf of a cell does not depend on: Shape of electrodes z Distance between the electrodes. z Amount of electrolyte z Current drawn from the cell The p.d. between the plates of a cell when current is drawn from it is called terminal voltage (V) or it is p.d. in closed circuit. It is also measured as potential difference across the external circuit. Distinction between e.m.f. and terminal voltage e.m.f. Terminal Voltage It is measured by the amount of work done It is measured by the amount of work done in in moving a unit positive charge in the moving a unit positive charge in the circuit complete circuit inside and outside the cell. outside the cell. It is the characteristic of the cell i.e. it does It depends on the amount of current drawn from not depend on the amount of current drawn the cell. More the current is drawn from the cell, from the cell. less is the terminal voltage. It is equal to the terminal voltage when cell is It is equal to the e.m.f. of cell when cell is not in not in use, while greater than the terminal use, while less than the e.m.f. when cell is in voltage when cell is in use. use. z 24 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 08: Current Electricity 25 Internal resistance: The resistance offered by the electrolyte inside the cell, to the flow of current is called the internal resistance of the cell (r) V = Ir. Internal resistance depends on 1 ). A Nature and concentration of the electrolyte: More the concentration more is the internal resistance. (r c) Surface area: Larger the area of the electrodes, less is the internal resistance (r 1 ) T Total resistance in a circuit = R + r, where R is the resistance in the external circuit. Temperature: Higher the temperature, less is the internal resistance. (r Current, I = E in external circuit R+r Terminal voltage, V = IR ER Er Voltage drop, Ir = R+r R+r Emf of the cell, E = V + Ir. E 1 V Internal resistance of the cell, r = Resistors in Series: In this combination, the resistors are joined R3 R2 R1 Q P one after the other as shown in the figure. V3 V2 V1 Resistors R1, R2 and R3 are connected in I series between the points P and Q. The Cell Key + ends of the combination are then connected to the terminals of the cell. V Let R be their combined resistance, suppose, V is the potential difference applied between the extreme ends P and Q so that V1, V2, V3 are the potential differences across the resistors R1, R2, R3 respectively. Thus: V = V1 + V2 + V3 + . According to Ohm s law, V = IR As the same current I passes through the various resistors. V1 = IR1, V2 = IR2, V3 = IR3 Substituting these values in (i), we get IR = IR1 + IR2 + IR3 or R = R1 + R2 + R3 Thus, when three resistors are connected in series the total resistance is equal to the sum of the individual resistances. If R1, R2 and R3 are 2 , 3 , and 4 respectively, total resistance. R = 9 Resistances in Parallel: In this combination, one end of each resistor is connected at one point (say A) and the other end of each resistor is connected at the other point (say B). The cell is then connected in between these points A and B is shown in the figure. Volume 2 of 2 Universal Tutorials X ICSE Physics 25 26 Let R be the equivalent resistance, V th he potential difference d ap pplied betwee en the ends A and B. e current I divides itself in nto various branches b as I1, I2 and I3 flowing thro ough the res sistors R1, The R2 and a R3 respe ectively. Tota al current, I = I1 + I2 + I3 Pote ential differen nce between n A and B. V = I1R1 = I2R2 = I3R3 As R is the equ uivalent resis stance and I the total cu urrent, then n: I= V R Sub bstituting in (i), we get V V V V = + + R R1 R2 R3 or 1 1 1 1 = + + R R1 R2 R3 Thus, when thre ee resistors are a connecte ed in parallel, the recipro ocal of the co ombined resistance is equal to the sum m of the recip procals of the e individual re esistances. If R1 = 2 , R2 = 3 and R3 = 4 then Equ uivalent resistance, R= 1 1 1 1 1 6 + 4 + 3 13 = = + + = 12 12 1 R 2 3 4 12 an the least of o the individ dual resistanc ce (2 ) = 0.92 , which is less tha 13 REVIEW QUESTIONS: Direct Questions: Q 1) De efine the term m current and d its S.I. unit t. 2) De efine the term m electric pot tential. State e its S.I. unit. 3) De efine the term m resistance. State and define d its S.I. unit. 4) Tw wo wires, one e of copper and the othe er of iron, are e of the sam me length and d same radiu us. Which will have more e resistance? ? Give reason n. 5) What is the ne ecessary condition for a conductor c to obey Ohm s law? 6) What is an ohmic conduct tor? Give one example of o an ohmic conductor. c D Draw a graph h to show its current-volta age relations ship? 7) Na ame the factors on which h the resistan nce of a wire e depends. How H does the resistance of a wire de epend on the e factors state ed by you? 8) Write an expression conn necting the resistance r and resistivity y. State the meaning of f symbols us sed. 9) A cell is used to send cur rrent to an external circu uit. How does s the voltage e across its terminals co ompare with its i e.m.f ? Un nder what co ondition is the e e.m.f. of a cell equal to its terminal voltage? 10) Na ame a subs stance of wh hich the res sistance rem mains almost t unchanged d by the inc crease in tem mperature. 11) Na ame the mate erial used for making con nnection wire es. Give a re eason for you ur answer. 12) Na ame the mate erial used for making a fu use wire. Giv ve a reason. 13) What is a supe erconductor? ? 14) Dif fferentiate be etween the e.m.f. e and ter rminal voltag ge of a cell. 15) State two facto ors on which the internal resistance of o a cell depe ends. 16) On n what factor rs does the resistivity r of a wire depend? Does it depend on s size and sha ape of the wire? 26 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 08: 0 Current Electricity E 27 N NUMERICAL S: Class Work: W 1) A current of 1. .6 mA flows through a co onductor. If charge c on an n electron is 1.6 10 19 coulomb, fin nd the numb ber of electr rons that will pass each second th hrough the c cross sectio on of that [1016] co onductor. 2) An n electric bulb draws 1.2 A current at 6.0 V. Find the t resistanc ce of filament t of-bulb. [5 ] 3) Ca alculate the current c flowin ng through a wire of resi istance 5 2 connected c to o a battery of f potential [0.6 A] dif fference 3 V. . 4) Tw wo wires of the t same ma aterial and same s length have radii r1 and r2 resp pectively. Co ompare (i) the eir resistance es, (ii) resisti ivity [(i) r22 : r12 (ii) 1 : 1] 5) A wire w of 3 ohm m resistance e and 10 cm length is stre etched to 30 cm length. A Assuming tha at it has a un niform cross section, s wha at will be its new n resistanc ce? [27 ] 8 8 6) What length of copper wir re of resistivi ity 17 10 m and ra adius 1 mm is required so s that its sistance is 1 . [184.7 [ m] res 7) A dry battery of e.m.f. 3.0 0 V supplies current thro ough a circuit in which the resistanc ce can be ch hanged. A high resistanc ce voltmeter r is connecte ed across th he battery. A As the curre ent in the cir rcuit is increa ased, the vol ltmeter readi ing decrease es. Explain th his. When the current is 1.5 A, the [0.2] vo oltmeter reads 2.7 V. Find d the internal l resistance of o the battery y. 8) A battery of e.m.f. 15 V and intern nal resistanc ce 3 ohm is s connected d to two res sistors of res sistances 3 ohm o and 6 ohm in series. Find: a) the current through t the battery, b b) the p.d. betw ween the terminals of the e battery. [(a) ) 1.25 A (b) 11.25 1 V] nal resistance e r sends cur rrent 1.0 A when w it is con nnected to an n external 9) A cell of e.m.f. and intern sistance 19 . But it sen nds current 0.5 0 A when it t is connecte ed to an exte ernal resistan nce 8.9 . res Ca alculate the values v of and r. [E = 2; r = 0.1 ] 10) Th hree resistors s each of 2 are conne ected togethe er so that their total resis stance is 3 Draw a dia agram to sho ow this arran ngement and check it by calculation. c [2 2 in parallel; 1 series] 11) Yo ou have thre ee resistors of values 2 , 3 , and 5 . How wi ill you join th hem so that t the total res sistance is le ess than 2 ? ? [in n parallel] 12) In the circuit sh hown below, calculate the value of x if the the e equivalent resistance between b A an nd B is 4 [1 ] 13) Ca alculate the effective e resistance between the poin nts A and B in the circuit show wn in fig. [4.5 ] 14) A circuit consists of 1 ohm m resistor in series with a parallel arrangement of 6 ohm an nd 3 ohm sistors. Calculate the tota al resistance e of the circui it. Draw a dia agram. res [3 ] 15) In the network shown in ad djacent fig. ca alculate the eq quivalent resi istance betw ween the poin nts a) A and B, b) A and C [1.5 5 , 2 ] 16) Volume 2 of 2 Calculate the t resistanc ce between the t points X a and Y in the netw work fig show wn below: Universal Tu utorials X IC CSE Physics s [2 2 , 8 ] 27 28 17) A resistor of 6 is connect ted in series with another r resistor of 4 . A poten ntial differenc ce of 20 V is applied acro oss the comb bination. Calc culate the cu urrent in the circuit c and th he potential difference d cross the 6 resistor. [2 2 A, 12 V] ac 18) A particular res sistance wire e has a resistance of 3.0 ohm per me eter find: a) The total res sistance of th hree lengths of this wire each e 1.5 m long, in paral llel. b) The potenti ial difference e of the batt tery which gives a curre ent of 2.0 A in each of the t 1.5 m len ngth when connected c in n parallel to o the battery y (assume that t the res sistance of battery b is ne egligible). c) The resistan nce of 5m le ength of a wire w of the sa ame material, but with tw wice the area a of cross se ection. [(a) 1.5 5 (b) 9 V (c) 7.5 ] 19) Ca arefully study y the circuit diagram d show wn below and calculate [11.28 ] the e value of resistor x. our cells eac ch of e.m.f. 2 V and internal resis stance 0.1 are connected in se eries. The 20) Fo co ombination is s connected in series to an ammeter r of negligible e resistance, , a 1.6 res sistor and an n unknown re esistor R1. Th he current in the circuit is s 2 A. Dr raw a labelled circuit diag gram for the above arrang gement and calculate: a) the total res sistance in th he circuit, b) the t total e.m m.f. c) the value of f R1, d) the t potential difference a across R1 2 + R1) (b) ) 8 V (c) 2 (d) 4 V] [(a) (2 X 3 P 21) Fiv ve resistors each e 3 , ar re connected d as shown in the diagram. Calculate th he resistance e: a) Between th he points P and Q, b) Between the e points X an nd Y. 3 A 3 3 Q Y [(a) 2 (b) 8 ] 3 22) Fo our resistors of resistanc ces 0.5 , 1. .5 , 4 and d 6 are connected in s series to a battery b of e.m m.f. 6V and negligible n int ternal resista ance. Calcula ate: a) the current drawn d from cell c [(a b) p.d. at the end e of each resistor. r a) 0.5 A (b) 0 0.25 V, 0.75V V, 2V, 3V] Home Work: W 1) Fin nd the potential differenc ce required to o pass a curr rent of 0.2 A in a wire of r resistance 20 0 . [4 V] 2) A car bulb connected to a 12 volt battery draws 2 A current whe en glowing. W What is the resistance of the filament of the bulb b? Will the resistance be b more, sam me or less w when the bu ulb is not owing? [6 resistor will be less] glo 3) In an experime ent of verifica ation of Ohm m s law, follow wing observa ations are obt tained. Po otential differ rence 0.5 1.0 1.5 2.0 2.5 5 Cu urrent I (in am mpere) 0.2 0.4 0.6 0.8 1.0 0 Dr raw a charac cteristic V I graph g and us se this graph h to find: 28 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 08: 0 Current Electricity E 4) 5) 6) 7) 8) 29 a) potential diff ference V wh hen the curre ent I is 0-5 A, A b) current I when the poten ntial differenc ce V is 0.75 V, V n circuit. [(a) 1.25 V (b) 0.3 A (c c) 2.5 ] c) resistance in A given wire is s stretched to o double its le ength. How will w its resista ance change e? [4 times] A wire w of 9 ohm m resistance e having 30 cm c length is tripled t on itse elf. What is it ts new resist tance? [1 ] Th he diagram below b in fig. shows s a cell of e.m.f. = 2 volt and in nternal res sistance r = 1 ohm to an external l resistance R = 4 ohm m. The am mmeter A measures m the e current in t circuit and the voltmeter V me easures the terminal volt tage across the cell. Wha at will the readings of the ammete er and voltme eter when (i) ) the key K is s open; (ii) the key K is closed. [(i) A = 0 V = 2 ( (ii) A = 0.4, v = 1.6 V] A cell of e.m.f. 1.8 V and in nternal resist tance 2 is connected a ammeter of o resistance e 0.7 and a resistor of in series with an 5 as show wn in fig. 4.5 a) What would be the reading of the am mmeter? b) What is the potential diff ference across the termin nals of the ce ell? [(a) 0.25 A (b) ( 1.3 V] A cell of em.f. 15 V and inte ernal resistance 10 is co onnected to a resistor of 5 5 with an ammeter in se eries [0.1 A] Fig g. What is the reading of the ammete er? 9) Tw wo resistors having resis stance 5 and a 10 ar re connected d in parallel. Find their equivalent e res sistance. [3.3 ] 10) Fo our resistors each of resis stance 2 are a connected in parallel. What is the effective res sistance. [0.5 ] 11) Ca alculate the equivalent e re esistance of the t fol llowing comb bination of re esistors r , r2, r3 and r4 if r1 = r2 = r3 = r4 = 2.0 between the e points A an nd B. [5 ] 12) A combination consists of three resisto ors in series. Four similar sets are co onnected in parallel. p If e resistance of each resis stor is 2 ohm m, find the res sistance of th he combination. the [1.5 ] 13) A wire of unifo orm thickness s with a resis stance of 27 7 is cut into o three equa al pieces and d they are lel. Find the resistance of f the parallel combination n. joined in parall [3 ] alculate the effective e resistance between the poin nts A 14) Ca an nd B in the ne etwork show wn below in fig g. [1.0 ] 15) Ca alculate the resistance r be etween the po oint X an nd Y in the ne etwork fig sh hown below: [0.71] 16) Tw wo resistors of o 2.0 and d 3.0 are connected c fir rst in series, and then in parallel with a battery of 6.0 V and negligible n internal resista ance. For each case draw w a circuit d diagram and calculate the e current thro ough the bat ttery. [in series 1.2 A, in par rallel 5 A] 17) Volume 2 of 2 s the current t flowing thro ough each of the resistors s A and What is B in the e following circuit c shown in fig. [A = 2 A B = 1 A] Universal Tu utorials X IC CSE Physics s 29 30 18) Th he figure give en shows a circuit c diagram having a battery b of 24 V and ne egligible inter rnal resistanc ce. Calculate e: (a) ) reading of ammeter a (b) readings of V1, V2 and V3. [(a) 2A A (b) V1 = 3V V, V2 = 4 V, V3 = 17 V] 19) Calculate the equivale ent resistance e between th he points X and a Y work shown below: in the netw [1 ] 20) In the figure sh hown, calcula ate: f combined resistance r of f 40 i) the value of and R, usin ng the reading of the two meters. 0.4 A f R. ii) the value of [(i) 10 (ii) 13.3 (iii) 0.3 A] iii) the current flowing throu ugh R. s a current of o 0.6 A through a 2 coil c and a current of 0.3 A through a 8 coil. 21) A cell supplies alculate the e. e m. f and th he internal re esistance of the t cell. Ca [3.6 V, 4 ] 22) Th he diagram shows s the circuit c diagra am containing g 12 cells, each e of e.m.f. 1.5 V and d internal res sistance 0.25 5 . Calculat te the: a) total interna al resistance. b) total e.m.f. c) total external resistance e. d) reading sho own by ammeter. 1 and 8 resistors. e) Current in 12 or. f) p.d. across 2.2 resisto ential across s the terminals of battery. g) drop in pote [(a) 3 (b) 18 V (c) 9 (d) 1.5A (e) 0.75 A (f) 3.30 V (g g) 4.5 V] APPLICATION TYPE: T 1) A substance has zero resis stance below w a temperatu ure of 1 K. What W is such a substance e called? 2) A cell of e.m.f f. E and internal resistan nce r is used d to send current to an e external resis stance R. Write expressions for (a) th he total resistance of circ cuit, (b) the current drawn n from the ce ell, and (c) the e p.d. across s the cell. 3) Th he V-I graph for a series s combinatio on and for a parallel com mbination of two res sistors is sh hown in Fig. Which of th he two, A or B, represe ents the para allel co ombination? Give G a reaso on for your an nswer. 4) What happens s to the resist tance as the conductor is s made thinn ner? 5) What happens s to the resist tance if the le ength of the wire is const tant and the radius is dou ubled? 6) What happens s to the resist tance if the radius r of the wire is const tant and the length is dou ubled? 7) i) The voltage e applied to a given rheo ostat is made e four times its initial valu ue. By what factor will the resistan nce of the rhe eostat chang ge? ii) What are th he conditions s under which charges ca an move in a conductor? iii) How will yo ou maintain a potential dif fference betw ween the end ds of a condu uctor? iv) ) By which na ame is joule/ /coulomb cal lled? v) Name the device d used for f measurin ng: a) electric current c b) potential diff ference 30 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 08: 0 Current Electricity E 31 8) Th he diagram alongside a rep presents a cir rcuit for the verification v o Ohm s law. of i) State the fu unction of ea ach compone ent used in the t circuit diagram. ii) Account fo or the flow of an electr ric current through t a metallic con nductor. iii) Name the method m used d to verify Oh hm s law. vi) ) What is the e nature of the t graph wh hen plotted against V and I? v) Which phys sical quantity y does the slo ope of the gr raph represe ent? ou are given one 10 an nd two 5 resistors. r Sho ow using dia agrams how y you will conn nect them 9) Yo to obtain an ef ffective resist tance of: b) 12.5 c) 2 a) 20 r of 5 eac ch. How will you arrang ge them so that the 10) i) You are given three resistances r equivalent resistance a) is maxim mum? b) is minimum? ? c) has h a value between b (a) and (b) ams for each h case mentioned in (i) and calculate the resistance in each ca ase. ii) Draw diagra de an electric c circuit like the one show wn alongside in order to 11) A student mad i) connect the e four cells in n parallel ii) to measure e current thro ough two lam mps L1 and L2. iii) to measure e potential dif fference acro oss two lamp ps L1 and L2 a) Are the four cells in series s or par rallel? b) Are the two lamps in n series or pa arallel? c) Is there an error in this circuit? If so, point out the erro or and draw w a diagram m showing th he correct wa ay to connec ct this circu uit. Us se the appropriate circuit symbols in your y diagram m. 12) Ho ow does temperature affe ect the resist tance in case e of i) metals ii) alloys iii) semiconduc ctors iv) carbon 13) i) Two wires iron and co opper of equal length and a thicknes ss are joined d in parallel l and the combination n is put across a battery. The copper r wire glows but b not the ir ron wire. Why y? ii) Wire the ex xpressions fo or electric pow wer in terms of: c) I and R a) V and I b) V and R 14) What are ohmic and non-o ohmic conductors? Give one exampl le of each an nd draw the graphs to sh how their curr rent-voltage relationship. 15) Give three cha aracteristics of o a series ci ircuit. 16) Th he figure giv ven shows a battery hav ving an e.m.f. of 9 V and int ternal resista ance of 0.6 , connecte ed to three resistors A, B an nd C. Calcula ate the curren nt in each resistor. 17) Given below is s the circuit diagram d in which w three re esistances 1 , 2 and 3 3 are conne ected to a ce ell of e.m.f. 2 V and intern nal resistance e 0.5 . i) Calculate th he total resis stance of the circuit. e reading of the ammeter? ? ii) What is the iii) What will be the amm meter reading if an ex xactly simila ar cell is connected in series with h the given cell? c Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 31 32 18) Fo our cells, each of e.m.f 1.5 V and internal i resis stance 2.0 are conne ected in parallel. The ba attery of cells s is connecte ed to an exter rnal resistance of 2.5 . Calculate: a) the total res sistance of th he circuit. b) the current flowing in the external circuit, and the e reading of the ammeter. c) the drop in potential acr ross the term minal of the cells. 19) Study the circu uit diagram in n the figure carefully c and calculate: a) the current in main circu uit, b) the current in each of th he resistors in parallel circ cuit. 20) Fo or the combin nation of resi istors as sho own in the cir rcuit, fin nd the equiva alent resistan nce between (a) A and B, , and (b) C and a D. 3 21) Fo our resistanc ces are conne ected as sho own in the diagram below w. A potentia al difference is applied be etween the po oints P and S. S a) Calculate the t equivalent resistanc ce between the points Q and R. b) What is the e equivalent resistance between b the points P and S? ent flowing th hrough the 3 resistanc ce is 2A, wh hat is the cur rrent through h the 2 c) If the curre resistance? ? e current thro ough the 4 resistance? d) What is the e current thro ough the 1 resistance? e) What is the e potential dif fference betw ween P and S? S f) What is the 22) A battery of e.m.f. 3 V is connected in series with h an ammeter, a 10 co oil of wire and with a pa arallel combination of resistance r o 3 and 6 . of 6 Draw a circuit dia agram for th he above arr rangement. What W is the: a) resistance of o the paralle el combinatio on? b) reading on the t ammeter r? c) potential dif fference acro oss the 3 resistor? r d) current flowing through 3 resistor? ? MIS SCELLANEO OUS: 1) Ho ow is the elec ctric potentia al difference between the e two points defined? d State its unit. 2) Ex xplain the sta atement the potential diff ference betw ween two poin nts is 1 volt . 3) Ex xplain the an nalogy betwe een the flow w of charge (or current) in a conduc ctor under a potential dif fference with h the free fall of a body un nder gravity. 4) Na ame the part ticles which are respons sible for the flow f of curre ent in a meta al. Explain th he flow of cu urrent in a me etal on the ba asis of move ement of the particles nam med by you. 5) Ho ow does the resistance of o a wire depe end on its radius? Explain your answ wer. 6) Ho ow does the resistance of o a wire depe end on its, le ength? Give a reason for your answer r. 7) Ho ow does the resistance of o a metallic wire w depend on its tempe erature? Explain with reas son. 8) State Ohm s law and dra aw a neat labelled circ cuit diagram m containing a battery, a key, a vo oltmeter, an ammeter, a a rheostat r and an unknown n resistance to t verify it. 32 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 08: 0 Current Electricity E 33 9) De escribe an experiment e w with a neat labeled l circu uit diagram to t verify Ohm s law. Dra aw graph sh howing the change c in po otential diffe erence acros ss the ends s of a condu uctor and the current flo owing in it. What W does the e slope of the e graph repre esent? 10) Which of the fo ollowing state ements does s not represe ent Ohm s law w? a) current /pote ential difference = consta ant b) potential diff ference/curre ent = constan nt c) potential diff ference = current x resist tance d) current resis stance x pote ential differen nce 11) What are non-ohmic resist tors? Give on ne example and a draw a graph g to sho ow its current t voltage rel lationship. 12) Which of the fo ollowing is an n ohmic resis stance? a) diode c) diamond b) germanium d d) n nichrome. 13) fig g. below sho ows the I V characterist tic curves fo or four resist tors. Identify y the ohmic and nonoh hmic conduct tors. Give a reason r for an nswer. 14) Dr raw a V-I gra aph for a co onductor obe eying Ohm s law. What does d the slo ope of V-I gr raph for a co onductor repr resent? 15) Ho ow does the e resistance of a wire depend d on temperature? ? Draw a V-I graph to show the va ariation of res sistance with h temperature e at two diffe erent tempera atures. 16) Ex xplain the ter rm resistivity. . 17) Th he unit of res sistivity is ___ _______ 18) State the order of resistivity y of a metal, a semiconductor and an n insulator. 19) Which will have higher resi istivity: a con nductor or an n insulator? 20) Fo or which of th he following substances, s resistance decreases d with increase in temperature? d) p a) copper b) mercury c) carbon c platinum. 21) Ex xplain the me eaning of the e terms e.m.f f., terminal vo oltage and in nternal resistance of a cell. 22) Ex xplain why th he p.d. acros ss the termin nals of a cell is more in an open circ cuit and reduced in a clo osed circuit. 23) Write the expr ressions for the t equivale ent resistance e R of three resistors R1, R2 and R3 joined in (a) ) parallel, (b) ) series. 24) Ho ow would you connect tw wo resistors in series? Draw D a diagra am. Calculat te the total equivalent e res sistance. 25) Sh how by a dia agram how tw wo resistors R1 and R2 are a joined in parallel. Obt tain an expre ession for the e total resista ance of combination. 26) Tw wo resistors are a joined wi ith a battery such that a) same curren nt flows in ea ach resistor. b) potential diff ference is sa ame across each e resistor r. c) equivalent re esistance is less than eit ther of the tw wo resistance es. e) equivalent resistance r is more than either e of the tw wo resistanc ces. State how are the resistors s connected in each of the above cas se. 27) An n electric bulb is rated 10 00 W 250 V. V What does s this imply? 28) An n electric sto ove is rated 3 kW, 250 V. V Explain whether w or no ot this stove e should be used u in a cir rcuit containing 13 A fuse e. i) Two fuse wires w are rate ed 15 A and 5 A. which of f the two will be thicker a and why? ii) In which cir rcuit cables are a the follow wing used? b) A 5 A rating fuse. a) A 15 A rating fuse iii) Why it is da angerous to replace r fuse wire by a co opper wire? Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 33 34 29) 30) 31) 32) 33) 34) 35) 36) 37) 38) 39) 40) 41) 42) iv) The flexible lead used for connecting an electrical appliance to the mains, is made up of three separate wires coloured red, black and green. Which one of these wires is meant for earthing? i) Name four devices which make use of only heating affect of current. ii) Name two devices which in addition to heat, produce light energy. iii) Give three characteristics of a material used for making heating appliances. If you look at pin socket with earth socket at the top, then which of the lower socket is live the one to the left or the one to the right of the socket? Which has a higher resistance, a 240 V, 40 W lamp or a 240 V 100 W lamp? Give reasons: i) The wires leading to an electric iron remain cold, while the element gets red hot. ii) Tungsten is used in an electric lamp iii) Nichrome is used as a heating element. iv) Copper wires are used for wiring. Make a table with the names of three electrical appliances used in your home in one column, their power, voltage rating and approximate time for which each one is used in the other columns. A lady received a shock when she happened to touch an electric kettle even though the switch was off. i) State the possible reasons for the shock ii) Point out the possible defect with the electric wiring inside the kitchen. A light bulb manufacturer makes two different 60 W and 200 W bulbs. Each bulb has a different length of wire coiled to make its filament. The filaments are made of the same metal and are at the same temperature. Explain how the longer filament has the same resistance as the shorter filament. You have resistors of value 2 , 3 and 5 . How will you join them so that the total resistance is less than 2 ? Write an expression for the resistance of a conducting wire in terms of length and area of crosssection. Explain why the potential difference across the terminals of a cell is more in an open circuit and it is reduced in a closed circuit. OR Explain why the potential difference across the terminals of a cell is less than its e.m.f. when current is drawn from it. On what factors does the resistance of a conductor depend? Show by a diagram how resistors R1, R2 and R3 are joined in parallel. Obtain an expression for the total resistance of the combination. OR Show that when the resistance is put in parallel, the total resistance is less than the least of the total individual resistance. PREVIOUS YEAR BOARD QUESTIONS 1) An electrical appliance is rated at 1000 kVA, 220 V. If the appliance is operated for 2 hours, calculate the energy consumed by the appliance in (i) kWh (ii) joule. [2012] 2) Calculate the equivalent resistance between P and Q in the diagram. [2012] 34 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 08: 0 Current Electricity E 35 3) i) A cell is se ending curren nt in an exte ernal circuit. How does th he terminal v voltage compare with the e.m.f. of o the cell? ii) What is the e purpose of using a fuse in an electrical circuit? iii) What are th [2012] he characteri istic propertie es of fuse wire? 4) i) Write an ex xpression for r the electrica al energy sp pent in the flo ow of current t through an electrical appliance in n terms of I, R and t. ii) At what volt tage is the alternating cu urrent supplie ed to our hou uses? iii) How should [2012] d the electric c lamps in a building b be connected? 5) Th hree resistors s are connec cted to a 6 V battery as shown in the adjacent t diagram. Ca alculate i) the equivale ent resistanc ce of the circuit, ii) total curren nt in the circu uit, fference acro oss the 7.2 resistor. iii) potential dif [2012 3 2 2 30 alculate the equivalent e re esistance bet tween A 6) Ca an nd B from the e following diagram: B A 6 6 [2010] 4 4 7) i) Draw a graph of Potential difference e (V) versus Current (I) fo or an ohmic r resistor. ii) How can yo ou find the re esistance of the t resistor from f this grap ph? iii) What is a non ohmic n re esistor? [2010] 8) Th hree resistors s are connec cted to a 12V V battery + ( ) as s shown in the figure give en below: A 12V i) What is th he current th hrough the 18 ohm 12 res sistor? 18 ii) What is the e potential difference d ac cross the parallel com mbination of 6 ohm and 12 ohm 6 6 resistor? [2010] iii) What is the e current thro ough the 6 oh hm resistor? 9) i) Sketch a graph g to sho ow the chan nge in poten ntial differenc ce across th he ends of an a ohmic resistor and d the current flowing in it. . Label the ax xes of your graph. g (2008) ii) What does the slope of the graph re epresent? hree resistors s of 6.0 , 2.0 2 and 4.0 0 respective ely are 10) Th joined together as shown in the figure e. The resisto ors are co onnected to an ammeter r and to a cell c of e.m.f. 6.0V. ca alculate: (2008) i) the t effective resistance of o the circuit ii) the current drawn d from the cell 11) What is the ma (2007) ain energy tra ansformation n that occurs s in: Charging g of a battery y 12) Ca alculate the value v of the resistance r w which must be e connected to a 15 res sistance to provide p an (2007) eff fective resist tance of 6 . (2006) 13) Me ention two fa actors on whi ich the resist tance of a wi ire depends. 14) Fo our resistanc ces of 2.0 each are joined end to end, to form m a square ABCD. Calc culate the (2005) eq quivalent resi istance of the e combinatio on between any a two adjac cent corners s. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 35 36 15) In the figure gi iven alongsid de, A, B and d C are three e ammeters. Th he ammeter r B reads 0.54A. (All l the ammeters have ne egligible resis stance) Calculate: a) The reading gs in the amm meters A and d C. b) The total res sistance of th he circuit. 16) Ca alculate the equivalent e re esistance of the t co ombination of f resistors r1, r2, r3 and r4. (2005) [1995] 17) A wire of unifo orm thickness s with a resis stance of 27 7 is cut into o three equa al pieces and d they are joined in parall [2006] lel. Find the resistance of f the parallel combination n. 18) Ca alculate the value v of the resistance which w must be b connected to a 15 resistance to t provide an n effective res sistance of 6 . [2006] 19) Th he V I graph for a seri ies combina ation and for a parallel co ombination of f two resistor rs is as show wn in the figure below: Which of the two t A or B represents the t parallel combination? Give a [2007] rea ason for your answer. 20) eads 0.3 A. Calculate: C In the figure below the ammeter A re a) the total t resistan nce of the cir rcuit b) the value v of R [2006] c) the current c flowin ng through R. R 21) A cell of e.m.f f 1.5 V and internal res sistance 1.0 is connec cted to two r resistors of 4.0 4 and 0.0 in series s as shown in the figure. 20 [2007] Ca alculate the: a) current in th he circuit. ference acro oss the 4.0 resistor. b) potential diff p when the current c is flow wing. c) voltage drop ference acro oss the cell. d) potential diff 36 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 08: Current Electricity 37 ANSWERS: Previous Year Board: 1) Given : P = 1000 kVA = 103 kW. V = 220 volt, t = 2 h i) Energy consumed = Pt = 10 2 kWh = 2 10 kWh ii) Since 1 kWh = 3.6 106 J Energy consumed = 2 103 3.6 106 = 7.2 109 J 2) Equivalent resistance of 10 and 10 in series = 10 + 10 = 20 Equivalent resistance of 20 and 5 in parallel [Fig. (i)] = 20 5 =4 20 + 5 Equivalent resistance of 3 . 4 and 2 in series between P and Q [Fig. (ii)] = 3 + 4 + 2 = 9 3) i) The terminal voltage is less than the e.rn.f. of the cell. ii) The propose of using a fuse wire in an electrical circuit is to limit the current in the circuit and to safeguard it and the appliances connected in it from being damaged. iii) A fuse wire must have a low melting point and high resistivity. 4) (i) Electrical energy spent= I2Rt (ii) At 220 volt. (iii) In parallel. 6) 3 8) i) The equivalent resistance of 8 and 12 in parallel is R = R 1R 2 8 12 = = 4.8 R1 + R 2 8 + 12 Total resistance of circuit (7.2 and 4.8 in series) = 7.2 + 4.8 = 12 ii) Total current in circuit l = Total e.m.f . 6V = = 0.5 A Total resis tan ce 12 iii) Potential difference across R (= 7.2 ) resistor, V = IR = 0.5 7.2 = 3.6 V 9) 10 10) 3 , 2A 14) 2 15) (a) 0.18 A (b) 0.36 A (c) 4 17) 3 20) 20 30 , 0.2A, 0.1 A 21) 0.06A, 0.44 V, 0.06 V, 0.06 V Volume 2 of 2 Universal Tutorials X ICSE Physics 37 38 Chapter 9: Electrical Power and Household Circuits Chapter Map: Introduction Measurement of Electrical Energy Electrical Power Commercial Unit Power Rating House Hold Wiring Transmission of Power Types of wiring Tree system Ring system Essential Components Fuse Switch Earthing Three Pin Plug Colourcoding of Wires Introduction: When electric current is passed through a metallic wire, having a resistance the wire gets heated and electrical energy gets converted to heat energy. This heat is used in electric heater, electric toaster etc. Expression for the Measurement of Electrical Energy: P. D = W Work ;V= Q Ch arg e or W = VQ But Q = I t Q Q = , t hence W = VIt This work measures electrical energy which is supplied by the source. In S.I unit work is in J, current in A, p.d. in V and time in s. therefore, 1 J = 1 V 1 A 1s By ohm s law, V = IR 38 W = I2Rt or I= V R Universal Tutorials X ICSE Physics W= V 2t R Volume 2 of 2 Chapter 9: Electrical Power and Household Circuits I= 39 V V 2t therefore W = R R Thus electrical energy supplied by the source is W = VIt = V 2t = I2 Rt. R The heat produce in a wire depends on Strength of the current (H i2) Time of passage of current (H t) Resistance of wire (H R) H = I2Rt = VIt = V 2t R (Joule s law of heating effect) 1J = 0.24 cal Electrical Power: Power is the rate of doing work. It is the rate at which electrical energy is dissipated. In electrical circuits, power is the rate at which electrical energy is supplied by the source. Power = i.e., P= energy time W VIt = t t P = VI = I2R = V2 R 1 SI unit is watt. (Js ) If a P.D. of 1 volt causes a current of 1A to flow through a circuit, per second, the electrical power consumed is 1 watt. Bigger Units: z z z 1 kW = 103 W 1 MW = 106 W 1 Horse power = 746 W. Commercial Unit of Electrical Energy: Electrical energy is measured in watt hour (Wh) or kilo watt hour (kWh) One watt hour is the electrical energy spent by an appliance of power 1 watt when used for 1 hour 1 Wh = 1 60 60 = 3600 J kWh is the electrical energy consumed by an electrical appliance of power 1kw used for 1 hour. 1kWh = 1000 3600 1kWh = 36 105 J = 3.6 106 J. Power Rating: Generally all electrical appliances are rated with power and voltage. e.g. 100 W-250 V bulb. This means that the given bulb gives an output power of 100 W when put on a 250 volts line. V2 P = R ; V = P; = . v P Thus we can find the amount of safe current the bulb can withstand. With these ratings, the resistance and current are calculated by Volume 2 of 2 Universal Tutorials X ICSE Physics 39 40 Power rating of Some Common Appliances: The electric meter measures the electrical energy consumed in kWh. Calculation: Energy in kWh = Power ( watt ) time (hrs) V I t (hour ) = 1000 1000 Appliance Power (in watt) Voltage (in volt) Electric bulb 15 200 220 Electric fan 60 100 220 Television set 120 220 Refrigerator 150 220 Electric iron 700 220 Electric mixer 750 220 Room heater 1000 220 Geyser 1500 220 Electric kettle 2000 220 Electric oven 3000 220 Household Wiring System: Transmission of Power: Generally A.C. is supplied at 220V in India. Electric power is usually generated at places which are very far from the areas where it is consumed. Voltage during transmission is stepped up so that current is low and energy loss (I2Rt) during transmission decreases. The power is transmitted over long distance by transmission line wires. 33 kV TO HEAVY INDUSTRY 11 kV TO LIGHT 33 kV INDUSTRY 11 kV 132 kV Step up 11 kV BOILER TURBINE GENERATIORS Step down GRID SUB STATION MAIN SUB STATION Step down Step down INTER SUB STATION 220 V CITY SUB STATION CITY CONSUMERS POWER STATION 40 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 9: 9 Electrical Power P and Ho ousehold Circuits 41 Powe er Distribu ution: There T are two types of ho ousehold wir ring: The e Tree sys stem: z z In this sys stem differen nt branch lin nes are taken from the distrib bution board to different parts p of the house. h The swit tches and sockets s sho ould have proper current ca arrying capac city. Dis sadvantag ges: z z z z It requires s plugs and sockets s of dif fferent sizes for different current carry ying capacities. When the e fuse in on ne distributio on line blows s, it disconn nects all the appliances from the supply line e in that distribution line. This wiring is expensiv ve. If a new appliance a is to be installe ed requiring higher curre ent, say 15 A A, new line wires w from the applia ance up to th he distributio on box could d be quite expe ensive and in nconvenient. . The e Ring Sys stem z z Wires from m the main fuse box run n around all the main room ms of the ho ouse and co ome back to the fuse box. A separat te connection is taken from the live wire w of the ring g for each ap ppliance. Adv vantages of the ring g system: z z z In the ring g system the current ca an travel to an a individual appliance t through two separate paths. Thus the conne ection for each appliance e is effectively through a thick wire. Therefore, T the wire used u for ring main is of a lower curre ent carrying capacity c than n that which would be required for f a direct co onnection to the mains. This T reduces s the cost of w wiring consid derably. Plugs and d sockets all of the same e size can be b used, but each socket t should hav ve its own fuse of rat ting suitable for the appliance to be connected c wi ith it. In installin ng a new app pliance in a room, a new w line up to th he distributio on box is not required. The appliance can be directly connected to the ring circuit in t that room. The only considera ation is that th he total load on that ring circuit shoul ld not exceed d the main fu use rating (viz. 30 A). Supp ply to a Ho ouse: T To supply po ower to a hou use from the e power sub s station, eithe er the overh head wires on o poles are e u used or an un nderground cable c is used d. The T cable ha as three wires s: z Live (or phase) wire z Neutral wire w and z Earth wire e. The T neutral and a the eart th wires are connected together at th he local substation so th he neutral w is at the earth potent wire tial (i.e. at 0 V). Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 41 42 Before the electric line is connected to the meter in a house, a fuse of high rating (= 50 A) is connected in the live wire connected at the pole or before the meter. This is called the company fuse or pole fuse. After the company fuse, the cable is connected to a kWh meter. From the meter, connections are made to the distribution board through a main fuse and a main switch. The main switch is a double pole switch. It has an iron covering. The covering is earthed. This switch is used to cut the connections of the live as well as the neutral wires simultaneously. The earth wire from the meter is locally earthed. Household Wiring is Done in Parallel: In parallel arrangement, each appliance works at same voltage. For example if several bulbs are connected in parallel, each bulb glows at the same voltage. Therefore, the glow of a bulb is unaffected if another bulb is switched on or off. In parallel arrangement if one bulb or appliance is switched off or fuses, the other bulbs (or appliances) continue to glow (or operate). Each appliance draws current as per its current rating. On the other hand if the electric appliances (say, bulbs) would have been connected in series, then z While one bulb glows and another bulb is connected to the same circuit, the resistance of the circuit will increase, hence it will reduce the current in the bulbs so each bulb will glow less bright, and If one bulb fuses, the circuit breaks and the other bulbs also ceases to glow. Essential Components of Household Wiring: Live Wire Fuse: Holder Socket The use of a fuse is to safeguards the circuit and the appliances connected in the circuit from being T1 damaged when excess current enters the circuit. Broken The fuse is a short piece of wire made of a material of Fuse Wire high resistance and of low melting point. So that it may T2 easily melt due to overheating when excessive current passes through it. Generally an alloy of lead and tin is used as the Live Wire material of the fuse wire. The fuse wire is stretched between the two metallic terminals T1 and T2 in a porcelain holder. This holder fits into a porcelain socket which is connected in the live wire of the circuit. When current in the circuit exceeds the specified value (due to any reason such as voltage fluctuation, short circuiting) the fuse wire gets heated and melts. As a result, the circuit breaks, no current flows and the appliance or the circuit is saved. The fuse is always connected at the entry point of the circuit, so that the fuse may melt first before the appliance is damaged. These days most of the appliances such as television etc. are provided with a cartridge type fuse. This provides additional safety to the appliance. The fuse is always connected in the live wire of the circuit. If the fuse is put in the neutral wire, then even when the fuse burns (current stops flowing), the appliance remains connected to the live wire and the person touching the appliance will get shock. 42 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 9: Electrical Power and Household Circuits 43 These days miniature circuit breakers (MCB) are used for each lighting circuit. They switch off the circuit in very short time duration ( 25 milli second) in case of short circuiting. Switches: A switch is a device to close or open a circuit. Switches are of various types and they are made in many designs. The most common among these is the single pole switch used in houses. When the switch is in off position, no current flows through the appliance and its live and neutral wires are at the same (or zero) potential If the switch is connected in the neutral wire, then even when the switch is off (i.e. the circuit is open), the appliance remains connected to the live wire and it is at the supply voltage. If we come in contact with the live wire of the appliance, we may get a fatal shock. Dual Switch z z z Dual control switches are the double pole type switches which are used at the top and bottom of a staircase, and at opposite ends of a long corridor etc. With such switches, the appliance (say, a bulb) can be switched on or off from two different places. The working of a dual control switch is illustrated in Fig. Let a switch S1 be fitted at the bottom and a switch S2 at the top of the staircase. Fig (a) shows the off position of the bulb. The bulb can now be switched on independently by either the switch S1 or the switch S2. If the switch is operated, the connection ab is changed to bc , which completes the circuit and the bulb lights up [Fig (b)]. Similarly, on operating the switch S2, the connection bc changes to ba , which again completes the circuit [Fig. (c)]. Similarly, if the bulb is in on position as shown in Fig (b) or (c), one can switch off the bulb either from the switch S1 (by changing the connections bc to ba ) or the switch S2 (by changing the connections ab to bc ). Safety Precautions: z z A switch should not be touched with wet hands. Water forms a conducting layer between the hand and the live wire of the switch. The current passes to the hand and we may get a fatal shock. Earthing: In a house, the local earthing is made near the kWh meter. For this purpose a 2 3 meter deep pit is dug in the ground. A thick copper rod (or a thick copper wire) which is surrounded with a hollow insulating pipe, is put in the hole A thick copper (or iron) plate of dimensions 50 cm 50 cm is welded to the end of the copper rod and is buried in the ground. The plate is surrounded by a mixture of charcoal and salt to make a good earth connection. To keep the ground damp, water is poured through the pipe from time to time. This forms a conducting layer between the plate and the ground. Volume 2 of 2 Universal Tutorials X ICSE Physics 43 44 S Safety by th he local ear rthing: If due to some reason r such as short cir rcuiting, an excessive e c current flows s through the e live wire, it t will pass to o earth throu ugh the earth h wire if ther re is local e earthing, othe erwise it may y cause a fire e due to overheating of th he live wire. Ear rthing of an a Applian nce: z z z z z The outer r metallic bo ody (or meta allic case) of an electrica al appliance (such as ref frigerator, toaster, geyser, electr ric iron, elect tric cooler etc c.) which we e handle physically is also o earthed for safety reasons. For earthing of an ap ppliance, the earth wire (green ( or yel llow) of the c cable is connected to the outer metallic case e of the appliance. Safety by y earthing of o an applia ance: When the live wir re of a faulty y appliance comes in direct con ntact with it ts metallic case c due to o break of insulation af fter constant t use (or otherwise e), the whole appliance ac cquires the potential p of th he live wire. A person touching it will w get a fata al shock. Bu ut if the case e of the appliance is earthed, then as soon as a the live wire w comes in n contact wit th the metallic case of th he appliance, a heavy current flo ows (since th he metallic case c has alm most zero res sistance) and the fuse connected c with the appliance a blows off and appliance get ts disconnect ted. Thus, the person touc ching the def fective appliance does not n get a sho ock and the appliance a is saved from f being da amaged. Thre ee Pin Pl lug: Elec ctric applianc ces are prov vided with a cable havin ng a plug top at one 44 end to connect to t the electric c supply. The e plug is inse erted into th he socket, which w has three holes with w inner walls in the form m of metallic tubes. t The e upper bigge er hole in the e socket is fo or earth conn nection while the hole on the t right side e is for conne ection to the live wire and the hole on the left is fo or connection n to the neutr ral wire of ele ectric supply y. The e plug top has s three meta allic (usually brass) b pins in n an ebonite case. In a plug top, the top pin is for f the earthing, the live pin is on the e left and the neutral pin is s on the right t. In good g quality plug tops, th hese are marked as E, L and N resp pectively. Som me times the plug used fo or low power r connection has only the e live and the neutral pins. The e earth pin is thicker and longer l than the t other two o. The e earth pin is longer so that the earth connection c is s made first. This s ensures the e safety of th he user because if the ap ppliance is de efective, the f fuse will blow w off. The e earth pin is thicker so th hat even by mistake m it cannot be inserted into the live or neutr ral hole of the socket. Safe ety Precauti ion: While co onnecting a plug top, it is s necessary to t make the connections s tight. The e plug pins should s fit in the t socket ti ightly. For th his, plug pins s are split at the ends to provide spring action. An ny loose con nnection will give g rise to sparking s and burning of th he socket or the plug. The ere are two major m dangers s while using g electricity, namely n (i) fir re and (ii) an electric shoc ck. A fir re is caused due to over heating of liv ve wires (or cable) of cur rrent carrying g capacity higher than the current which can flow th hrough it at th he time of pe eak use or while w using all l the applianc ces. An electric e shoc ck may be ca aused due to o either poor insulation of o wires or t touching the electrical appliances with wet hands. Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 9: Electrical Power and Household Circuits 45 To avoid it, the insulation of wires must be of good quality and it should be checked from time to time particularly when they become old. An electrical appliance, switch, plug, socket, electric wire etc. should never be operated (or touched) with wet hands and they should always be kept in a dry condition. Colour Coding of Wires in a Cable: An electric appliance is provided with a three core flexible cable. The insulation on the three wires is of different colours. Wire Old convention New international convention Live Red Brown Neutral Black Light Blue Earth Green Green (or yellow) High Tension Wires: For high voltage and heavy current, high tension wires are used. These wires have low resistance and large surface area. Instead of a single thick wire, it is made by twisting a number of insulated then wires. This provides large surface area and radiate the heat quickly. REVIEW QUESITONS: Direct Questions: 1) State the S.I. units of (i) electrical power and (ii) electrical energy. 2) i) Define the household unit of electricity. ii) What is the voltage of the electricity that is generally supplied to a house? 3) Name the physical quantity which is measured in (i) kW, (ii) kWh. 4) Define the term kilowatt-hour and state its value in S.I. unit. 5) Distinguish between kilowatt and kilowatt-hour. Find the value of 1 kilowatt-hour in joule. 6) What do you mean by power rating of an electrical appliance? How do you use it to calculate the resistance of the appliance and the safe limit of current in it, while in use? 7) At what voltage is the electric power generated at the power generating stations 8) At what voltage and frequency is the a.c. supplied to our houses? 9) What is pole fuse? Write down its current rating. 10) State the function of each of the following in a house circuiting a) kWh meter, b) main fuse, and c) main switch. 11) Name the two systems of house wiring. Which of the two systems is economical? 12) What is a fuse? Name the material of fuse. 13) State two characteristics of a fuse wire. a) A fuse is a short piece of wire of material of high _______ and low ______ b) Fuse wire has a _______ a melting point and ______ it is made of an alloy of and if the current in a circuit rises too high, the fuse wire _______ c) A fuse is connected in ________ (parallel / series) with the _______ (earth/neutral/live) wire. 14) A fuse is always connected to the live wire of the circuit. Explain the reason. 15) What is the purpose of a switch in a circuit? Why is the switch put in the live wire? 16) Draw a labeled diagram of a three pin socket. 17) Under what circumstances does one get an electric shock from an electrical gadget? Volume 2 of 2 Universal Tutorials X ICSE Physics 45 46 18) Give two characteristics of a high tension wire 19) For earthing an electrical appliance, one has to remove the paint from the metal body of the appliance where the electrical contact is made. Explain the reason. NUMERICALS: Class Work: 1) An electric bulb is rated at 220 V, 100 W. What is its resistance? What maximum current can be passed through it? [484 , 0.45 A] 2) An electric heater is rated 220 V, 550 W. Calculate the electrical energy consumed in 3 hours. [1.65 kWh] 3) An electrical appliance having a resistance of 200 is operated at 200 V. Calculate the energy consumed by the appliance in 5 minutes. [60,000 J] 4) A current of 0.2 A flows through a wire whose ends are at a potential difference of 15 V. Calculate: i) the resistance of the wire, and ii) the heat energy produced in 1 minute. [(i) 75 , (ii) 180 J] 5) What is the resistance, under normal working conditions of a 240 V electric lamp rated at 60 W ? If two such lamps are connected in series across a 240 V mains supply; explain why each one appears less bright. 6) An electric bulb is marked 250 W, 230 V. i) How many joule energy does it consume in one hour? ii) How long would this lamp take to consume 1.0 kWh energy when connected to 230 V mains? [(i) 9 105 J (ii) 4 h] 7) A bulb is connected to a battery of p.d. 4 V and internal resistance 25 . A steady current of 0.5A flows through the circuit. Calculate: i) the total energy supplied by the battery in 10 minutes, ii) the resistance of the bulb, and iii) the energy dissipated in the bulb in 10 minutes. [(i) 1200 J, (ii) 5.5 , (iii) 825 J] 8) A battery of e.m.f. 15 V and internal resistance 2 is connected to two resistors of 4 ohm and 6 ohm joined (a) in series, (b) in parallel. Find in each case the electrical energy spent, per minute in 6 ohm resistor. [(a) 562.5 J, (b) 669.1 J] 9) An electric toaster draws 8 A current in a 220 V circuit. It is used for 2 h. Find the cost of operating the toaster if the cost of electrical energy is Rs 2.50 per kWh. [Rs. 8.80] 10) An electric heater rated 4 kW, 220 V is used for 12 h. Find the cost if one kWh of electrical energy costs Rs 3.25. [Rs.156] Home Work: 1) Calculate the current through a 60 W lamp; rated for 250 V. If the line voltage falls to 200 V, how is the power consumed by the bulb affected? [0.24 A, power consumed reduces to 38.4 W] 2) A bulb of 40 W is used for l25 h each day for 30 days. Calculate the electrical energy consumed. [15kWh] 3) An electric iron is rated at 750 W, 230 V. Calculate the electrical energy consumed by the iron in 16 hours. [12 kWh] 4) A bulb marked 12 V, 24 W operates on a 12 volt battery for 20 minutes. Calculate: i) the current flowing through it, and ii) the energy liberated. [(i) 2A, (ii) 28,800 J] 5) A bulb is connected to a battery of e.m.f. 6 V. A steady current of 05 A flows through the circuit. Calculate the total energy supplied by the battery in 5 minutes. [900 J] 6) Two bulbs are marked 60 W, 220 V and 60 W, 110 V respectively. Calculate the ratio of their resistances. [4:1] 46 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 9: 9 Electrical Power P and Ho ousehold Circuits 47 7) Th hree 250 W, 100 V heaters are conne ected in para allel to a 100 V supply. Ca alculate: i) the t total curr rent taken fro om the supply, ii) the resistanc ce of each he eater, and iii) the energy supplied s in kWh k to the th hree heaters in 5 hours. [( (i) 7.5 A (ii) 4 40 ohm (iii) 3.75 3 kWh] 8) Tw wo resistors of o 4 and 6 are conne ected in para allel. The com mbination is connected across a a6 vo olt battery of f negligible resistance. r C Calculate: (i) the power r supplied by y the battery y, (ii) the po ower dissipat ted in each re esistor. [ (i) 15 5 W, (ii) 9W, (iii) ( 6 W] 9) Water in an electric kettle connected to o a 220 V su upply took 5 minutes to r reach its boil ling point. Ho ow long would it have tak ken if the sup pply had been of 200 V? [6.05 minutes] 10) An n electric ket ttle is rated 2.5 2 kW, 250 V. Find the cost of running the kettle e for two hou urs at Rs. 2.4 40 per unit. [R Rs 12.00] 11) A geyser is rat ted 1500 W, 250 V. This geyser is connected to 250 2 V mains. Calculate: i) the t current drawn, d ii) the energy consumed c in 50 hours, an nd [(i) 6 A (ii) iii) the cost of energy e consu umed at Rs 2. 2 20 per kW Wh. ) 75 kwh (iii) Rs. 165] APPLICATION TYPE: T 1) Tw wo lamps, on ne rated 220 0 V, 50 W an nd the other rated 220 V, V 100 W, are e connected in series wit th mains of 220 2 V. Expla ain why does the 50 W lamp consume e more powe er. 2) Na ame the facto ors on which h the heat pro oduced in a wire w depends when curre ent is passed d in it and sta ate how does s it depend on o the factors s stated by you. y 3) Na ame the thre ee connecting g wires used d in a household circuit. Which two w wires are at the same po otential? In which w of the wires w is the switch connec cted? 4) Th he fig. shows s two ways of o connecting g three lamp ps A, B and C to a.c. supply of 220 V. Name the two o arrangeme ents. Which of o the em would yo ou prefer in a household d circuit? Giv ve reason fo or yo our answer. 5) Tw wo sets A and B of four bulbs b each, are a glowing in two separa ate rooms. W When one of the bulbs in set A is fuse ed, the other three bulbs also cease to t glow. But in i set B, whe en one bulb fuses, f the oth her bulbs continue to glo ow. Explain the t difference e. [Hint: In set A, the bul lbs are in series while in set B, the bu ulbs are in pa arallel]. 6) Tw wo fuse wires s of same len ngth are rate ed 5 A and 20 0 A. Which of o the two is t thicker and why? w 7) Give two reaso ons why a thick copper wire w is not use ed as a fuse wire. d 8 A . Can it t be used wit th an electric cal appliance of rating 5 k kW, 200 V? 8) A fuse is rated n electric ket ttle is rated 3 kW, 250 V. V Give reaso on whether this kettle ca an be used in n a circuit 9) An wh hich contains s a 13 A fuse e. 10) What should be b the curren nt rating of a fuse to be used with an electric appl liance of rating 15 kW 2 220 V? How is the fuse co onnected wit th the appliance? he diagram shows s a three e pin plug. La abel the thre ee pins. Why is the top pin 11) Th thi icker and lon nger than the e other two? Why W are the pins split at the ends? 12) Volume 2 of 2 The diagram below w shows a 3 terminal plu ug socket. hat is the pur rpose of the terminal t E? a) Wh b) To which part of o the applian nce is the terminal E conn nected? oined to L or r N, is the fus se connected d and why? c) To which wire jo Universal Tu utorials X IC CSE Physics s 47 48 13) The diagra am shows a three-pin t soc cket marked as 1, 2 and 3 3. Identify an nd write live (L), neutral (N) and d earth (E) ag gainst the co orrect numbe er. 14) Which of the cables, c one rated r 5 A oth her 15 A will l be of thicke er wire? Give e justification n for your an nswer. [Hint: To carry lar rger current, the re wire should be lo ow, so its are ea of cross should s be lar rge.] 15) An n electric hea ater is rated 220 2 V, 55 W. W Calculate the electrical energy cons sumed in 3 hours. h 16) A bulb marked d 12 V, 24 W operates on o a 12 volt t car battery for 20 minu utes. Calcula ate: (i) the cu urrent flowing g through it and a (ii) the en nergy liberate ed. 17) A bulb is connected to a battery of e.m m.f. 6 V. A steady current t of 0.5 A flo ows though th he circuit. Ca alculate the total t energy supplied s by the t battery in n 5 minutes. 18) A bulb is conn nected to a battery b of p.d d. 4 V and in nternal resis stance 2.5. . A steady current c of 0.5 5A flows thro ough the circ cuit. Calculate e: i) The T total ene ergy supplied d by the batte ery in 10 min nutes. ii) The resistan nce of the bulb. iii) The energy dissipated in n the bulb in 10 minutes. wo resistors of o 5 and 20 0 are conne ected in para allel. The com mbination is connected across a a6 19) Tw vo olt battery of f negligible resistance. r C Calculate: (i) the power r supplied by y the battery y, (ii) the po ower dissipat ted in each re esistor. 20) Water in an electric kettle connected to o a 220 V su upply took 5 minutes to r reach its boil ling point. Ho ow long would it have tak ken if the sup pply had been of 200 V? 21) Tw wo lamps, on ne rated 220 0 V, 50 W an nd the other rated 220 V, V 100 W, are e connected in series wit th mains of 220 2 V. Expla ain why the 50 W lamp co onsumes more energy. 22) An n electric ket ttle is rated 2.5 2 kW, 250 V. Find the cost of runn ning the kettl le for two ho ours at 60 pa aise per unit. 23) A geyser is ra ated 1500 W, W 250 V. Thi is geyser is connected to t 250 V ma ains. Calculate: (i) the cu urrent drawn, (ii) the ener rgy consume ed in 50 hours, (iii) the cost of energy consumed at a Rs.2.20 pe er kWh. MIS SCELLANEO OUS: 1) De educe an ex xpression for r the electric cal energy spent s in flow w of current through an electrical ap ppliance. 2) Ob btain an expr ression for th he electrical power spent t in flow of cu urrent through a conducto or. 3) Ele ectrical powe er P is given by the expre ession P = (Q Q V) time e. a) What do the e symbols Q and V repres sent? b) Express the e power P in n terms of cu urrent and re esistance ex xplaining the e meaning of f symbols us sed there in. 4) When a curren nt I flows thro ough a resist tance R for time t, the ele ectrical energ gy spent is gi iven by: a) IRt c) IR2t b) I2Rt d) I2R/t 5) Ex xplain the me eaning of the e statement t the power of f an applianc ce is 100 W . 1 volt 1 ampere ... .... 6) Fil ll in the blank ks in the sen ntence: 1kWh h= 1000 7) Lis st the names s of three ele ectrical gadg gets used in your house. . Write their power, volta age rating an nd approxima ate time for which each h one is use ed in a day. Hence find the electrica al energy co onsumed by each e in a day y. 8) Ex xplain the tra ansmission of f electric pow wer from the generating station s to you ur house. 9) Why is the elec ctric power from the gene erating statio on transmitte ed at a high v voltage? 48 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 9: Electrical Power and Household Circuits 49 10) When the main switch of the house circuit is put off, it disconnects the a) live wire only b) neutral wire only c) earth wire only d) both the live and neutral wire 11) Where is the main fuse in a house circuit connected? 12) Draw a circuit diagram to explain the ring system of house wiring. What advantage does it have over the tree system? 13) Draw a labelled diagram with necessary switch, regulator etc., to connect a bulb, a plug socket and a fan with the mains. In what arrangement are these connected to the mains, series or parallel? 14) How are the light and other outlet points connected in a room? Show it by a diagram. Give a reason for your answer. 15) How should the electric lamps in a building be connected so that the switching on or off in a room has no effect on other lamps in the same building. 16) The bulbs, fans, heaters etc. in a house are always connected in parallel and not in series. Give two reasons. 17) Why is the fuse wire fitted in a porcelain casing? 18) How is a fuse put in an electric circuit? State the purpose of using a fuse in a circuit. 19) Describe with the aid of a diagram some form of a fuse which is used in the electric lighting circuit of a house. Explain why a fuse must not be replaced by an ordinary wire? 20) Explain the meaning of the statement the current rating of a fuse is 5 A . 21) Explain briefly how a fuse protects an electric circuit. 22) A switch is not touched with wet hands while putting it on or off. Give a reason for your answer. 23) Draw a circuit diagram using the dual control switches to light a staircase electric light and explain its working. 24) What purpose is served by the terminals of a three pin plug? Draw a diagram and name the pins. 25) According to international convention of colour coding of insulation of wires in a cable: a) live is red, neutral is blue and earth is brown. b) live is red, neutral is green and earth is black c) live is brown, neutral is blue and earth is black d) live is brown, neutral is light blue, and earth is green. 26) What is the international convention of colour coding in a cable? 27) What do you mean by the term local earthling Explain how it is done? 28) The metal case of an electric appliance earthed. Explain the reason. 29) The earthing of an electric appliance is us only if the fuse is in the live wire. Explain reason. 30) A power circuit uses a cable using three different wires. Name them. To which of the two wires should the heating element of an electric geyser be connected? To which wire should the switch and fuse be connected? 31) Why it is necessary to have an earthing installed in a power circuit, but not in a lighting circuit? 32) An electric bulb is rated 250 W, 230 V. What information does this convey? 33) Calculate the current through a 60 W lamp rated for 250 V. 34) An electric iron is rated at 750 W, 230 V, Calculate the electrical energy consumed by the iron in 16 hours. 35) An electrical appliance having a resistance of 200 is operated at 200 V. Calculate the energy consumed by the appliance in 5 minutes. 36) A current of 0.2 A flows through a wire whose ends are at a potential difference of 15 V. Calculate: (i) the resistance of the wire and (ii) the heat energy produced in I minute. 37) What is the resistance, under normal working conditions, of a 240 V electric lamp rated at 60 W? If two such lamps are connected in series across a 240 V mains supply, explain why each one appears less bright. Volume 2 of 2 Universal Tutorials X ICSE Physics 49 50 38) Tw wo bulbs are e marked 60 W, 220 V and a 60 W, 110 V respec ctively. Calcu ulate the rati io of their res sistances. 39) An n electrical appliance is ra ated 1500 W, W 250 V. This s appliance is i connected d to 250 V ma ains. Find i) the current drawn, ii) the electrica al energy consumed in 60 6 hours, iii) the cost of electrical ene ergy consum med at Rs.2.5 50 per kWh 40) i) State the purpose p of a fuse in an electric e circuit. Name the e material us sed for makin ng a fuse wire. ii) Mention two (2004) o factors on which the int ternal resista ance of a cell depends. 41) An n electric bulb rated 220 V, V 60 W is working w at full l efficiency. i) State the re esistance of the t coil of the e bulb. ii) Another ide entical bulb is s connected in series wit th the first on ne and the sy ystem is co onnected acr ross the ma ain as show wn alongside. a) State the e rate of con nversion of en nergy in each bulb. b) Calculat te the total po ower. c) What will be the tota al power, if th he bulbs are connected in n parallel? (2004) 42) Ex xplain briefly how a magn net can be de emagnetised d using an alt ternating cur rrent. 43) What is the voltage of the electricity e tha at is generally supplied to o a house? 44) Which of the tw wo wires of similar s dimen nsions, copp per or nichrom me, would yo ou use for th he electric he eater element? Give reas sons to justify y your answe er. 45) Tw wo fuse wires s of the sam me length are e rated 5A an nd 20 A. which of the two o fuse wires is thicker an nd why? 46) With reference e to the diagr ram given be elow, calculat te: i) The equivalent resistance between P and Q ii) The reading g of the amm meter. iii) ) The electric cal power between P and dQ 47) A cell of e. m. m f. 1.5 V and a internal resistance 10 ohms is co onnected to a resistor of f 5 ohms, with an ammete er in series (see figure). What is the rea ading of the ammeter? a 48) A geyser has a label 2 kW W, 240 V. What W is the cost of usin ng it for 30 minute, if th he cost of ele ectricity is Rs s.3 per comm mercial unit? 49) Ex xplain briefly the function of the follow wing in the ho ousehold wiring: i) A three pin n plug ii) Main switch 50) Fo our cells, eac ch of e.m.f. 1.5 V and in nternal resist tance 2.0 oh hm are connected in par rallel. The ba attery of cells s is connecte ed to an exter rnal resistance of 2.5 ohm m. Calculate: i) The T total res sistance of the circuit. ii) The current flowing in the external circuit, and iii) The drop on n potential ac cross the terminals of the e cells. 51) Ma ake a table with the nam mes of three e electrical appliances us sed in your home in one e column, the eir power, vo oltage rating g and approx ximate time for f which ea ach one is us sed in one day d in the oth her columns. 52) A family uses a light bulb of o 100 W, a fan f of 100 W, W and a hea ater of 1000 W W, each for 8 hours a da ay. If the cos st of electricity is Rs.2 pe er unit, what t is the expe enditure for the family pe er day, on ele ectricity? 50 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 9: 9 Electrical Power P and Ho ousehold Circuits 51 53) De efine the e.m m.f. (E) of a cell c and the potential p diffe erence (V) ac cross a resis stor (R) in ter rms of the wo ork done in moving a un nit charge. State S the rela ation betwee en these two o works and the work do one in movin ng a unit cha arge through h a cell connected acros ss the resist tor. Take the e internal res sistance of th he cell as r . Hence obta ain expressio on for the cur rrent i' in the circuit. 54) A bulb is marked 100 W, 220 V and an electric heater is marked d 2000 W, 22 20 V. i) What is the e ratio betwee en the resista ances of these two devic ces? ii) How does the t power voltage rating of a device help us to decide abo out the type of leads (connecting g wires) to be e used for it? ? iii) In which of the two devi ices, a thicke er connecting g wire or lead d is required? 55) i) Name the material m used d for making g a fuse wire. State two properties p of f the materia al of fuse wire which make it suita able for use. ii) Calculate the t electrical energy in SI units consumed by a 100 W b bulb and a 60 6 W fan connected in parallel for 5 minutes. 56) i) Under what t circumstanc ces does one e get an elec ctric shock fr rom an electr ric gadget? ii) What is me eant by earthing of an ele ectrical applia ance? How does d earthing g offer protec ction? 57) In the circuit sh hown calcula ate: i) The value of o the combined resistan nces of 40 ohm o and R, using the re eadings of th he two meters s. ii) The value of o R iii) The current t flowing thro ough R. 58) A wire of unifo orm thickness with a resistance of 27 7 is cut into o three equa al pieces and d they are joined in parall lel. Find the resistance of f the parallel combination n. 59) Me ention two fa actors on whi ich the resist tance of a wi ire depends. 60) In the figure be elow, the am mmeter A read ds 0.3 A. Calculate: i) The total re esistance of the t circuit. ii) The value of o R iii) The current t flowing thro ough R. 61) Fo our resistanc ces of 2.0 each are joined end to end, to form ma sq quare ABCD D. Calculate e the equiv valent resis stance of the t co ombination be etween any two t adjacent t corners. 62) In a three pin plug, why is the earth pin n made longe er and thicke er than the ot ther two pins s? 63) In the figure given alongsid de, A, B and C are three ammeters. Th he ammeter B reads 0.5 5 A. (All the ammeters have h negligib ble res sistance). Ca alculate: i) The reading gs in the amm meters A and d C. ii) The total re esistance of the t circuit 64) An n electric kettle is rated 2.5 kW, 250 V. find the e cost of running the ke ettle for two hours at 60 0 paise per unit. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 51 52 PREVIOUS YEAR BOARD QUESTIONS: 1) Two bulbs are marked 100W, 220V and 60W, 110V. Calculate the ratio of their resistance.[2010] 2) i) What is the colour code for the insulation on the earth wire? ii) Write an expression for calculating electrical power in terms of current and resistance. [2010] 3) i) Name two safety devices which are connected to the live wire of a household electrical circuit ii) Give one important function of each of these two devices. [2010] 4) i) An electric bulb is marked 100W, 250V. What information does this convey? ii) How much current will the bulb draw if connected to a 250V supply? [2010] 5) An electric heater is rated 1000 W 200 V. Calculate: i) the resistance of the heating element. ii) the current flowing through it. (2009) 6) i) Give two characteristic properties of copper wire which make it unsuitable for use as fuse wire. ii) Name the material which is used as a fuse wire? (2009) 7) i) Draw a labeled diagram of the staircase wiring for a dual control switch showing a bulb in the circuit (2008) ii) The electrical gadgets used in a house such as bulbs, fans, heater etc. are always connected in parallel, NOT in series. Give two reasons for connecting them in parallel iii) An electrical heater is rated 4 kW, 220 V. find the cost of using this heater for 12 hours if one kWh of electrical energy costs Rs. 3.25. 8) How does the heat produced in a wire or a conductor depend upon the: (2008) i) current passing through the conductor. ii) resistance of the conductor? 9) Of the three connecting wires in a household circuit: (2007) i) Which two of the three wires are at the same potential? ii) In which of the three wires should the switch be connected? 10) Define a kilowatt hour. How is it related to the joule? (2007) 11) i) What is meant by earthling of an electrical appliance? Why is it essential? (2007) ii) What will be the effect on the working of an electric bell if instead of a direct current, an alternating current us used? 12) Draw a labeled diagram of a three pin socket (2006) 13) Find the cost of operating an electric toaster for two hours, if it draws 8 A current on a 110 volt circuit. The cost of electrical energy is Rs.2.50 per kWh (2006) 14) In a three pin plug, why is the earth pin made longer and thicker than the other two pins? (2005) 15) An electrical appliance is rated 1500 W, 250 V. this appliance is connected to 250 V mains. Calculate: (2005) i) The current drawn ii) The electrical energy consumed in 60 hours. iii) The cost of electrical energy consumed at Rs.2.50 per kwh. 52 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 10: Electro Magnetism 53 Chapter 10: Electro Magnetism Chapter Map: Magnetic Effect of Current Magnetic Field due to Current Straight Wire Rules Circular Coil Electromagnet Force on current carrying conductor in a magnetic field D. C. Motor Fleming s left hand rule Working Electromagnetic Induction Faraday s Laws Fleming s right hand rule Lenz s rule A. C. Generator Transformer Magnetic Effect of Current: Hans Oersted, in 1820, by his experiments first observed that when an electric current is passed through a conducting wire, a magnetic field is produced around it. If a compass needle is placed either above or below a current carrying wire, the needle is found to deflect in a definite direction. The direction of deflection of needle is reversed if the direction of current in the wire is reversed. Thus, a current (or moving charge) produces a magnetic field. This is called the magnetic effect of current. If current is alternating then magnetic field generated will also alternate. Magnetic Field Due to Current: 1) Straight Wire: Compass Needle Current A Rheostat B Key Iron Filings Cardboard Volume 2 of 2 Universal Tutorials X ICSE Physics 53 54 Rules s to Find the Direc ction of Magnetic M Field: F T The direction n of magneti ic field (or magnetic m field d lines) prod duced due to o flow of current in a c conductor ca an be determined by any of the follow wing two rules s: Rig ght Hand Thumb T Ru ule: z If we hold d the current carrying con nductor in the e right hand such that the thumb points s in the direc ction of the current, then n the fingers en ncircle the wire w in the direction of th he magnetic field lines. Direct tion of magne etic field lines Maxwell s Co ork Screw w Rule: z Imagine a right han nded cork screw place ed with its axis coinciding g with the current carrying wire. It is now n rotated such that it ad dvances in the t direction n of current, the directio on in which the e thumb rota ates gives th he direction of magnetic field lines. 2) Circular C C Coil: Loop irection Di of f Current A Ca ard Board P C Q Key Rheost tat 54 Battery y T polarity at the faces of loop depe The ends on the direction o current and it is determ of mined by the clock rule. Clock C rule ( (clockwise cu urrent south h pole and anticlockwise current n north pole) L Looking at th he face of th he loop, if the e current aro ound that f face is in ant ti clockwise e direction, th he face has the north p polarity, whil le if the current at that face is in clockwise c A Anticlockwise e d direction; the e face has the e south polarity. Current (a) If a conducting wire is wound w in the form of a cylindrical c c whose diameter coil d is le ess in comp parison to the length, t coil is cal the lled a soleno oid. A current carrying soleno oid behaves s like a b magnet with bar w fixed pol larities at its ends. Its strength can c be increa ased by incr reasing t current the Its direction can be reve ersed by rev versing t direction of current. the Universal Tu utorials X IC CSE Physics s Clockwise C Current (b) Volu ume 2 of 2 Chapter 10: 1 Electro M Magnetism 55 Electromagn net: An electromagne e et is a tempo orary strong magnet. It is s just a solen noid with wire e wound on a soft iron core e. Way ys of increas sing the magn netic field of an electromagnet. By B increasing g the number of turns By B increasing g the current t through the solenoid. Use es: z z z z z For lifting g and transp porting large masses of iron scrap, girders, plat tes etc., esp pecially to places wh here it is not convenient to t take the help of human n Labor. Elec ctromagnets are used to lift as much m as 20 2 22 tones of ir ron in a single lift. For separ rating magne etic substanc ces such as iron from oth her debris (e.g. for separ rating iron from the crushed c copp per ore in copper mines). . For remov ving pieces of o iron from wounds. w In severa al electrical devices d such h as electric bell, telegra aph, electric tram, electr ric motor, relay, mic crophone, lou ud speaker, etc. e In scientif fic research, to study the magnetic pr roperties of a substance i in a magnetic c field. (I shaped electromagnet) (U shape ed) Distinguish between elec ctromagnet and perman nent magnet t. Ans: Electrom magnet P Permanent Magnet M Made of soft iron Made of stee el Magnetic field M d is produce ed as long as a current fl lows in its co oils i.e. magn netic field is te emporary Magnetic fie eld produced is permanen nt. M Magnetic field d strength ca an be change ed Magnetic fie eld strength c cannot be changed P Polarity can be b reversed Polarity cann not be revers sed. Adv vantages of Electro omagnet Over O Perm manent Magnet: z z z An electro omagnet can n produce a strong s magne etic field, The stren ngth of the magnetic m field d of an electr romagnet ca an easily be c changed by changing the curren nt or the num mber of turns in the soleno oid. The polar rity of the ele ectromagnet t can be cha anged by rev versing the d direction of current c in the soleno oid. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 55 56 Forc ce on a current c c carrying g conduc ctor in a magne etic field Curr rent (I) Mag gnetic field st trength (B) Length of the wire (L) and an ngle between n the directio on of wire. F = KBIL sin F BIL sin F = 0 and if = 90 If = 0 nit of magne etic field (B) ) Un B= ne ewton F = IL amper m K = 1 in SI F = BIL (maxim mum) Current Magnetic field f Forc ce is directly proportional l to Force F = BIL N A 1 m 1 or tesla (T) or Weber m 2 D.C. Motor: An electric e moto or is a device e which conve erts electrica al energy into o mechanical energy. It works w on the e principle th hat when an n electric current is pas ssed through h a conducto or placed norm mally in a ma agnetic field, a force acts s on conducto or as a result of which the conductor begins to mov ve and work is obtained. The e direction of force is obta ained with the e help of Fleming s left ha and rule. Fle eming s lef ft hand Ru ule: z Stretch th he forefinger, , middle finge er and the th humb of your left hand m mutually perp pendicular to each ot ther. If the fo orefinger indicates the direction of ma agnetic field and the middle finger indicates the direction n of current, then the thumb will ind dicate the dir rection of motion (i.e. force) on conductor. The Main M Part ts of an Electric E Motor are: The T armature e coil ABCD mounted on an axle, The T split ring g commutator (i.e. a slip ring r made of copper divid ded in two pa arts S1 and S2), A pair of brus shes B1 and B2 and A horse shoe ele ectromagnet NS. The e coil ABCD is wound ro ound a soft iron core an nd is placed in between the pole pie eces of a pow werful horse shoe electro omagnet. The e coil is free to rotate abo out its axis. The e ends of the coil A and D are respect tively connec cted to the sp plit rings S1 a and S2 of a ri ing. Two o brushes B1 and B2 mad de of carbon (or copper), touch the split rings S1 and S2 respe ectively A d.c. source is co onnected acr ross the brus shes B1 and B2. Whe en the coil ro otates, the sp plit rings rotate, but the brushes do no ot move. Wo orking: z z z 56 The plane e of coil is ho orizontal and d the split ring g S1 touche es the brush B1 while the e split ring S2 touches the brush B2. The brush b B1 is connected d to the ano ode of d.c. battery while e the brush B2 is connec cted to catho ode. The curre ent flows in th he coil in dire ection ABCD. The arms s BC and DA D being pa arallel to the e magnetic field, experience no forc ce. According g to Fleming s left hand d rule, a force F acts on Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 10: Electro Magnetism z z z z z z 57 the arm AB in inward direction perpendicular to the plane of paper and a force F acts on the arm CD in an outward direction perpendicular to the plane of paper. The forces on the arms AB and CD being equal and opposite form an anti clockwise couple due to which the coil begins to rotate such that the arm AB goes in and the arm CD comes out. When the coil reaches the vertical position the couple becomes zero. But due to inertia of motion, the coil does not stop in this position. As the coil passes from its vertical position, the split ring S1 comes in contact with the brush B2 while the split ring S2 comes in contact with the brush B1. Now the current flows through the coil in direction DCBA and the forces acting on the arms DC and AB of coil again form an anti clockwise couple due to which the coil remains rotating in the same direction. Whenever the coil comes in vertical position, the direction of current through the coil reverses and the coil continues to rotate in the same direction. Ways of Increasing the Speed of Rotation of Coil: The speed of rotation of coil can be increased by z Increasing the strength of current, z Increasing the number of turns in coil, z Increasing the area of coil and z Increasing the strength of magnetic field (for which a soft iron core can be inserted within the coil). Electromagnetic Induction: Faraday s Laws: Whenever there is a change in magnetic flux linked with a coil or conductor an e.m.f. is induced. The induced e.m.f. lasts so long as there is a change in magnetic flux linked with the coil or conductor. This phenomenon is called electromagnetic induction. The magnitude of the e.m.f. induced is directly proportional to the rate of change of magnetic flux linked with the coil. If the rate of change of magnetic flux remains uniform, a steady e.m.f. is induced. The magnitude of e.m.f. induced depends on the following two factors: z The change in magnetic flux and z The time in which the magnetic flux changes. Direction of induced e.m.f. is given by z Fleming s right hand rule, and Lenz s law. Fleming s right hand rule: z Stretch the thumb, central finger and fore finger of your right hand mutually perpendicular to each other. If the forefinger indicates the direction of magnetic field and the thumb indicates the direction of motion of conductor, then the central finger will indicate the direction of induced current. Lenz s law: z It states that the direction of induced e.m.f. (or induced current) is such that it always tends to oppose the cause which produces it. Volume 2 of 2 Universal Tutorials X ICSE Physics 57 58 A. C. Genera ator (or Dynamo o): A dy ynamo is a device d which h converts mechanical en nergy into ele ectrical ener rgy using the e principle of electromagne etic induction. Whe en a coil (or conductor) is i rotated in a magnetic field, the ma agnetic flux l linked with it t changes and therefore an n e.m.f. is ind duced in the coil. The e main parts of o an a.c. generator are: The T field mag gnet The T armature e coil The T slip rings s, and The T brushes. Field magnet: This is a str rong horse s shoe perman nent gnet NS. An electromagn net run by d.c c. source can n also be use ed for the hig gh power gen nerators. mag Arm mature Coil: This is a so oft iron core on which a coil ABCD having h a larg ge number of o turns of insu ulated copper wire is wou und. This arm mature coil is s rotated rap pidly in the m magnetic field d between the poles of the field magnet t by the axle fixed to the armature. a Slip p rings: Thes se are the tw wo metallic rings S1 and S2 kept co a axially. They y rotate along g with the armature coil. Th he ends of th he armature coil are conn nected to the ese slip rings s S1 and S2. Brushes: Two brushes B1 and a B2 made of carbon press lightly y against the e slip rings S1 and S2 resp pectively. The external circuit c (i.e. loa ad) is conne ected betwee en the other ends of the brushes. The e brushes B1 and B2 do no ot rotate alon ng with the coil. c Wo orking: z z z z z z z z z z z 58 Initially let t the plane of o the coil be perpendicula ar to the mag gnetic field. Maximum m magnetic fie eld lines pas ss through th he coil. The end A of the coi il is connect ted to the slip ring S1 an nd the end D to the slip ring S2. The slip ri ings also rota ate with the coil. c The brush h B1 touches the slip ring S1 and the brush b B2 the slip ring S2. Let the co oil be rotated clockwise. As A the coil st tarts rotating, the magnetic flux linke ed with the coil decreases s and theref fore an e.m.f. is induced at the ends of the coil due to wh hich an indu uced current flo ows in the co oil. When the e coil gets ro otated by 90 0 the magn netic flux linked d with the clock become es zero and the e.m.f. indu uced in the coil c is maximum. By Fleming s right hand h rule, the t direction n of induced current c in the coil is along g ABCD. The induc ced current is s from A to B, B while in the e arm CD wh hich is movin ng inwards (o or into the plane of paper) p the ind duced curren nt is from C to t D. Therefore e in external circuit c the cu urrent flows from brush B2 to brush B1. On furthe er rotation of f the coil by 90 , the pla ane of the coil c again be ecomes norm mal to the magnetic field and the e magnetic flux linked wit th the coil be ecomes max ximum, so the e induced s to zero. e.m.f. falls Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 10: 1 Electro M Magnetism z z z z z z 59 As the co oil further ro otates, the magnetic m flu ux linked wit th the coil d decreases and a again induced current c flows in the coil in a direction DCBA. D The e.m.f f. again incre eases to the same s maxim mum value, but in opposite direction. Thus in one complete e rotation of coil, c we get one cycle of f alternating. e.m.f. in the e external circuit. Ma agnitude of the t e.m.f. is given by Faradays seco ond law and direction by Flemings right hand d rule. The altern nating e.m.f. thus produced has the same s frequen ncy as that of the rotation n of coil. If coil mak kes n rotation ns per secon nd then induc ced e.m.f. e = e0 sin 2 nt t and i = i0 sin n 2 nt. Frequency of a. c. in India is 50Hz z. Maximum va alue of e.m m.f. induce ed can be increased d by: z z z Increasing g the number of turns of the t coil Increasing g the area of f the coil Increasing g the speed of rotation of o the coil Distinguish between a.c. generator (or ( dynamic c) and d.c. motor. m Ans: D.C. Motor D Dynamo Converts me C echanical energy into th he electrical energy. C Converts electrical energy into mecha anical energy y. Works on the princ W ciple e electromagne etic induction n. of Works on th W he principle that force acts on a current c carrying cond ductor placed d in a magne etic field. Uses two separate coa U axial slip ri ings. Uses two parts of a slip ring i.e., split rings which U h act as a commutato or. A generator is a devic ce which c converts the mechanical m e energy. A d.c. motor r is a device e which con nverts the electrical e energy into th he mechanic cal energy. A generator works w on the principle o electromag of gnetic induction. A d.c. motor works on th he principle o of force actin ng on a c current carrying conducto or placed in a magnetic fi ield. In n a genera ator, the me echanical e energy is use ed in rotating g the coil in n a magne etic field so o as to p produce elect trical energy. or works on the electrica al energy is provided p In a d.c. moto by the d.c. source to flow b w current in the coil plac ced in a m magnetic fie eld due to which the coil rotate es (i.e., m mechanical energy e is obt tained.) A generator makes use e of two s separate coax xial slip rings s. A d.c. motor makes use of two parts s of a slip ring (i.e., s split rings) which act as a commutator. Tran nsformer r: A tr ransformer is s a device by b which the e amplitude of an alter rnating e.m.f f. can be incr reased or de ecreased. It works w on the principle of electrom magnetic ind duction (mutual induction) and make es use of two o coupled coi ils. The ere are two ty ypes of transformers : Step up S tran nsformer Step down S tr ransformer Step p up transf former: The transformer used to cha ange a low voltage alte ernating e.m m.f. to high voltage v alter rnating e.m.f. (of same frequency) f is s called a ste ep up transfo ormer. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 59 60 Step down transformer: The transformer used to change a high voltage alternating e.m.f. to low voltage A.C. e.m.f. (of same frequency is called a step down transformer. A transformer consists of thin rectangular frames of soft iron (or silicon steel) placed one above the other and insulated from each other by a paint or varnish. This forms a thick rectangular frame which is called the laminated core. On one arm of the core, a coil P of insulated copper wire is wound. This coil is connected with the alternating source of e.m.f. This is called the primary coil. On the other arm of the core, another coil S of insulated copper wire is wound. The induced alternating e.m.f. is obtained across the terminals of this coil (i.e. output is obtained at the ends of this coil). This is called the secondary coil. The ratio of number of turns Ns in secondary coil to number turns Np in primary coil (i.e. Ns, Np) is called the turns ratio. Step up transformer : Ns >1 Np Step down transformer Ns <1 Np N Es = s for ideal transformer Ep Np Uses of Transformers: z Transformers are used in electrical appliances which operate at a voltage other than the voltage supplied by the mains. Step up Transformers are Used in: z z z z Power transmission at the generating station, Television, Wireless sets, and X ray tubes to provide a high accelerating voltage. Step Down Transformers are Used in: z z z z Electric bells, Radio sets for valve heaters, and Power sub stations to step down the voltage before its distribution to the consumers. Distinction between the step-up and step-down transformers. Step-up transformer It increases the a.c. voltage and decreases the current. i.e., Es > Ep and Is < Ip It decreases the a.c. voltage and increases the current. i.e., Es < Ep and Is > Ip The turns ratio Ns / Np > 1 i.e., it has more number of turns in the primary coil. Its turns ratio Ns / Np < 1 i.e., it has less number of turns in the secondary coil than in the primary coil. The wire of primary coil is thicker than that in the secondary coil. The wire in the secondary coil is thicker than that in the primary coil. Uses: At power generating station, with X-ray tubes, television, etc. 60 Step-down transformer Uses: At power substation, with night electric bulb, ratio sets for valve heaters, transistors, etc. Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 10: 1 Electro M Magnetism 61 Energ gy losses s in a Transformer r: H Heating in the coils. Due to flow of cu urrent, the pr rimary and se econdary coils get heate ed up. It is r reduced by ta aking a thick k wire (low re esistance) for r the coil. Eddy E current ts in the core e. Due to flo ow of varying g current in the primary c coil, the mag gnetic flux t through the soft iron cor re changes, so eddy cu urrents (i.e. the t induced local curren nts in the c core)* flow in n the core and there is los ss in energy. . It is reduced d by laminating the iron core. c Hysteresis H lo oss in the co ore. Due to the flow of varying curr rent in the p primary coil, the core m magnetizes a demagne and etizes. It is re educed by ta aking the cor re of soft iron n. Magnetic M fiel ld link loss. There may be some los ss of energy y if the entire e magnetic field f lines p produced due to flow of current in the primary co oil are not lin nked with the e secondary coil. This lo oss of energ gy is reduced d by taking a closed core. . Distinction D be etween an AC generator and DC gen nerator. A generato AC or D generato DC or A generator is s a device which conve erts the e mechanica al energy int to the electr rical en nergy. A d.c. motor is a device w which conve erts the electric cal energy in nto the mechanical energ gy. A generator works w on th he principle of ele ectro-magne etic induction. A d.c. motor works s on the principle of force e acting on a current carrying conductor placed in a magne etic field. In a generator r, the mecha anical energy y is us sed in rotating the coil in a magne etic fie eld so as to produce p elect trical energy. he electrical energy is provided p In a d.c. motor, th by the d.c. source to flow curre ent in the coil l placed in a magnetic m field d due to which the coil rotates. (i.e., mechanical m energy is obta ained) A generator makes m use of o two separ rate co oaxial slip ring gs. A d.c. motor make es use of two o parts of a slip s ring (i.e., split rings) wh hich act as a commutator r. REVIEW QUESTIONS: Direct Questions: Q 1) Dr raw a diagram m showing th he magnetic field lines du ue to a straig ght wire carry ying current. 2) State a law, wh hich determines the direc ction of magn netic field aro ound a curre ent carrying wire. w 3) Dr raw a labele ed diagram showing s the magnetic fie eld lines of a loop carrying current. Mark the dir rection of cur rrent and the e direction of f magnetic fie eld by arrows s in your diag gram. 4) Dr raw a diagra am to repre esent the magnetic field d lines along g the axis o of a current t carrying so olenoid. 5) Na ame and stat te the rule by y which the direction d of magnetic m fiel ld in a curren nt carrying so olenoid is de etermined. 6) Th he following diagram sho ows a spiral coil wound on a hollow ca ardboard tub be. A magne etic compass is placed close to it. When the current flows by closing the key, how will the co ompass need dle be affecte ed? Give reason. 7) State two ways s by which th he magnetic field due to a solenoid ca an be made stronger. 8) Why does a cu urrent carryin ng freely suspended solenoid rest alo ong a particular direction? ? 9) What effect will w there be on a magne etic compass s when it is s brought ne ear a current t carrying so olenoid? 10) Yo ou are required to make e an electro omagnet from m a soft iron n bar. Draw w a circuit diagram to rep present the process. p In your y diagram m you are req quired to sho ow a cell, an n insulated co opper coil the e soft iron ba ar and a swit tch. Label the e poles of the e electromag gnet. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 61 62 11) Show with the aid of a diagram how a wire wound on a U-shaped piece of soft iron is used to make it an electromagnet. Complete circuit diagram and label the poles of the electromagnet. 12) What is an electromagnet? State two factors on which the strength of magnetic field of an electromagnet depends. 13) State two ways through which the strength of an electromagnet can be increased. 14) State three uses of an electromagnet 15) State two advantages of an electromagnet over a permanent magnet. 16) Why is soft iron used as the core of the electromagnet in an electric bell? 17) State three factors on which the magnitude of force on a current carrying conductor placed in a magnetic field depends. 18) Name and state the law which is used to determine the direction of force on a current carrying conductor placed in a magnetic field. 19) State the unit of magnetic field in terms of the force experienced by a current carrying conductor placed in a magnetic field. 20) What is an electric motor? State its principle. 21) What energy conversion does take place during the working of a d.c. motor? 22) Draw a labelled diagram of a d.c. motor and explain its working. When will the couple acting on the coil be (a) maximum, and b) minimum? 23) State two ways by which the speed of rotation of an electric motor can be increased. 24) State two factors on which the magnitude of induced e.m.f. depend. 25) What kind of energy change takes place when a magnet is moved towards a coil having a galvanometer at its ends? Name the phenomenon. 26) How would you demonstrate that a momentary current can be obtained by the suitable use of a magnet and a coil of wire? What is the source of energy associated with the current so obtained? 27) State Fleming s right hand rule. 28) What is Lenz s law? 29) State the principle of a simple a.c. generator. What determines the frequency of a.c. produced in a generator? 30) Draw a labelled diagram of a simple a.c. generator. 31) What energy conversion does take place in a generator when it is in use? 32) State two differences between a d.c. motor and an a.c. generator. 33) State one advantage of using a.c. over d.c. 34) For what purpose are transformers used? Can they be used with a direct current source? 35) How is the e.m.f. in the primary and secondary coils of a transformer related with the number of turns in these coils? 36) Give two points of difference between a step- up transformer and a step-down transformer. 37) What is the function of a transformer in an a.c. circuit? How do the input and output powers in a transformer compare? Name two causes of energy loss in a transformer. 38) Name the two kinds of energy losses that take place in the core of a transformer. How are they minimised? NUMERICALS: Class Work: 1) A primary of 800 turns is connected to a 220 V a.c. supply and the secondary has 8 turns. What will be the output voltage? [2.2 volt] 2) A transformer is designed to work from a 240 V a.c. mains and to give a supply of 8 V to ring a house-bell. The primary coil has 4800 turns. How many turns would you expect the secondary to have? [160] 62 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 10: 1 Electro M Magnetism 63 Home Work: W 1) Th he input and output volta ages of a tra ansformer ar re 220 V and d 44 V respectively. Find: (a) the [(a) 1:5 (b) tur rns ratio, (b) the current in input Circu uit if the outp put current is 2 A. ( 0.4 A] APPLICATION TYPE: T 1) Ho ow is the ma agnetic field due to a st traight curre ent carrying wire w affected d if current in wire is (a) ) decreased, , (b) reversed d? 2) A straight wire e lying in a horizontal h pla ane carries a current from m north to so outh. (a) Wh hat will be the e direction of o magnetic field f at a poi int just unde erneath it? (b b) Name the law used to o arrive at the e answer in part p (a) and state s it. 3) What will happ pen to a com mpass needle e when the compass c is placed p below w a wire and a current is made to flow w through the e wire? Give a reason to justify your answer. a 4) Th he diagram below show ws two stra aight wires A and B carrying c equ ual cu urrents. Draw w the pattern of magnetic field lines ar round them and a mark the eir dir rections. What will be the e resultant? Ma agnetic field at a point K equidistant from f the wire es A and B? 5) A wire, bent in nto a circle, carries current in an an nticlockwise direction. W What polarity does this fac ce of the coil l exhibit? 6) What is the dir rection of ma agnetic field at a the centre of a coil car rrying current t in i) clockwise, c ii) anticlockwise e direction? 7) Th he adjacent diagram d show ws a coil wo ound around a soft iron bar b XY. State the pola arity at the end X and Y as the switch s is pre essed. Su uggest one way of incr reasing the strength of electromagn net so for rmed. Y AT 8) What name is given to a cylindrical c co oil diameter less than its s length? If a piece of so oft iron is pla aced inside this t coil and current is passed in the e coil from a battery, wha at name then n given to the e device so obtained o ? Give G one use of this devic ce. 9) Fig g shows the current flow wing in the coil c of wire wound w around the soft iro on ho orse shoe core. State the polarities de eveloped at the t ends A and a B. 10) Ho ow is the wo orking of an electric bell affected, it alternating current be u used instead d of direct cu urrent? 11) State when ma agnitude of force f on a current carrying conducto or placed in a magnetic field f is (a) ze ero, (b) maxim mum. 12) What happens s when a cur rrent is passe ed through a flat coil ABC CD freely suspended bet tween the po ole pieces of a U-shaped permanent magnet with the plane of f coil pa arallel to the magnetic field. a) When will th he coil come to rest? b) Name an instrument which w makes s use of the principle stated ab bove. 13) A coil ABCD mounted m on an a axle is placed betwee en the poles N and S of a permanen nt magnet as s shown in fig g. a) In which direction will the coil beg gin to rotate when curre ent is passe ed through th he coil in direction AB BCD by conn necting a bat ttery at the ends A and D of the coil? b) Why is a co ommutator necessary n for continuous s rotation of the t coil? Sho ow the comm mutator in the diagram m at the appr ropriate posit tion. 14) Th he following diagram sho ows a coil of f several tur rns of coppe er wire co onnected to a sensitive ce entre-zero galvanometer r G near a magnet m NS S. The coil is s free to mov ve. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 63 64 15) 16) 17) 18) i) Describe th he observatio on if the coil is i rapidly mo oved in the di irection of ar rrow. ii) How would the observa ation be altere ed if a) the coil has h twice as many turns. b) the coil was w made to move three times as fas st? Th he following diagram shows s a fixe ed coil of several turn ns connecte ed to a cen ntre zero ga alvanometer G and magn net NS which h can move. De escribe the observation o in the galvan nometer if i) the magnet t is mo oved rapidly in the direction of arrow. . , ii) the mag gnet is kept still s aft ter it has moved m into th he coil, iii) the magnet is then rapid dly pu ulled out of th he coil. Th he diagram shows s a coil connected to o a centre ze ero galvanom meter G The e galvanomet ter shows a deflection to o the right when w the nort th pole N of f a powerful magnet is m moved to the e right as sh hown. i) Explain why y does the deflection occ cur in the galvanometer ii) State whet ther the cu urrent in the e coil is clockwise or anticlockwis se when view wed from the e end P. G iii) State the observation o i G when the in t coil is moved m away from north pole N of the magne et keeping the t magnet stationary. iv) ) State the observation o in n G when bo oth the coil and a the mag gnet are mov ved to the rig ght at the same speed. Th he following diagram d sho ows a coil X connected to o a sensitive e centre-zero o galvanomet ter G and a coil c P connec cted to a d.c c. supply thro ough a switch h S. De escribe the observation n when the switch S is (i) closed su uddenly, (ii) then t kept cl losed, (iii) fin nally opened d. Name and sta ate the law which w explain ns the above observations. In an a.c. gen nerator the speed at which w the coil rotates is doubled. H How would this affect ma aximum outp put voltage. b) the a) the frequenc cy of output voltage, v t maximum m output volt tage. 19) Co omplete the following f dia agram of a tr ransformer and name the e parts labelled A and B. B Name par rt you have drawn to co omplete the diagram. d Wh hat is the ma aterial of this s part? Is tra ansformer a step-up s or st tep-down? Give G reason. 20) Th he diagram below b shows the core of a transformer and its inpu ut and output connections. a) State the material m used for core and d describe its s structure. b) Complete the t diagram of the trans sformer and connections s by labeling all parts join ned by you. c) Name the transformer: step up or step s down? 21) Th he figure alon ngside shows s the simple form of a DC C motor i) State the di irection of ro otation of coil. ii) How can yo ou determine e the direction of rotation? ? iii) How can th he speed or the power of the motor be e increased? ? 22) Na ame the devi ice which co onverts electr rical energy into i mechanical en nergy. 23) i) Why does a current car rrying wire move when placed in a ma agnetic field? ? 64 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 10: 1 Electro M Magnetism 65 ii) What angle e should the wire (condu uctor) make with the fiel ld so that the force actin ng on the wire is (a a) maximum (b) zero 24) AB BCD is a rect tangular coil kept betwee en the pole piece p of a pow werful magnet as shown in figure i) Explain what happens when a curr rent is passe ed through the t coil in the directio on of the arro ows. Mark th he direction of o the forces s on each arm. ii) What happe ens to the moment m of the e couple as the t coil turns s from the position ind dicated to that t when it ts plane is perpendicula ar to the magnetic fie eld? Why do oes the motio on continue beyond b this position? p 25) AB B is a wire which w hangs s in a magne etic field and d carries a current c as sh hown in figure e. It is free to o swing. i) Mark the direction of the e current in AB. A ii) Describe th he motion of AB A as long as a key K is pressed. iii) What is the e purpose of the variable resistor R? 26) Ac ccording to Faraday s F co oil and magnet experiment, state and d explain what happens when the no orth pole of a magnet is moved m i) towards the e face of the coil connected to a galva anometer, he coil conne ected to a ga alvanometer. (Draw neat t and correctly labeled ii) away from the face of th sketches). 27) A fixed copper r coil having many turns s is connecte ed with a galvanometer. Answer the following qu uestions: i) A magnet is s moved rapidly in the dir rection of arr row. ii) The magne et is stopped within the co oil. iii) The magne et is rapidly moved m away from the coil l. iv) ) What polari ity is induced d at point Y when w a) The mag gnet is move ed towards th he coil? b) The mag gnet is move ed away from m the coil? v) Name the la aw applied to o find the polarity at point Y. vi) ) Name the phenomenon p n that occurs. MISCELL LANEOUS EX XERCISE: 1) State the energy change which takes s place when a magnet is moved in nside a coil having a ga (2005) alvanometer at its ends. Name N this ph henomenon. (2005) 2) Dr raw a labelled diagram of f an A. C gen nerator. 3) i) State the fu unction of a split s ring in a D. C motor. ii) Mention tw wo reasons why a soft t iron core is used wi ithin the co oil of a mov ving coil (2005) galvanomet ter. elled diagram m to show the e various components of f a step dow wn transform mer. 4) i) Draw a labe (2004) m differenc ce between a step up an nd step down n transforme er. ii) State the main s by which th he emf in an A. C genera ator can be in ncreased 5) State two ways (2006) antages of a electromagn net over a ba ar magnet. 6) State two adva imilarities be etween a DC motor and an a AC genera ator. 7) State two dissi raw a represe entative diag gram of a dc motor. Labe el the followin ng in your dia agram: 8) Dr ii) The armature iv) Wire brus i) The T field magnet iii) Commu utators shes Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 65 66 9) i) State two characteristic c cs of a primary coil of a step up tra ansformer w when compar red to the secondary coil. ii) With about a D. C. moto or, state: a) The ener rgy change that t takes pla ace. b) The princ ciple on whic ch it operates s. 10) De escribe an ex xperiment to o demonstrat te that there is a magnetic field arou und a current carrying co onductor. 11) State two differences betw ween an elect tromagnet an nd a permanent magnet. 12) Na ame two app pliances in wh hich an elect tric motor is used. u 13) State Faraday s laws of ele ectromagneti ic induction. 14) De escribe briefl ly one way of o producing an induced current. Sta ate one facto or that determ mines the ma agnitude of induced e.m.f. What facto or determines the directio on of induced d e.m.f.? 15) Why is it more difficult to move m a magn net towards a coil which has h a large n number of tur rns? 16) Ex xplain why an a induced current c mus st flow in su uch a direction so as to o oppose the e change pro oducing it. 17) Ex xplain the sig gnificance of o Lenz s law w to show th he conservat tion of energ gy in electro omagnetic ind duction. 18) What is the effect on the magnitude of e.m.f. ge enerated in an a a.c. gene erator if the speed of rot tation of the coil of gener rator is increa ased? 19) Su uggest two ways w in an a.c c. generator to produce a higher e.m.f. 20) De escribe a ste ep-down transformer an nd explain ho ow it works. . State two characteristics of the pri imary coil as s compared to t its secondary coil. 21) Dr raw a labelle ed diagram of o a device you y would us se to transfo orm 200 V a. .c. to 15 V a.c. a Name the e device and d explain how w it works. Give its two us ses. PR REVIOUS YE EAR BOARD D QUESTION NS 1) i) What is an a.c. generator or dynamo o used for? ii) Name the principle p on which w it work ks. 2) Th he adjacent diagram sho ows a curre ent carrying loop or a circular c coil pa assing throug gh a sheet of f cardboard at a the points M and N. Th he sheet of ca ardboard is spinkled unifo ormly with iro on filings i) Copy the diagram and draw an arro ow on the circular coil to o show the direction of f current flow wing through it. ii) Draw the pattern p of ar rrangement of the iron filings f when current is passed thro ough the loop p. [2012] [2012] (2009) 3) Th he figure sho ows an electr romagnet. X i) What W will be the polarity at a the end X? ii) Suggest a way w by which h the strength h of the elect tromagnet ref ferred to in th he question, may be incr reased. Y () 4) i) Why does a magnetic needle show w a deflectio on when bro ought close to a current t carrying conductor? (2008) t into a circle e carries cur rrent in an an nticlockwise direction. W What polarity does this ii) A wire bent face of the coil exhibit? iii) Draw a sim mple sketch of o a step dow wn transforme er. Label the different par rts in the diag gram. 66 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 10: Electro Magnetism 67 5) i) What is the name given to a cylindrical coil whose diameter is less in comparison to its length? ii) If a piece of soft iron is placed inside the current carrying coil. What is the name given to the device? (2008) 6) i) State two factors on which the strength of an induced current depends. ii) When a solenoid that is carrying current is freely suspended, it comes to rest along a particular direction. Why does this happen? (2007) 7) i) What will happen to a compass needle when the compass is placed below a wire and a current is made to flow through the wire? Give a reason to justify your answer. (2006) ii) What energy conversion takes place during the working of a d.c. motor? 8) Name the chief energy transformation that occurs: (2005) i) In a Loudspeaker; ii) In an Electrical cell (primary) Volume 2 of 2 Universal Tutorials X ICSE Physics 67 68 Chapter 11: Calorimetry Chapter Map: Introduction Heat capacity Specific heat capacity Relation between heat capacity Specific heat capacity Calorimeter Principle of mixtures Change of phase Melting or fusion Vaporisation/ Boiling Latent heat Change in Phases of ice Introduction: Heat is a form of energy. SI unit is joule. The other unit is calorie. One calorie of heat is the quantity of heat required to raise the temperature of 1g of water from 14.5 C to 15.5 C. 1 cal = 4.18 J or 4.2 J 1k cal = 1000 cal = 4200 J. The average internal KE of the molecules of a substance is a measure of temperature of that substance. Temperature determines the direction of heat flow. Differences between heat and temperature: Heat The kinetic energy due to random motion of the molecules of a substance is known as its heat energy. The quantity which determines the direction of flow of heat between two bodies kept in contact is called temperature. The S.I. unit of heat is joule (J). The S.I. unit of temperature is Kelvin (K). It is measured by the principle of calorimetry. It is measured by a thermometer. It is an additive quantity. 68 Temperature It is not an additive quantity. Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 11: Calorimetry 69 Heat Capacity: (C ) Definition: The heat capacity of a body is the amount of heat required to raise its temperature through 1 C or 1 K. C = Q . T 1 1 Unit: JK ; J C , 1 1cal K = 4.2 JK 1. Specific Heat Capacity: Definition: The specific heat capacity of a substance is the amount of heat energy required to raise the temperature of unit mass of that substance through 1 C (or 1K). i.e. Specific heat capacity, c = Amount of heat energy sup plied Mass Rise in temperatur e If Q amount of heat energy is supplied to a body of mass M so that its temperature rises through T C or T K, then its specific heat capacity c= Q M T Units of Specific Heat Capacity: The S.I. unit of specific heat capacity is joule per kilogram per kelvin (J kg 1 K 1) or joule per kilogram per degree celsius (J kg 1 K 1) The other units of specific heat capacity are cal g 1 C 1 and kilo-cal kg 1 C-1 1 kilo cal kg 1 C 1 = 4.2 103 J kg 1 K 1 Water has an unusually high specific heat capacity (= 4200 J kg 1 K 1). Higher the specific heat, greater is the amount of heat given or taken by unit mass of the substance for a unit degree change in temperature. Relationship between Heat Capacity & Specific Heat Capacity: If Q joule of heat energy is supplied to M kg of a substance so that its temperature rises through T K, then its specific heat capacity c is given as c = Q Jkg 1 K 1 M T Thus, Q = M c T joule or Heat energy required = Mass specific heat capacity change in temperature The heat energy required to change the temperature of the body by 1 K is called the heat capacity of the body. Heat capacity = Q = MC T Thus, Heat capacity = Mass Specific heat capacity Volume 2 of 2 Universal Tutorials X ICSE Physics 69 70 Heat Capac city Spe ecific heat C Capacity It is the amou unt of heat energy e required to rai ise the temperature of en ntire body by y 1 C. unt of heat energy requ uired to It is the amou se the tempe erature of un nit mass of th he body rais by 1 C. It depends d on the mass of the body. It does not depe end on the m mass of the body. b Its s unit is J K 1 Its unit is J kg 1 K 1 He eat capacity (C ) = mass (m) specif fic heat ca apacity (c). Spe ecific heat ca apacity (c) heat capacity (C ) = mass m Calo orimeter: A ca alorimeter is a cylindrica al vessel which is used to o measure the amo ount of heat gained g or los st by a body when it is mixed with oth her body. It is made up of thin copper sheet s because copper c is a good condu uctor of heat, so it soon n acquires the t temperature of its contents, and copper c has low l specific heat capaci ity so the he eat capacity of c calorimeter is s low and th he amount of f heat energy taken by the c calorimeter f from its cont tents to acqu uire the tem mperature of its c contents is negligible. The e outer and in nner surface es are polish hed so as to reduce the loss of heat due to radia ation. It is plac ced inside a wooden jac cket. The space between n the calorim meter and th he jacket is filled f with som me poor cond ductor such as a wool, cotto on etc. to av void heat loss s by conduct tion. It is cov vered with a wo ooden lid to avoid heat loss by conv vection. The lid has two holes, h one fo or the stirrer (used for prop per mixing th he contents of o the calorim meter) and th he other for the thermom meter (to measure the temperature of it ts contents). : Instead of metallic calo orimeter, an expanded polystyrene p c cup can also o be used. It I has low Note: heat capacity c due e to its small l mass and it t provides ve ery good hea at insulation. . As a result, , the heat energ gy taken by calorimeter c can be neglec cted. Princ ciple of Mixture es (or Principle of o Calor rimetry): : The e principle of measureme ent of heat is s based on th he law of conservation o of energy. When a hot body is mixed (o or is kept in contact) c with a cold body y, heat energ gy passes fro om the hot bo ody to the cold d body, till bo oth the bodie es attain the e same temp perature. If no n heat ener rgy is exchan nged with the surroundings s, i.e. if the system s is fully y insulated, then t Hea at energy lost t by the hot body b = Heat energy gained by the co old body This s is called the e principle of f mixtures or the principle e, of calorime etry. Let a substance e A of mass m1, specific heat capacit ty c1 and at a higher tem mperature T1 be mixed with h another substance B of o mass m2, specific hea at capacity c2 and at a lower tempe erature T2 (whe ere T2 < T1). If the final te emperature of o mixture be ecomes T, then Fall in te emperature of o A = T1 T of B = T T2 Rise in temperature t Heat energy lost by A = m1 c1 fall in temp perature = m1 c1 (T T1 T) .... (i) 70 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 11: Calorimetry 71 Heat energy gained by B = m2 c2 rise in temperature = m2 c2 (T T2) .... (ii) If no heat energy is lost to the surroundings, then by the principle of mixtures, Heat energy lost by A = Heat energy gained by B m1 c1 (T1 T) = m2 c2 (T T2) The above expression can be used to calculate the unknown quantity. Measurement of Specific heat of a solid: Let c J kg 1 K 1 be the specific heat capacity of solid, while cc J kg 1 K 1 and cw J kg 1 K 1 be the specific heat capacities of the material of calorimeter and of water respectively. Mass of water = (M2 M1) kg Rise in temperature of water = (T T1) C . (i) Heat energy gained by water = (M2 M1) cw (T T1) J Heat energy gained by calorimeter = M1 cc (T T1) J .(ii) Fall in temperature of solid = (T2 T) C ..(iii) Heat energy lost by solid = Mc (T2 T) J Assuming no loss of heat, by the principle of mixtures, Heat energy lost by solid = Heat energy gained by water + Heat energy gained by calorimeter or Mc (T2 T) = (M2 M1) cw (T T1) + M1 cc (T T1) [(M2 M1 )c wM1c c ]( T T1 ) c= J kg 1 K 1 M( T2 T ) Specific heat capacity of a solid or liquid by electrical method: In this method, heat energy is supplied by an electric heater of known power W. The rise in temperature T in time t for a known mass M of the given substance is noted. Assuming that there is no loss of heat, the energy supplied by the heater (= Wt) is equal to the energy used (= Mc T) in raising the temperature of the substance. Thus, Wt = M c T Wt J kg 1 K 1 M T This method is suitable for good conductors of heat such as copper, silver, aluminum, mercury etc. specific heat capacity of the substance C = Natural Phenomena and Consequences of High Specific Heat Capacity of Water: Some consequences of high specific heat capacity of water are given below. The Climate Near the Seashore is Moderate: The specific heat capacity of water is very high (= 1000 cal kg 1 C 1 or 4200 J kg 1 K 1). It is about five times as high as that of sand. Hence the heat energy required for the same rise in temperature by a certain mass of water will be nearly five times more than that required by the same mass of sand. Similarly, a certain mass of water will give out nearly five times more heat energy than that given by sand of the same mass for the same fall in temperature. As such, sand (or earth) is heated or cooled more quickly as compared to water. Thus, a large difference in temperature is developed between the land and the sea due to which land and sea breezes are formed. These breezes make the climate near the seashore moderate. Volume 2 of 2 Universal Tutorials X ICSE Physics 71 72 Hot Water Bottles are Used for Fomentation: The reason is that water does not cool quickly due to its large specific heat capacity, so it provides heat energy for a long time. Water is Used as an Effective Coolant: By allowing water to flow in pipes around the heated parts of a machine, heat energy from such parts is removed (e.g. radiators in car and generator are filled with water). Water in pipes extracts more heat without much rise in its temperature because of its large specific heat capacity. In cold countries, water is used as heat reservoir for wine and juice bottles to avoid their freezing. The reason is that water can provide more heat to the bottles placed in it because of its high specific heat capacity and so they do not cool down further to freeze. Farmers fill their fields with water to protect the crops from frost: Water, having high specific heat capacity, does not allow the temperature in the surrounding area of plants to fall up to 0 C. In the absence of water, if on a cold night, temperature falls below 0 C, the water in the fine capillaries of plants will fall to 0 C and it will freeze, so the veins will burst due to increase in volume of water on freezing. As a result, plants will die and the crops will be destroyed. Some Examples of High and Low Thermal Capacity: The base of a cooking pan is made thick: By making the base of the cooking pan thick, its thermal capacity becomes large and it imparts sufficient heat at a low temperature to the bread for its proper baking. The base of an electric iron is made thick and heavy: By doing so, the thermal capacity of iron becomes large and it may remain hot for a long duration. The vessel used for measurement of heat (i.e. calorimeter) is made of thin sheet of copper: The reason is that the specific heat capacity of copper is low and by making the vessel thin, its thermal capacity becomes low so that it takes a negligible amount of heat from its contents to attain the temperature of the contents. Change of Phase (State): The process of change of a substance from one state to another at a constant temperature is called the change of phase. The change from solid to liquid phase at a definite temperature is known as melting, while the reverse change from liquid to solid is called freezing. The change from liquid to vapour is known as vaporisation, while the reverse change from gas (or vapour) to liquid is called condensation (or liquefaction). The direct change from solid to vapour is called sublimation and the reverse change from vapour to solid is called solidification. Melting and Fusion: The change from solid to liquid phase on heating at a constant temperature is called melting. 72 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 11: Calorimetry 73 The constant temperature at which a solid changes to liquid is called the melting point of the TEMPERATURE ( c) Y D B C LIQUID MELTING SOLID A A 20 40 60 80 100 TEMPERATURE ( c) 20 40 60 80 100 solid. In the graph, BC is parallel to time axis indicating that the temperature remains constant during change of phase. In graph (i) AB indicates vise in temperature i.e. solid absorbs energy while in graph (ii) AB indicates fall in temperature or cooling. At C, in graph (i) all solid has melted while at C in graph (ii) all liquid has solidified. Thus, the melting point and the freezing point are the same. Y LIQUID C B FREEZING SOLID D X 0 30 60 90 120 150 180 210 240 TIME(S) i) (Heating Curve) X 0 30 60 90 120 150 180 210 240 TIME(S) ii) (Cooling Curve) Change in Volume on Melting: 3 Some substances like ice contract on melting. Experimentally it is found that 1.091 cm volume of 3 ice at 0 C on melting becomes 1 cm of water at 0 C. On the other hand, substances like wax expand on melting. 1.161 cm3 of solid wax on melting at 64 C becomes 1.166 cm3 of molten wax. Effect of Pressure on the Melting Point: The melting point of a solid depends on the external pressure on its surface. The melting point of the substances which contract on melting (like ice, iron etc.) decreases by the increase in pressure. For example, the melting point of ice decreases by 0.0072 C for every one atmosphere rise in pressure. On the other hand, the melting point of the substances (such as wax, lead etc.) which expand on melting increases by the increase in pressure. Effect of Impurities on the Melting Point: The melting point of a substance decreases by the presence of impurities in it. This fact is used in making the freezing mixture by adding salt to ice. The freezing mixture is used in preparing kulphies . The melting point of ice decreases to 22 C on mixing salt to it properly. Vaporisation or Boiling: The change from liquid to gaseous (or vapour) phase on heating at a constant temperature, is called vaporisation. The particular temperature at which vapour pressure of the liquid equals external pressure is called the boiling point of liquid. For example, 100 C water Volume 2 of 2 Boiling Absorption of heat Rejection of heat condensation 100 C steam Universal Tutorials X ICSE Physics 73 74 Hea at energy is absorbed a at a constant te emperature during d vaporization. Exp periment (heating ( c curve for water): w z z z z z Take som me water (a at room tem mperature say, s 20 C) in a flask and su uspend a the ermometer in n it. Heat the flask and note its tem mperature after every half f minute till water starts boiling. Dur ring boiling, one o can see e the format tion of bubb bles throughou ut water whic ch indicates that the boil ling occurs thr roughout the e volume of water. w Plot a gra aph for tempe erature again nst time, by taking t the te emperature o on Y-axis and d the time on X axis s. The graph is shown in figure. It is called c the hea ating curve fo or water. From the graph it is observed that t initially the temperature of water rise es continuou usly in the part AB where w it is in the liquid ph hase. At B, th he boiling sta arts and the temperature does not rise furthe er in part BC, , although he eat energy is s being contin nuously abso orbed. The part BC B thus repr resents boilin ng and the particular p tem mperature at the point B (= ( 100 C) is the boiling point of f water. At C, C the entire water conve erts into the vapour. The e part CD represent ts the increas se in tempera ature of vapo our. Cha ange in Vo olume on Boiling: z All liquids s expand on boiling. For example, 1 cm c 3 of water r at 100 C b becomes 176 60 cm3 of steam at 100 C. Effe ect of Pressure on the Boiling Point: z z z z The boilin ng point of a liquid inc creases with h increase in i pressure and decrea ases with decrease in pressure. The boilin ng point of pure p water at one atmos spheric press sure (= 760 mm of Hg) is 100 C. Water boi ils at a temp perature high her than 100 C, if the atm mospheric pr ressure is hig gher than 760 mm of o Hg and bo oils at a temperature low wer than 100 C, if the atm mospheric pr ressure is less than 760 mm of Hg. H In a press sure cooker, the water bo oils at about t 120 C to 12 25 C due to increase in pressure, as the ste eam is not allowed a to es scape out of f it. The vap pour pressure e inside the pressure cooker be ecomes near rly 1.75 time es the atmos spheric press sure. Thus c cooking of ve egetables etc., beco omes much easier and faster f in it si ince they ge et sufficient h heat before the t water boils. At high altitudes, such h as hills and d mountains, , atmospheric pressure is s low, theref fore water e lower than n 100 C and d so it does not provide the required d heat for boils at a temperature cooking. Thus T cooking g there becomes very dif fficult and tak kes much lon nger time. Effe ect of Imp purities on n the Boiling Point: z The boilin ng point of a liquid increases by the addition of impurities to o it. If a little common salt is add ded to water r, the water boils b at a tem mperature hi igher than 10 00 C. This is s why the addition of o salt makes s the cooking g faster. Latent Heat: : Hea at energy is absorbed a by a solid durin ng melting an nd an equal amount a of he eat energy is s liberated by the liquid dur ring freezing, , without sho owing any ris se or fall in te emperature. S Similarly, heat energy is ab bsorbed by a liquid durin ng vaporisatio on and an eq qual amount t of heat energy is liberat ted by the vapo our during condensation c n, without showing any rise or fall in temperature e. Thus, wat ter at 0 C has more heat energy e than ice at 0 C. 74 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 11: 1 Calorimet try 75 Sinc ce this heat t energy is not externa ally manifested by any rise or fall in temperat ture. It is cons sidered to be e hidden in th he substance e and is calle ed the latent heat. Late ent heat whe en expressed d for unit mas ss of the sub bstance is called the spec cific latent he eat. Defin nition: S Specific laten nt heat of fus sion of a sub bstance is defined as the e heat energ gy required to t convert u mass of substance at unit a its melting point from so olid to liquid without chan nge in tempe erature. Specific S laten nt heat is den noted by the symbol L. Thus, T specific c latent heat Heat energy e exchanged for th he change of phase Mass If Q amount of heat ene ergy is abso orbed or libe erated by mass m of a substance during d its c change of ph hase at a con nstant tempe erature, then specific latent heat L= Q m Unit U of spec cific latent heat: h The S.I I. unit of specific latent heat is J kg 1. Other comm mon units 1 a cal g and are d kilo-calorie e kg . They T are rela ated as 1 cal g 1 = 4.2 J g 1 L= Specific Laten nt Heat of f Vaporisa ation: T The specific latent heat of o vaporisatio on of a subs stance is def fined as the heat energy y required f converting for g unit mass of the substa ance at its boiling b point from f the liquid to the vap pour state w without change in temper rature. Spe ecific Late ent Heat of o Vaporisa ation of St team: z z z The speci ific latent hea at of vaporisa ation of steam is 226800 00 J kg 1 (= 5 540 cal g 1). This means that 2268 8000 J of he eat energy is needed to convert c 1 kg g of water at 100 C to steam at 100 C or 2268000 1 of heat energ gy is rejecte ed when 1 k kg of steam at 100 C condense es to form wa ater at 100 C C. Hence, 1g g steam at 10 00 C contain ns 2268 J of heat energy more than 1 1g water at 100 C. Chan nge in Phases P o Ice: of Figu ure represen nts a temperature-time graph to illustrate the change in ph hases of 1 g of ice initially at 10 C when w it is heated at a constant rate e. The e part AB represents s the cont tinuous rise e in temperature till it reaches the e melting point (i.e.0 C) at a B. The e heat energy y absorbed in n this part is mc c T = 1 2.1 1 10 = 21 J (si ince specific heat capacit ty of ice = 2.1 J g 1 K 1). The e part BC rep presents the melting of ice at a cons stant temperature 0 C C. The melt ting is comp pleted at C. The A heat energy abs sorbed in this s part is mL = 1 336 =336J. The e part CD rep presents the continuous rise in tempe erature of wa ater till it rea aches the boiling point of water w (i.e. 100 0 C) at D. The e heat energ gy absorbed in this part is mc T = 1 4.2 100 1 = 420 J (since spe ecific heat capa acity of wate er is 42 J g 1 K 1). Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 75 76 The part DE represents the vaporisation of water at a constant temperature 100 C which is completed at E. The heat energy absorbed in this part is mL = 1 2268 = 2268 J. The part beyond E represents the rise in temperature of steam. The melting of ice occurs at 0 C and the boiling of water occurs at 100 C. Further, we notice from the graph that the part DE is longer (more than six times) than the part BC. This is because the specific latent heat of vaporisation is much more as compared to the specific latent heat of melting. Since heat is supplied at a constant rate, the ratio in lengths DE and BC is equal to the ratio of specific latent heat of vaporisation to the specific latent heat of melting, i.e., 2268 : 336 or nearly 6.75 : 1. Natural Consequences of High Specific Latent Heat of Fusion of Ice: Snow on mountains does not melt all at once: The reason is that the ice has a high specific latent heat of fusion (equal to 336000 1 kg 1). It is due to this fact that it changes into water slowly as it gets heat energy from the sun. If latent heat would not have been so high, all the snow would have melted very quickly even with a small amount of heat energy and there would have been floods in rivers. Water in lakes and ponds in cold countries does not freeze all at once: The reason is that the specific latent heat of fusion of ice is very high so to freeze water, a large quantity of heat has to be withdrawn, and hence it freezes slowly and thus keeps the surroundings moderate. Drinks get cooled more quickly by adding pieces of ice at 0 C than the ice-cold water at 0 C: This is because 1 g of ice at 0 C takes 336 J of heat energy from the drink to melt into water at 0 C. Thus drink loses an additional 336 J of heat energy for 1g ice at 0 C than for 1g ice-cold water at 0 C. When ice in a frozen lake starts melting, its surroundings becomes very cold: The reason is that the heat energy required for melting the frozen lake is absorbed from the surrounding atmosphere. As a result, the surrounding temperature falls and it becomes very cold. It is generally more cold after a hail storm (when ice melts) than during or before the hail storm: The reason is that after the hail storm: the ice absorbs the heat energy required for melting from the surroundings, so the temperature of the surroundings further falls down and we feel more cold. Consequences of High Specific Latent Heat of Steam: Steam causes more severe burns than boiling water, both being at the same temperature (i.e. 100 C): This is because steam first condenses into water at 100 C by imparting its latent heat (= 2268 J g 1) and then its effect is the same as that of boiling water. Thus 1 g of steam at 100 C imparts 2268 J of heat energy in addition to the heat energy imparted by 1 g of water at 100 C. As a result, more severe burns are caused. Steam is used for running trains or machines: The reason is that because of high specific latent heat of steam, the heat energy released during condensation of steam is very large which gets converted into the mechanical energy. In cold countries, steam pipes are used for heating the buildings more effectively than the hot water pipes at 100 C: The reason is that 1g of steam at 100 C imparts an additional 2268 J of heat energy than that imparted by 1g of hot water at 100 C. Measurement of specific latent heat: z z 76 The specific latent heat of fusion of ice or vaporisation of steam can be determined by the electrical method. In this method, the heat is supplied by an electric heater of known power W to melt the ice or to vaporise water into steam for a known time t. Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 11: Calorimetry z 77 Assuming that there is no loss of heat energy, the energy supplied by the heater (= Wt) is equal to the energy (= mL) used up in melting the mass m of ice at 0 C or in boiling off the mass m of water into steam at 100 C. Thus, Wt = m L Specific Latent heat L = Wt m REVIEW QUESTIONS: Direct Questions: 1) Define the term heat. 2) Name the S.I. unit of heat. 3) Define the term calorie. How is it related to joule? 4) Define the term heat capacity and state its unit. 5) Define the term specific heat capacity and state its unit. 6) Write the approximate value of specific capacity of water in S.I. unit. 7) What do you mean by the following statements: i) the heat capacity of a body is 501 K 1 ii) the specific heat capacity of copper is 0.4 J g 1 K 1? 8) Give one example each where high specific heat capacity of water is used (i) in cooling, (ii) as heat reservoir. 9) What is a calorimeter? Why is it made of copper? Give two reasons. 10) State the effect of presence of impurity on the melting point of ice. Give one use of it. 11) What is the effect of increase in pressure on the boiling point of a liquid? 12) Will the boiling point of water at Delhi be the same, less or greater than that at Shimla? Give a reason. 13) It is difficult to cook vegetables on hills and mountains. Explain the reason. 14) Define the term specific latent heat of fusion of ice. State its S.I. unit. 15) Which has more heat: 1g of ice at 0 C or 1g of water at 0 C? Give reason. 16) Ice cream appears colder to the mouth than water at 0 C Give reason. NUMERICALS: Class Work: 1) Calculate the heat capacity of a copper vessel of mass 150 g if the specific heat capacity of copper is 410 J kg 1 K 1. How much heat energy will be required to increase its temperature from 25 C to 35 C? [61.5 J K 1, 615 J] 2) 45 g of water at 50 C in a beaker is cooled when 50 g of copper at 18 C is added to it. The contents are stirred till a final constant temperature is reached. Calculate this final temperature. The specific heat capacity of copper is 0.39 J g 1 K 1 and that of water is 4.2 K g 1 K 1 and that of water is 4.2 J g 1 K 1. State the assumption used. [47 C] 3) Calculate the amount of heat energy required to raise the temperature of 100 g of copper from 20 C to 70 C. Specific heat capacity of copper = 390 J kg 1 K 1 [1950 J] 4) Find the time taken by a 500 W heater to raise the temperature of 50 kg of material of specific heat capacity 960 J kg 1, from 18 C to 38 C. Assume that all the heat energy supplied by the heater is given to the material. [32 min] 5) A piece of iron of mass 2.0 kg has a thermal capacity of 966 J K 1. Find: i) heat energy needed to warm it by 15 C, and ii) its specific heat capacity in S.I. unit. [(i) 14490 J (ii) 483 k kg 1 K 1] 6) 10g of ice at 0 C absorbs 5460 J of heat energy to melt and change to water at 50 C. Calculate the specific latent heat of fusion of ice. Specific heat capacity of water is 4200 J kg 1 K 1 [336 J g 1] Volume 2 of 2 Universal Tutorials X ICSE Physics 77 78 7) A molten metal of mass 150 g is kept at its melting point 800 C. When it is allowed to freeze at the same temperature, it gives out 75,000 J of heat energy. What is the specific latent heat of the metal? If its specific heat capacity is 200 J kg 1 K 1, how much additional heat energy will it give out in cooling to 50 C? [500 J g 1, 25,500J] 8) In an experiment, 17 g of ice is used to bring down the temperature of 40 g of water at 34 C to its freezing temperature. The specific heat capacity of water is 4.2 J g 1 K 1. Calculate the specific latent heat of ice. State one important assumption made in the above calculation. [336 J g 1] 9) A piece of ice of mass 40 g is added to 200 g of water at 50 C. Calculate the final temperature of water when all the ice has melted. Specific heat capacity of water = 4200 J kg 1 K 1 and specific [28.33 C] latent heat of fusion of ice = 336 103 J kg 1 10) 2 kg of ice melts when water at 100 C is poured in a hole drilled in a block of ice. What mass of water was used? Given specific heat capacity of water = 4200 J kg 1 K 1 Lice = 336 103 J kg 1. [1.6 kg] 11) The amount of heat energy required to convert 1 kg of ice at 10 C to water at 100 C is 2,70,100 J. Calculate the specific latent heat of ice. Specific heat capacity of ice =2100 Jkg 1 K 1, specific heat capacity of water = 4200 J kg 1 K 1. [3,36,000 J kg 1] 12) 1kg of ice at 0 C is heated at a constant rate and its temperature is recorded after every 30 seconds till steam is formed at 100 C. Draw temperature-time graph to represent the change in state. 13) 200 g of a solid at 100 C are dropped into a copper calorimeter of mass 100 g containing water weighing 150 g at 40 C. The final temperature reached is 50 C. Calculate the specific heat capacity of solid. Given that the specific heat capacity of copper is 0.4 J g 1 C 1 and of water is 4.2 J g 1 C 1. [0.67 J/g C] 14) A piece of brass of mass 200 g and at 100 C, is placed in 400 g of turpentine oil, contained in a copper calorimeter of mass 50 g at 15 C. The final temperature recorded is 23 C. Find the specific heat capacity of turpentine oil. Take specific heat capacity for brass = 370 J kg 1 K 1 and specific heat capacity of copper = 390 J kg 1 K 1. [1731 J/kg K] 15) Calculate the mass of steam which should be passed through 1 kg of water at 0 C, contained in a vessel of mass 1000 g (specific heat capacity 0.15 cal g 1 C 1 such that final temperature is 20 C. Take specific latent heat of steam 540 cal g 1 and specific heat of water I cal g 1 C 1. [4.2 kg] Home Work: 1) 200 g mass of a certain metal at 83 C is immersed in 300 g of water at 30 C. The final temperature is 33 C. Calculate the specific heat capacity of the metal. Assume that the specific [0.378 J g 1 K 1] heat capacity of water is 4.2 J g 1 K 1. 2) The temperature of 600 g of cold water rises by 15 C when 300 g of hot water at 50 C is added to it. What was the initial temperature of the cold water? [5 C] 3) 1300 J of heat energy is supplied to raise the temperature of 0.5 kg of lead from 20 C to 40 C. Calculate the specific heat capacity of lead. [130 J kg 1 K 1] 4) An electric heater of power 600 W raises the temperature of 4.0 kg of a liquid from 10.0 C in 100 s. Calculate: i) the heat capacity of 4.0 kg of liquid, ii) the specific heat capacity of the liquid. [ (i) 1.2 104 J K 1 (ii) 3 103 J kg 1 K 1] 5) 0.5 kg of lemon squash at 30 C is placed in a refrigerator which can remove heat at an average rate of 30 J s 1. How long will it take to cool the lemon squash to 5 C? Specific heat capacity of [29 min 10 s] squash = 4200 J kg 1 K 1. 6) How much heat is released when 5.0 g of water at 20 C changes into ice at 0 C? Take specific [2100 J] heat capacity of water = 4.2 K g 1 K 1, specific latent heat of fusion of ice = 336 J g 1. 78 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 11: Calorimetry 79 7) A refrigerator converts 100 g of water at 20 C to ice at 10 C in 73.5 min. Calculate the average rate of heat extraction in watt. The specific heat capacity of water is 4.2 J g 1 K 1, specific latent heat of ice is 336 J g 1 and the specific heat capacity of ice is 2.1 J g 1 K 1. [10 W] 8) Find the result of mixing 10 g of ice at 10 C with 10 g of water at 10 C. Specific heat capacity of ice = 2.1 J g 1 K 1, specific latent heat of ice = 336 J g 1 and specific heat capacity of water = 4.2 J g 1 K 1. [0.625 g ice will melt Temperature will remain 0 C] 9) What will be the result of mixing 400 g of copper chips at 500 C with 500 g of crushed ice at 0 C? Specific heat capacity of copper = 0.42 J g 1 K 1, specific latent heat of fusion of ice = 340 J g 1. [247 g ice will melt and temperature will remain 0 C] 10) Calculate the total amount of heat energy required to convert 100 g of ice at 10 C completely into water at 100 C. Specific heat capacity of ice = 2.1 J g 1 K 1, specific heat capacity of water = 4.2 J g 1 K 1, specific latent heat of ice = 336 J g 1. [2.701 105 J] 11) 1.0 kg of water is contained in a 1.25 kW kettle. Calculate the time taken for the temperature of water to rise from 25 C to its boiling point 100 C. Specific heat capacity of water = 4.2 J g 1 K 1. [4 min 12 s] 12) Calculate the mass of steam at 100 C required to raise the temperature of 200 g of water from 60 C to 100 C. Take specific latent heat of steam = 2268 J g 1 and specific heat capacity of water = 4.2 J g 1 C 1. [14.8 g] 13) 0.50 kg of lead at 327 C is cooled to 27 C, when it gives off 22,500 calories of energy. Calculate the specific heat capacity of lead in: (a) Calories, and (b) Joules. [150 cal / kg C, 627 J/ kg C] 1 1 14) A vessel of mass 80 g (specific heat capacity = 0.8 J g C ) contains 250 g of water at 35 C. Calculate the amount of ice at 0 C, which must be added to it, so that the final temperature is [98.29 J] 5 C. Take specific latent heat of ice = 340 J g 1 15) A metal drill of power output 500 W drills a hole in a lead cube of mass 0.25 kg in 6.5 s. The specific heat capacity of lead is 130 J kg 1 C 1.Calculate: a) The heat generated by the metal drill in one second, [500 J] b) The heat generated by the metal drill in 6.5 seconds, [3250 J] c) If t C is the rise in temperature of the lead cube, the heat absorbed by the cube in terms of t . [32.5 t] d) The value of t. [100 C] Assume that all the heat generated by the drill is absorbed by the cube. APPLICATION TYPE: 1) Same amount of heat is supplied to two liquids A and B. The liquid A shows a greater rise in temperature. What can you say about the heat capacity of A as compared to that of B? 2) Why do the farmers fill their fields with water on a cold winter night? 3) Water is used in hot water bottles for fomentation. Give a reason. 4) Water is used as an effective coolant. Give reason. 5) A mass m1 of a substance of specific heat capacity c at temperature T1 is mixed with a mass m2 of other substance of specific heat capacity c2 at a lower temperature T2. Deduce the expression for the temperature of the mixture. State the assumption made, if any. 5) In an experiment to determine the specific heat capacity of a solid, the following observations were made: Mass of calorimeter + stirrer = x kg Mass of water = y kg Initial temperature of water = T1 C Mass of solid = z kg Temperature of mixture = T C Temperature of solid = T2 C Specific heat capacity of calorimeter and water are c1 and c2 respectively. Express the specific heat capacity c of the solid in terms of the above data Volume 2 of 2 Universal Tutorials X ICSE Physics 79 80 6) A heater of po ower P watt raises r the temperature of m kg of a liquid by T K in time t s. Express the e specific he eat capacity of o liquid in terms of above e data. 7) Th he fig shows the variation n in temperat ture with time when some wax cools from the liqu uid phase to the solid pha ase. (i) In wh hich part of th he curve, the e wax is in liq quid phase? ii) What does the part QR RS of the curve represen nt? iii) At which point on the curve, th he wax will b be in the liquid as well as s solid phase e? iv) In which part of the curve, the wax w is in solid d phase? 8) 1 kg of ice at 0 C is heat ted at a con nstant rate and a its tempe erature is rec corded after r every 30 s till steam m is formed d at 100 C. Draw a tem mperature-tim me graph to represent th he change of phase. 9) Why do bottle ed soft drinks s get cooled d, more quic ckly, by the ice i cubes tha an by the ice ed water, bot th at 0 C? 10) It is i generally cold c after a hailstorm h than during and d before the hailstorm. h Gi ive reason. 11) Th he temperatu ure of surrou undings star rts falling wh hen ice in a frozen lake e starts melt ting. Give rea ason. 12) Ex xplain the following: i) The surroun ndings beco ome pleasan ntly warm when w water in i a lake st tarts freezing g in cold countries. ii) The heat supplied to a substance during d its ch hange of stat te, does not t cause any rise in its temperature e. 13) Th he diagram below sho ows the cha ange of ph hases of a su ubstance on a temperatur re-time graph h. a) What do the e parts AB, BC, B CD and DE D represent? b) What is the melting poin nt of the subs stance? c) What is the boiling point t of the subst tance? 14) If equal amounts of heat is i supplied to t equal qua antities of oil and water, which subst tance will rec cord a highe er rise in temp perature? Ex xplain. 15) i) Water W has a very high sp pecific heat capacity. c Writ te its value in n the followin ng units: a) j /g C b) cal /g C w this propert ty of water is useful: ii) Explain how a) To farmers b) In moderatin ng climate tors d) To aquatic plants p and an nimals c) In car radiat 16) Ja ames joule demonstrated d d that almos st all of mechanical ener rgy would be e converted into heat en nergy. Is it possible to co onvert all he eat energy in nto mechanic cal energy? Support you ur answer wit th a suitable example. 17) i) A hot stone is placed in a pail of cold water. Is wo ork done by the t stone on the water? ii) If the pail in the above e example is s thermally insulated, i w what happens s to the tota al internal en nergy of the stone s and wa ater while a) the stone co ools down b) the t water wa arms up? 18) Why does the e temperatur re of body re emain steady when the body is und dergoing a change c in sta ate? 19) State whether heat is being g evolved or absorbed by y the body in n each of the following ex xamples: i) Melting ii) Condensation iii) Formation of M of sn now C n of steam f ice vi) Liquefaction of solid iv) ) Sublimation n of iodine v) Evaporation E of o alcohol 20) Would you select a liquid having h a high specific he eat capacity as a thermo ometric liquid? Explain yo our answer. 21) If a nail polish remover spil lls on the skin, the skin fe eels cool. Wh hy? 22) Dr raw a graph to show tem mperature ch hange during g the time when w ice at 10 C is he eated and ch hanged into steam s at 100 0 C. 23) Dr raw a graph to show the e volume ch hange during g the time when w ice at 10 C is he eated and ch hanged into steam s at 100 0 C. 80 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 11: Calorimetry 81 MISCELLANEOUS: 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) 26) Define one kilo-calorie of heat. How is the heat capacity of a body relate* the specific heat capacity of its substance The specific heat capacity of water is 4200 J kg 1 K 1. What information does statement convey? Write the expression for the heat energy Q received by the substance when m kg of substance of specific heat capacity c J kg 1 K 1 is heated through T C. Discuss the role of high specific heat capacity of water with reference to climate in coastal areas. Why is the base of a cooking pan made thick and heavy? What is the principle of method of mixture (or principle of calorimetry)? Name the law on which this principle is based. Describe a method to determine the specific heat capacity of a solid, like a piece of copper. How will you determine the specific heat capacity of a liquid like olive oil by the method of mixtures? What do you understand by the change of phase of a substance? Is there any change in temperature during the change of phase? Does the substance absorb or liberate any heat energy during the change of phase? Explain the terms melting and melting point. Describe an experiment to show that there is absorption of heat energy when the ice melts. State the effect of increase of pressure on the melting point of ice. The melting point of naphthalene, a crystalline solid, is 80 C and the room temperature is 30 C. A sample of liquid naphthalene at 100 C is cooled down to the room temperature. Draw a temperature-time graph to represent this cooling. Explain the terms boiling and boiling point. How is the volume of water affected when it boils at 100 C? How is the boiling point of water affected when some salt is added to it? Write down the approximate range of temperature at which the water boils in a pressure cooker. What do you understand by the term latent heat? Write the approximate value of specific latent heat of ice. The specific latent heat of fusion of ice is 336 J g 1. Explain the meaning of this statement. Which requires more heat: 1 g ice at 0 C or 1g water at 0 C to raise its temperature to 10 C? Explain your answer. A metal piece of mass 20 g is heated to a constant temperature of 100 C. Then it is dropped in a calorimeter of mass 50 g and specific heat capacity 0.42 J g 1 K 1, containing 50 g of water at 20 C. After stirring the water, the highest temperature recorded is 22 C. Calculate the specific heat capacity of metal. Specific heat capacity of water = 4.2 J g 1 K 1. What mass of liquid A of specific heat capacity 0.84 J 1 g 1 K 1 and at a temperature 40 C must be mixed with 100 g of a liquid B specific heat capacity 2.1 J g 1 K 1 and at 20 C, so that final temperature of mixture becomes 32 C? How much boiling water (at 100 C) is needed to melt 2 kg of ice so that the mixture, which is all water is at 0 C. Given: Sp. heat capacity of water = 4.2 Jg 1 K 1, Sp. latent heat of ice= 336Jg 1. The heat energy released by the condensation of m1 g of steam at 100 C into water at 100 C is used to convert m2 g of ice at 0 C into water at 0 C. If there is no heat loss to the surroundings, find: i) the heat energy lost by steam in terms of m1, ii) the heat energy gained by ice in terms of m2 iii) the ratio m2 : m1 by forming a heat energy equation Take specific latent heat of vaporisation of steam = 2268 kJ kg 1, specific latent heat of fusion of ice = 336 kJ kg 1 and specific heat capacity of water = 4200 J kg 1 C 1. Volume 2 of 2 Universal Tutorials X ICSE Physics 81 82 27) A thermos flask of negligible heat capacity contains 100 g ice and 30 g of water. Calculate: i) the mass of steam at 100 C needed to condense in the flask so as to just melt the ice, ii) the amount of water in the flask after condensation. Is it possible to condense the water formed back to ice by adding ice at 0 C? Give reason for your answer. Take: Lsteam = 2260 J g 1, Lice 336 g 1, Cwater = 42 J g 1 C 1. 28) The amount of heat energy required to convert 1 kg of ice at 10 C to steam at 100 C is 30,37,000 J. Calculate the specific latent heat of vaporisation of steam. (Specific heat capacity of ice = 2100 J kg 1 K 1; specific heat capacity of water = 4200 J kg 1 K 1; specific latent heat of ice = 336,000 J kg 1) 29) 2 kg of ice melts when water at 100 C is poured in a hole drilled in a block of ice. What mass of water was used? Given: specific heat capacity of water = 4200 J kg 1 K 1, Lice = 336 103 Jkg 1. 30) Find the result of mixing 10 g of ice at 10 C with 10 g of water at 10 C. (specific heat capacity of ice = 2.1 Jg K 1, specific latent heat of ice = 3361 g and specific heat capacity of water = 4.2 Jg 1 K 1). 31) Why does the temperature of the surroundings start falling when the ice of a frozen lake start melting? temp 32) A piece of ice is heated at a constant rate. The variation of E temperature with heat input is shown in the graph below: 100 C i) What are represented by AB and CD? D A o ii) What conclusion can you draw regarding the nature B 10 heat of ice from the above graph? o 33) If there is no heat loss to the surroundings, the heat released by the condensation of m1 g of steam at 100 C into water at 100 C can be used 10 convert m2 g of ice at 0 C into water at 0 C. i) Find: (a) the heat lost by steam in terms of m1. (b) the heat gained by ice in terms of m2. ii) Form a heat equation find the ratio of m2 : m1. (Specific latent heat of vaporization of steam = 2268 kJ/kg; Specific latent heat of fusion of ice = 336 kJ/kg; Specific heat capacity of water = 4200 J/kg C) 34) i) In winter, the weather forecast for a certain day was severe frost . A wise farmer watered his fields the night before to prevent frost damage to his crops. Why did he water his fields? ii) 10125 J of heat energy boils off 4.5 g of water at 100 C to steam at 1 00 C. Find the specific latent heat of steam. (2004) 35) A thermos flask of negligible heat capacity contains 100 g of ice and 30g of water. i) Calculate: a) The mass of steam at 100 C needed to condense in the flask to just melt the ice; b) The amount of water in the flask after condensation. (Specific latent heat of vaporization of steam = 2260 J/g; Specific latent heat of fusion of ice = 336 J/g; Specific heat capacity of water = 4.2 J/g C.) ii) Is it possible to condense the water formed, back to ice by adding ice at C? Explain, giving a suitable reason to justify your answer. (2004) 36) i) State the principle of calorimerty. ii) Express 1 kWh in terms of S.I. unit of energy. iii) Which of the two, 1 g of ice at 0 C or 1 gm of water at 0 C contains more heat? Give a reason for your answer. (2005) 37) i) Define specific heat capacity of a substance. State its SI unit. ii) Give one example each where high specific heat capacity of water is used: (a) In cooling (b) As heat reservoir. 38) A vessel of negligible heat capacity contains 40 g of ice in it at 0 C. 8 g of steam at 100 C is passed into the ice to melt it. Find the final temperature of the contents of the vessel. Specific latent heat of vaporization of steam = 2268 J/g; Specific latent heat of fusion of ice = 336 J/g and Specific heat capacity of water = 4.2 J/g C) 82 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 11: Calorimetry 83 39) i) Explain, why one feels ice-cream at 0 C colder than water at 0 C? ii) Draw the diagram of the ring main circuit. 40) In a laboratory experiment to measure specific heat capacity of copper, 0.02 kg. of water at 70 C was poured into a copper calorimeter with a stirrer of mass 0.16 kg, initially at 15 C. After stirring, the final temperature reached to 15 C. Specific heat of water is taken as 4200 J/kg C. i) What is the quantity of heat released per kg of water per 1 C fall in temperature? ii) Calculate the heat energy released by water in the experiment in cooling from 70 to 45 C. iii) Assuming that the heat released by water is entirely used to raise the temperature of calorimeter from 15 C to 45 C, Calculate the specific heat capacity of copper. 41) i) It takes a much longer time to boil off (change to steam) a certain quantity of water, rather than to bring it to its boiling point from room temperature, say 25 C. Explain the reason for this. ii) A hot solid of mass 60 g at 100 C. Is placed in 100g of water at 18 C. The final steady temperature recorded is 20 C. Find the specific heat capacity of the solid. 42) i) Draw a labelled diagram of the apparatus you would use to determine the specific latent heat of vaporisation of steam by the method of mixtures. ii) State two precautions you would take, while performing the experiment with the apparatus. 43) Calculate the mass of steam at 100 C that must be passed into 8.4 kg. of water 30 C to raise the temperature of water to 80 C. [Sp. heat capacity of water = 4.2 J/g C, Sp. latent heat of vaporisation of steam = 2268 J/g] 44) Why do the surroundings become pleasantly warm when freezing starts in cold countries? PREVIOUS YEAR BOARD QUESTIONS: 1) Differentiate between the heat capacity and specific heat capacity. [2012] 2) A hot solid of mass 60 g at 100 C is placed in 150 g of water at 20 C. The final steady temperature recorded is 25 C. Calculate the specific heat capacity of the solid. [Specific heat [2012] capacity of water = 4200 J kg 1 C 1]. 3) i) Write an expression for the heat energy librated by a hot body. 4) 5) 6) 7) 8) 9) 10) ii) Some heat is provided to a body to raise its temperature by 25 C. What will be the corresponding rise in temperature of the body as shown on the Kelvin scale ? iii) What happens to the average kinetic energy of the molecules as ice melts at 0 C ? [2012] A piece of ice at 0 C is heated at a constant rate and its temperature is recorded at regular interval till steam is formed at 100 C. Draw a temperature-time graph to represent the change in phase. Label the different parts of your graph. [2012] 40 g of ice at 0 C is used to bring down the temperature of a certain mass of water at 60 C to 10 C. Find the mass of water used. (Specific heat capacity of water = 4200 J kg 1 C 1 specific latent heat of fusion of ice = 336 103 J kg 1) [2012] Differentiate between heat and temperature. [2010] Define Calorimetry. [2010] 200g of hot water at 80 C is added to 300 g of cold water at 10 C. Calculate the final temperature of the mixture of water. Consider the heat taken by the container to be negligible. [specific heat capacity of water is 4200 J kg 1 C 1] i) Explain why the weather becomes very cold after a hail storm. ii) What happens to the heat supplied to a substance when the heat supplied causes no change in the temperature of the substance? [2010] i) When 1 g of ice at 0 C melts to form 1 g of water at 0 C then, is the latent heat absorbed by the ice or given out by it? [2010] ii) Give one example where high specific heat capacity of water is used as a heat reservoir. iii) Give one example where high specific heat capacity of water is used for cooling purpose. Volume 2 of 2 Universal Tutorials X ICSE Physics 83 84 Ten ( C) 11) 250g of water at 30 C is present in a copper vessel of mass 50g. Calculate the mass of ice required to bring down the temperature of the vessel and its contents to 5 C. Specific latent heat of fusion of ice = 336 103 J kg 1 Specific heat capacity of copper vessel = 400 J kg 1 C 1 Specific heat capacity of water = 4200 J kg 1 C 1 [2010] 12) Why do piece of ice added to a drink cool it much faster than ice cold water added to it? (2009) 13) 40 gm of water at 60 C is poured into a vessel containing 50 gm of water at 20 C. The final temperature recorded is 30 C. Calculate the thermal capacity of the vessel. (Take specific heat capacity of water as 4.2 J/gm C) (2009) 14) In what way will the temperature of water at the bottom of a waterfall be different from the temperature at the top? Give a reason for your answer. (2008) 15) A certain quantity of ice at 0 C is heated till it changes into steam at 100 C. draw a time temperature heating curve to represent it. Label the two phase changes in your graph (2008) 16) a) i) Define heat capacity of a given body. What is its S. . unit? ii) What is the relation between heat capacity and specific heat capacity of a substance? b) A piece of ice of mass 40 g is dropped into 200 g of water at 40 C. Calculate the final temperature of water all the ice has melted. (Specific heat capacity of water = 4200 J/kg C, (2008) Specific latent heat of fusion of ice = 3366 103 J/kg) 17) a) i) What is meant by specific heat capacity of a substance? (2007) ii) Why does the heat supplied to a substance during its change of state not cause any rise in its temperature? b) A substance is in the form of a solid at 0 C. the amount of heat added to this substance 120 and the temperature of the substance are 100 80 plotted on the following graph: 60 If the specific heat capacity of the solid 40 substance is 500 J/kg C, find from the 20 graph: 0 i) The mass of the substance; 2000 3000 1000 Amount of heat ii) The specific latent heat of fusion of the substance in the liquid state. 18) a) Give two reasons as to why copper is preferred over other metals for making calorimeters b) i) Define specific latent heat of vaporization of a substance ii) What is the principle of calorimetry? (2006) c) Explain why water is used in hot water bottles for formation and also as a universal coolant 19) Calculate the heat energy that will be released when 5.0 kg of steam at 100 C condenses to form water at 100 C express your answer in S. . unit. (Specific latent heat of vaporization of steam is 2268 kJ/kg) (2005) 84 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 11: 1 Calorimet try 85 ANSWERS: Previous s Year Boar rd: 1) He eat capacity of a body is the amount t of heat ene ergy required d to raise the e temperature of body by y 1 K. Its unit is J K 1. It depends on mass m of the body. b Sp pecific heat capacity c of material m of a body is the e amount of heat energy y required to raise the tem mperature of f unit mass of o that body by 1 K. Its unit u is J kg 1 K 1. It is ind dependent of mass of the e body. 60 2) Given: Mass of o solid = 60 g = kg g; Initial temp perature of so olid = 100 C; Mass of wa ater = 150 1000 150 g= kg; Initial tempe erature of wa ater = 20 C; Final steady temperature of mixture e = 25 C; 1000 Sp pecific heat capacity c of so olid = c J kg 1 C 1; Spec cific heat cap pacity of wate er = 4200 J kg k 1 C 1 He eat given by solid = mass s of solid specific heat capacity c of solid s fall in t temperature = 60 c (100 ( 25) = 4.5 cJ 1000 He eat taken by water = mas ss of water specific hea at capacity of f water rise e in temperat ture 150 4200 0 (25 20) = 3150 J 1000 1 t is no lo oss of energy y, heat given n by solid = heat h taken by y water or 4.5 c = 3150 0 If there = c= 3150 = 700 J kg 1 C 1 4.5 3) i) Heat energ gy liberated = mass of body b specific heat cap pacity of mat terial of body y fall in temperature e = mc t e rise in temp perature is sa ame on both the Celsius and Kelvin s scales) ii) 25 K (since ge kinetic ene ergy of molec cules does not n change. iii) The averag emperature-time graph is s shown 4) Te 5) Given, mass of o ice = 40 g = 40/1000 kg, initial temperature of o ice = 0 C, initial tempe erature of wa ater = 60 C, final tempera ature of mixt ture = 10 C. Le et m kg of wa ater be used. Heat energy y given by water w = m 4200 (60 10) = 210000 m J 40 He eat energy ta aken by ice in n melting = 336 103 J 1 1000 40 He eat energy ta aken by melte ed ice to rais se temperatu ure = 4200 (10 0 0) J 1000 40 40 To otal heat ene ergy taken by y ice = 336 103 + 4200 4 (10 0) = 15120 J 1000 1000 If there t is no lo oss of energy y, heat energ gy given by water w = total heat energy taken by ice e or 210000 m = 15120 or m = 7.2 10 3 kg (or 72 2 g) Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 85 86 Chapter 12: Thermionic Emission and Radioactivity Chapter Map: Introduction Thermionic Emission Factors Requisites of a good emitter Hot Cathode Ray Tube Principle Construction Uses Television Tube Isotopes Isobars Radioactivity Alpha, Beta, Gamma Properties of Alpha, Beta, Gamma Nuclear changes with Alpha, Beta, Gamma emission Uses Radioisotopes Harmful effects and safety precautions Background Radiations Introduction: Bound and Conduction Electrons: z z z z 86 The electrons which are in orbits close to the nucleus are called the bound electrons because they are tightly bound to the nucleus by the strong attractive force between the positive charge of nucleus and the negative charge of electrons. On the other hand, electrons in outer orbits are weakly attracted by the nucleus and so they are loosely bound. When a solid is formed, the loosely bound electrons of the outermost orbit leave their individual atom, but remain within the boundary of the solid and become a part of the solid as a whole. They are called the conduction (or free) electrons. Metals contain a very large number of free electrons, whereas non-metals have a negligible or very few free electrons. Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 12: Thermionic Emission and Radioactivity 87 Emission of Electrons from Metals: z z z The free electrons, in a metal, move randomly throughout its volume, but they do not have sufficient kinetic energy to leave the metal surface. These electrons can be made to leave the metal surface if sufficient energy is supplied to them from outside so that they acquire the necessary kinetic energy to get emitted. The escape of electrons from the metal surface is called the electron emission. Work Function: z The minimum amount of energy required to emit electrons from a metal surface is called the work function (or threshold energy) of that metal. It is generally expressed in electron volt (eV) where 1 eV = 1.6 10 19 J. Work function for some metals: Metal Platinum (Pt) Tungsten (W) Chromium (Cr) Zinc (Zn) Sodium (Na) Potassium (K) Barium coated tungsten Work function in eV 6.2 4.52 4.37 4.24 2.3 2.26 1.6 From the above table it is evident that the alkali metals (such as Na, K) and tungsten coated with barium or cesium have low work function. Thermionic Emission: The emission of electrons from a metal surface when heat (or thermal) energy is imparted to it, is called the thermionic emission. The emission of electrons from a heated metallic surface (or filament) is very much similar to the evaporation of liquid molecules from the surface of a liquid. On heating a metal, the kinetic energy of free electrons inside the metal increases and they overcome the force of attraction of their nucleus and escape out from the metal surface. Factors Affecting the Rate of Thermionic Emission: The number of electrons emitted per second (i.e. the rate of emission of electrons from a surface) depends on the following three factors: The nature of the metal surface: Lower the work function of the metal, greater is the rate of emission of electrons from its surface. The temperature of the surface: Higher the temperature of the surface, greater is the rate of emission of electrons from the surface. The surface area of metal: Larger the surface area of metal emitting the electrons, greater is the rate of emission of electrons. Requisites for a Good Electron Emitter: To obtain a good supply of electrons, the substance used as an electron emitter must have the following two properties Volume 2 of 2 Universal Tutorials X ICSE Physics 87 88 The work function of the substance should be low so that the electrons may be emitted from it even when it is not heated to a high temperature. The melting point of the substance should, be quite high so that it may not melt when heated for thermonic emission. Tungsten is the most suitable substance to be used as an electron emitter because of its high melting point which is 3655 K. But it has a high work function of 4.52 eV. It needs to be heated up to a temperature of nearly 2500 K to emit electrons. To avoid this difficulty, thoriated tungsten (tungsten coated with carbon and thorium) is used as an electron emitter since it has a work function of only 2.6 eV and it needs to be heated to only 2000 K to emit electrons. If tungsten is coated with barium and strontium oxide, the work function further reduces to 1eV and it is sufficient to heat it only to 1000 K to obtain a good supply of electrons. Thus, tungsten coated with barium and strontium oxide is preferred to be used as an electron emitter. Hot Cathode Ray Tube: Principle: z It works on the following three principles: thermionic emission, deflection of the electron beam by the electric and magnetic fields, and fluorescence produced by the electron beam on a fluorescent screen. Deflecting System Electron Gun L.T. (6V) c C G A1 A2 Y Y X b a X d Grid + + High Tension (1000V) Construction: z Fluorescent screen It is a long hollow evacuated glass tube (pressure nearly 0.00 1 mm of Hg) containing the three main components: the electron gun, the deflecting system, and a fluorescent screen. The Electron Gun: z z z 88 It is the part of cathode ray tube which gives out a fine beam of electrons. Figure shows separately the electron gun. It consists of a cathode C which is indirectly heated by a filament F connected to a low tension battery (L.T.) of about 6 V. The cathode emits electrons on heating (i.e. the electrons are emitted by thermionic emission). These electrons are focused and accelerated by a series of cylindrical anodes A1 and A2 respectively. The anodes A1 and A2 have a small hole along the axis and they are maintained at a high positive potential with respect to the cathode, by means of a high tension battery (H.T.) of about 1000 V. Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 12: 1 Thermion nic Emission and Radioact tivity z 89 In betwee en the cathod de and anod de there is a grid G on which w a variable negative e potential is applied d with respec ct to the cath hode C. By varying v the negative n pote ential on the e grid, the number of o electrons reaching the e anode and d striking the e screen ca an be chang ged which changes the t brightnes ss of the pat ttern on the screen. This s entire arrangement is known k as an electro on gun. Note: : On increas sing the filament curren nt (i.e., using g a hotter filament), fi the e rate of em mission of electr rons from the e cathode wi ill increase. On O the other hand, by inc creasing the accelerating g potential on an node, the energy of elect trons reachin ng the anode e will increas se due to wh hich the elec ctrons will strike the screen with w a greate er speed. The e Deflectin ng System m: z z z z z z It is the pa art of cathod de ray tube which w deflects s the electron beam. It co onsists of tw wo pairs of plates; on ne kept horizontal called d the Y-plat tes and the other kept v vertical calle ed the Xplates. Th he plates are placed with their planes parallel to th he beam. When a potential p diffe erence is applied across s any pair of plates, the e electric field is set up between the t plates wh hich deflect the t electron beam. b The Y-pla ates deflect the beam in n the vertica al direction cd, c while the e X plates deflect the beam in th he horizontal direction ab b as shown in figure. When no potential dif fference is applied acros ss these plates, the beam m strikes the e centre 0 of screen and a steady bright spot t is seen due e to fluoresce ence on the s screen. Sometime es a cathode e ray tube is used where e the electron n beam is de eflected by applying a a magnetic field. The magn netic field is applied from m outside by keeping the e tube either in between the poles of a horse eshoe magne et or by keep ping the tube e inside a sole enoid carryin ng the direct current. Def flection of f electrons s by the electric and d magneti ic fields: z z z z z In an elec ctric field pro oduced betwe een the two plates (one kept at a positive pote ential and the e other kept at negative potential), , the electron beam gets s deflected towards t the positive plate p (i.e. in a direction opposite o to th he direction of the elec ctric field). If the elec ctric field is normal n to the direction of o motion of electrons as shown in n figure, the e electrons of o the beam follow a parabolic p pat th AB as the ey enter at the point A within the e electric field and then on emerging g out of the electric field d at the poin nt B, they follow a st traight path BC B in the direction in whi ich they are deflected d by the electric field. As a resu ult the electr ron beam st trikes the fluorescent scr reen at a po oint C, at a deflected position. The T amount of deflection n OC produc ced depends on the inten nsity of electric field E applied be etween the plates. p More the electric field f more is the deflectio on. In a magn netic field, th he electrons of the elect tron beam get deflected in a directio on given by y Fleming s left hand rule (remembe ering that th he direction of motion of o electrons is opposite to t the directio on of current t). If the direction of mag gnetic field is s normal to th he direction of o electron beam as shown s in fig gure, the electron beam follows a circular path h PQ as it enters e within the magnet tic field at the point P an nd then they follow a stra aight path QR Q on emerging out of the magnetic field f at the point p Q so as s to strike the e screen at a point R, at a deflec cted position n. Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 89 90 z z The deflection OR is more if the intensity of magnetic m fiel ld is more. H However, the e electron beam rem mains undeflected if the direction of f electron be eam is parallel to the direction of magnetic field. The deflection caused d by the mag gnetic field is much large e as compar red to that caused c by the same length of ele ectric field. The e Fluoresc cent Scree en: z z z z z z It is the part p of cathode ray tube on which th he pattern is formed by t the electron beam on striking it. The end of the tube is made flat lik ke a screen and a it is coat ted from insi ide with a fluorescen nt material (such as zinc z sulphide e or barium m platinocyanide or zinc silicate containing g traces of manganese m e etc.) The elect tron beam on striking the screen giv ves a bright spot due to o fluorescenc ce on the screen. If a varyin ng potential is i applied on n the deflecting plates, th he beam def flects according to the variation of o potential on o the plates s and therefo ore the spot traces t out a pattern on th he screen in accorda ance with the e variation of f potential. Thus the electrical sig gnal applied on o the deflec cting plates changes c into o the visual pattern p on the screen n. The tube is evacuated d so that the e electrons may m move without w being obstructed by the air molecules s. Further th he inner surf face of the tube t betwee en the electro on gun and the screen is coated with graph hite and it is earthed. Thi is shields the e electron be eam from any y electric fiel ld outside the tube and a also increases the life e of the tube e. Uses of a cath hode ray tube: t A cathode ray tube is used u mainly y to convert the electric cal signal int to a visual signal by a applying the electrical sig gnal on the deflecting plat tes. Some S of its uses u as follow ws: z To investigate the wa ave form of an a unknown alternating potential by applying it on o the Yplates and d a known pe eriodic time base b potentia al on the X-p plates. z In determ mining the fr requency of f an alternat ting potentia al by applyin ng it on one pair of deflecting plates and comparing it t with the fre equency of other alternat ting potential l on other ates. pair of pla z For check king the wave e form of an electrical sig gnal. z For measuring the sho ort time inter rvals. z In televisio on as a pictu ure tube. Use of o cathod de Ray Tu ube in Television: T The electron in Televisio on tube is no ot deflected b electric fie by eld but by ma agnetic field The T magnetic field is pro oduced by current c coils p placed outsid de the tube The T accelera ating voltage e is much higher h than 1 1000V i.e. 20 0 to 25 kV. The T fluoresce ent screen is s covered by y glass plate f protection for n. The T magnetic c field deflec cts the electro on beam along x and y axis a and scan n the screen. . 90 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 12: Thermionic Emission and Radioactivity 91 A scan of full screen involves a sweep of 525 horizontal lines (each sweep requires 1 sec) 25 The signal received by the antenna is amplified and then applied to the picture tube The intensity and hence brightness depends on the signal received Different spots of varied brightness combine to form the picture Illusion of moving picture is due to persistence of vision Note: A colour TV screen has 3 phosphor dots. Red, Blue, Green. When an electron beam strikes a spot it glows red, blue or green If it strikes more than one spot secondary colour cyan, magenta and yellow is seen Isotopes: The atoms of the same element whose nuclei have the same atomic number Z, but differ in their mass number A, are called the isotopes. Protium 1 1H (Z=1, A=1) 1 0 No. of protons No. of neutrons 2 H Deuterium 1 (Z=1, A=2) 1 1 3 H Tritium 1 (Z=1, A=3) 1 2 Isobars: The atoms of different elements whose nuclei have the same mass number A, but differ in their atomic number Z, are called the isobars. 23 23 23 For example, 11 Na and 12 Mg are the isobars. 11 Na contains 11 protons and 12 neutrons inside its nucleus, while Mg contains 12 protons and 11 neutrons inside its nucleus. The total number of protons and neutrons is 23 in each. 23 24 Isotones have same number of neutrons eg. 11 Na 12 Mg Radioactivity: Henry Becquerel discovered the phenomenon of radioactivity in 1896. The substances which disintegrate by the spontaneous emission of radiations are called the radioactive substances. Some of the radioactive substances are uranium, radium, polonium, thorium, actinium etc. All the elements of atomic number higher than 82 (i.e. lead) exhibits the phenomenon of radioactivity. These are called the natural radioactive substances. Any physical changes (such as change in pressure and temperature) or chemical changes (such as excessive heating, freezing, action of strong electric and magnetic fields, chemical treatment, oxidation etc.) do not alter the activity (or the rate of disintegration) of the radioactive substance. Thus the phenomenon of radioactivity cannot be due to the orbital electrons which could easily be affected by such changes. The radioactivity should therefore be the property of the nucleus. Definition: Radioactivity is a nuclear phenomenon. It is the process of spontaneous emission of or and radiations from the nuclei of atoms during their decay. From a radioactive substance containing a very large number of atoms, there is no way to know (or predict) when or which nucleus of the atom will decay at any moment, hence the radioactive decay is a random phenomenon. Volume 2 of 2 Universal Tutorials X ICSE Physics 91 92 Radi ioactivit ty as Em mission of Alph ha ( ), Beta B ( ) & Gamm ma ( ) Radia ations: In 1903, Ruthe erford studie ed experimentally the nature of radiations emitte ed by the rad dioactive sub bstances. He found that when w the rad diations give en out by a radioactive r subs stance are subjected to a magnetic field in n direction perp pendicular to o their path, , they separ rate into thr ree distinct cons stituents as shown s in figu ure. Tho ose which tur rn to the left are positively charged an nd are called d the alpha (or ) particle es. Those whic ch turn to the e right are ne egatively cha arged and are e called the beta a (or ) partic cles. The e particles are deviated d more than the partic cles. Those whic ch pass undeviated, are uncharged (neutral) and d are called the gamma (or ) ra adiations. -radiations are the elec ctromagnetic waves sim milar to light and are therefore not affected by the magnetic m field. Similarly, if the radiations r giv ven out by a radioactive substance are a subjected d to an electr ric field in direction perpen ndicular to their path, th hey separate e into three constituents s as shown in figure. Tho ose which tur rn towards th he negative plate p are the positively ch harged alpha ( ) particles s. Tho ose which tur rn towards th he positive plate p are the negatively charged beta ( ) particle es. Those whic ch pass undeviated are the uncharged gamma ( ) radiations s. The beta p particles are e deviated mor re than the alpha particles. Disti inction Between B n The Pr roperties s of , and R Radiation ns: P Property Nature e Speed Rest mass m Charge e Specific charge (q/m m) Wavele ength Effect of electric an nd magne etic fields Ionizing power Penetr rating pow wer (range e in cm) Stopping substance Biological damage 92 particle Stream of o positively charged particles i.e. helium nuclei Nearly 107 ms 1 4 times the t mass of pr roton i.e., 6.68 8 10 27 kg Positive charge = +3.2 10 19 C (or +2e) 4.79 10 07 C Kg 1 Less def flected Maximum m (10,000 time es of ) Small (3 8 cm in air) Thin pap per, human skin Cause so ome damage particle e radiation Strea am of negative ely charg ged particles, i.e. energ getic electrons s. Abou ut 90% of the speed s of light or 2.7 108 ms 1 Equa al to mass of electron e i.e., 9.1 9 10 31 kg Nega ative charge = 1.6 10 19 C (or e) 1.76 1011 C Kg 1 More e deflected in direction d oppo osite to alpha particles p Less than alpha pa article (100 times of ) Large e (up to few metre m in air) Abou ut 1 mm of lead or about 5 mm of Al Caus se more dama age Hig ghly energetic c ele ectromagnetic radiation. Universal Tu utorials X IC CSE Physics s 8 1 3 10 ms (in vacuum) No o mass o charge No 13 10 m or 10 3 naffected Un Min nimum Ve ery large (up to o a few hundred metre in n air) Ab bout 30 cm of iron i or few me etre of concret te Ca ause immense e damage Volu ume 2 of 2 Chapter 12: Thermionic Emission and Radioactivity 93 Changes within the Nucleus in Alpha,Beta & Gamma Emission If the nucleus of a radioactive element X of mass number A and atomic number Z emits an a- particle, a new element Y (daughter nucleus) is formed which has mass number equal to. (A 4) and atomic number equal to (Z 2). This change can be expressed in the form of a reaction as follows: A ZX A 4 4 Z 2 Y + 2 He ( particle) In emitting a particle, the number of nucleons in the nucleus (i.e. mass number A) remains the same, but the number of neutrons is decreased by one and the number of proton (i.e. atomic number Z) is increased by one. A ZP 0 A Z + 1 Q + 1 e There is no change in the mass number A and atomic number Z of the nucleus in gamma emissions. A ZX A Z X+ In a single radioactive decay, and particles are never emitted simultaneously, There will be either an emission or a emission, which may be accompanied by the emission. The daughter product may again be radioactive and it may decay by emitting either the particle or the particle. This process continues till a stable nucleus is formed. Uses of Radioactivity Radio Isotopes: Medical use: z z z z Many diseases such as leukemia, cancer, etc., are cured by radiation therapy. Radiations from cobalt -60 are used to treat cancer by killing the cells in malignant tumour of the patient. The salts of weak radioactive isotopes such as radio-sodium chloride, radio iron, radioiodine are used for diagnosis. Such radio isotopes are called the tracers. These are used to detect the suspected brain tumours and blood clots before they become dangerous. The tracers are also used to study the natural process in the body. For example, radiosodium chloride is injected with common sodium chloride in the human body and the radioactivity of the blood at different parts of the body is tested to study the blood circulation. The process is called radio cardiology. rays emitted by the radio isotopes are used to sterilize bandages, dressings, syringes and other equipments to make them free of germs. This method is quicker, more reliable and cheaper than sterilization by heat. Scientific Use: z z z Alpha particles emitted from the radio isotopes are used as projectiles for nuclear reactions. The scattering of alpha particles from the nucleus helps us in understanding the nature of nuclear forces. The radioactive tracers are used in agriculture science to study the growth of plants with respect to the type of chemical manure used (e.g. how readily a plant takes in the phosphate and to which part of the plant the phosphate goes). The age of rocks and hence the age of earth is determined by the study of disintegrated bits and the remaining radioactive substance in a sample. The process is called carbon dating. Volume 2 of 2 Universal Tutorials X ICSE Physics 93 94 Industrial Use: z z z z Radio isotopes are used as fuel for atomic energy reactors. Radio isotopes are used by engineers to measure the life of an engine and also to trace obstructions in oil, gas or water pipes. The ionising effect of radiations from radio isotopes is used in making certain luminescent signs and atomic batteries. The radiations emitted from the radio isotopes are used for controlling the thickness of paper, plastic and metal sheets during their manufacture. Harmful Effects and Safety Precautions: Alpha has the lowest penetrating power, while gamma has the highest penetrating power, so gamma radiations are more harmful. When they fall on the human body, they kill the living tissues and cause radiation burns (or damage). The biological effects of nuclear radiations are of three types: short term recoverable effects, long term irrecoverable effects Genetic effects. The first two effects are limited to the individuals who are actually exposed to the radiations, while the third effect appears in the later generations. The exposure to radiations can be acute if there is an accidental burst of radiation from an unshielded source. Similarly, it is chronic in case of radiographers or persons working in atomic energy establishments when they get exposed to radiations. Therefore, the people working with radioactive materials are required to follow strictly the safety rules which are given below. Safety Rules: z z z z z They should put on special lead lined aprons and lead gloves. They should handle the radioactive materials with long lead tongs. The safety limit for each type of radiation is known and care is taken that no one is exposed beyond the safety limit in any case. For this, special film badges are used which are tested from time to time to know the amount of radiations to which a particular person has been exposed. The radioactive substances are kept in thick lead containers with a very narrow opening so as to stop radiations, coming out from other directions. A container should he such that it absorbs the radiations which strike on its walls. Alpha particles can easily be stopped by a thin metal sheet, but for particles we need a thicker metal sheet, whereas gamma radiations need very thick lead sheets. Background Radiations: Background radiations are the radioactive radiations (such as , and ) to which we all are exposed even in the absence of an actual visible radioactive source. Its total dose is not very large, so it does not cause any serious biological damage to us. There are two sources of background radiations: Internal source: the radioactive substances such as potassium (K-40), carbon (C-14) and radium are present inside our body, and External source: the radiations coming from the environment, such as cosmic rays from high altitudes and local terrestrial radiations from the radioactive rocks in the earth s crust. 94 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 12: Thermionic Emission and Radioactivity 95 It is not possible for us to keep ourselves away from the back ground radiations. But the total radiations from both the internal and external sources forming the background radiations, do not exceed the maximum permissible level for human safety. REVIEW QUESTIONS: Direct Questions: 1) Define the term work function of a metal. 2) Name the unit in which work function of a metal is generally expressed. How is it related to the S.I. unit joule? 3) State two factors on which the rate of emission of electrons from a heated surface depends. 4) State two properties which a substance used as thermionic emitter should possess. 5) Draw a simplified labelled diagram of a hot cathode ray tube and briefly explain its working. 6) Why is graphite coating made on the inner side of the cathode ray tube? 7) What are the three main components of a cathode ray tube? Explain the function of each part. 8) State two uses of a cathode ray tube. 9) How is a cathode ray tube used to convert an electrical signal into a visual signal? 10) Name one device in which the cathode ray is used and explain its role in the device mentioned by you. 11) What are isotopes? Give one example. 12) What is radioactivity? Name two radioactive substances. 13) Compare the ionising powers of , , and radiations. 14) What is the nature of , and radiations? State four properties of each. 15) What is the composition of , and radiations? Which one has the least penetrating power? 16) What happens to the mass number of an element when (i) an -particle, (ii) a -particle, and (iii) -radiation is emitted? 17) State, giving reasons, whether the following nuclear disintegrations are allowed or not (star indicates an excited state). Given reason if it is not allowed. i) A ZX A Z X+ ii) A ZX A 4 Z 2 X+ 2 He 18) An element P disintegrates by -emission and the new element suffers two further disintegrations, both by -emission, to form an element Q. Explain the fact that P and Q are the isotopes. 19) A nucleus of an element X which has the symbol particle. The final nucleus is a bY Find 202 84 X emits an alpha particle and then a beta a and b. 20) Complete the following nuclear changes: a) 27 12 Mg Al c) b aX X + e) a xP Q + 0 1 b) 24 11Na Mg + 0 1 4 d) a x P Q + 2 He f) 238 92 P Q R S 21) State the medical use of radioactivity. 22) What do you mean by background radiations? Name its sources. Is it possible for us to keep ourselves away from it? 23) Name the radiation which produces maximum biological damage. Give reason. What precautions are necessary while handling the source of this radiation? 24) State three safety precautions that you would take while handling the radioactive substances. Volume 2 of 2 Universal Tutorials X ICSE Physics 95 96 APPLICATION TYPE: T 1) Why are the materials m of lo ow work func ction preferre ed as electron n emitters? 2) Na ame a subst tance used as cathode for thermion nic emission. State its w work function n and the tem mperature to o which it is to o be heated for electron emission. 3) In a cathode ra ay tube, give e reason for the t following: i) the t filament is made of tu ungsten. ii) the cathode plate is coat ted with an oxide o of bariu um or strontiu um. iii) the anode is s kept at pos sitive potentia al with respe ect to the cath hode. iv) ) the screen is coated wit th barium pla atinocyanide. . v) a grid is kep pt at negative e potential be etween the cathode c and anode. a 4) In a hot catho ode ray tube, what is the e effect on th he beam of particles if ( (i) a hotter fi ilament is us sed, (ii) the anode voltage e is increased? 5) De escribe briefl ly the effect of (a) a mag gnetic field, (b) an electr ric field, on a stream of electrons. e Dr raw diagrams s in support of o your answ wer. 6) Th he diagram below show ws a simplif fied version of a ca athode ray tube in which the t arrow ind dicates a bea am of ch harged partic cles approach hing an elec ctric field betw ween the e plates A an nd B. a) Name the charged partic cles. b) State the am mount of cha arge on each particle. c) State the ap pproximate vo oltage V1 use ed to heat th he filament. d) State the ap pproximate potential p diffe erence V2 app plied betwee en the anode e and filamen nt. e) What will happen h to th he beam wh hen it passe es through the t electric f field? Draw separate dia agram to exp plain your an nswer. 7) Sh hows a simplified version n of a device known as an a electron gun inside a c cathode ray tube. X is a cathode c whic ch is heated by means of f an electric current in the e filament F. Y is a metal llic hollow cy ylinder. F is connected c to the terminals T1 and T2, X to the ter rminal T3 and d Y to the ter rminal T4. a) State : i) the t purpose of F, ii) the purpose of X, and iii) the purpose e of Y. b) Label the sc creen with the letter S. c) If you have two batteries s available one o giving 6 V and the ot ther 1000 V, show on the e diagram ho ow to connec ct these batt teries betwee en the terminals T1, T2, T3 and T4 in n order to pr roduce an ele ectron beam from the gun. d) Draw a pair of plates P1 P and P t: the t electric field f in the diagram d and d the deflecti ion of the pa articles emitte ed by X in be etween the plates p P1 and P2 8) A radioactive substance s is oxidised. What W changes s would you expect to tak ke place in the nature of radioactivity y? Explain yo our answer. 9) A radioactive source s emits three types of radiations s. Name them m. i) Name N the radiations whic ch are charged. ii) Name the ra adiation whic ch is most pe enetrating. adiation whic ch travels wit th the speed of light. iii) Name the ra ) Name the ra adiation whic ch has the la argest mass. iv) v) Name the ra adiation cons sisting of the same kind of o particles as the beam o of electrons. 96 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 12: Thermionic Emission and Radioactivity 97 10) A radioactive source emits three types of radiations. i) Which radiation has zero mass. ii) Name the radiation which has the lowest Ionizing power. iii) Name the radiation which has the lowest penetrating power. iv) Give the charge and mass of particles composing the radiations in case (iii). v) When the particle referred to in case (iii) becomes neutral, they are found to be the atoms of a rare gas. Name this rare gas and draw a model of its neutral atom. vi) From which part of the atom do these radiations come? 11) The adjacent diagram shows a radioactive source S placed in a thick walled lead container. The radiations given off are allowed to pass through a magnetic field. The magnetic field (shown as x) acts perpendicular to the plane of paper inwards. Arrows show the paths of the radiations A, B and C. i) Name the radiations labelled A, B and C. ii) Explain clearly how you used the diagram to deduce what radiation A, B or C is 12) The fig. shows a mixed source R of alpha and beta particles in a thick lead container. The particles pass through a magnetic field in a direction perpendicular to the plane of paper Inwards as shown by x. State and show in the diagram how the particles get affected. 13) Fig. shows a radioactive source S in a thick lead container. The radiations pass through an electric field between the plates A and B. Complete the diagram to show the paths of , and radiations. Why is the source S kept in a thick lead container? 14) In fig shows a radioactive source S in a thick lead container. The radiations pass through an electric field between the plates A and B. Complete the diagram to show the paths of , and radiations. Why is the source S kept in a thick lead container? 15) Explain why alpha and beta particles are deflected in an electric or a magnetic field, but gamma rays are not deflected in such a field. 16) An -particle captures an electron. What does it change to? 17) An -particle captures two electron. What does it change to? 18) Define the terms (i) atomic number and (ii) mass number. Are these quantities conserved in a radioactive -decay? 19) A certain radioactive nucleus emits a Particle that leaves its mass unchanged, but increases its atomic number by one. Identify the particle and write its symbol. 20) What changes occur in the nucleus of a radioactive element when it emits (a) an alpha particle, (b) a beta particle, (c) gamma radiation? Give one example, in each case (a) and (b) in support of your answer. 21) What happens to the atomic number of an element when (i) an -particle, (ii) a -particle, and (iii) -radiation are emitted? 22) i) An atomic nucleus A is composed of 84 protons and 128 neutrons. The nucleus A emits an -particle and is transformed into a nucleus B. What is the composition of B? ii) The nucleus B emits a -particle and is transformed into a nucleus C. What is the composition of C? iii) What is the mass number of the nucleus A? iv) Does the composition of nucleus C change if it emits a y-radiation? Volume 2 of 2 Universal Tutorials X ICSE Physics 97 98 23) A certain nucleus A (mass number 238 and atomic number 92) is radioactive and becomes a nucleus B (mass number 234 and atomic number 90) by the loss of a particle. i) What particle was emitted? ii) Explain how you arrived at your answer. iii) State the change in the form of a reaction. 24) A nucleus of radioactive phosphorus has atomic number 15 and mass number 32. i) If a stable isotope of the above mentioned nucleus has one neutron less, what are the atomic number and the mass number of the isotope ? ii) If the radioactive isotope emits a -particle, what are the atomic number and the mass number of the new nucleus? b 25) Two radioactive nuclei are represented by a x P and y Q where a, b are the mass numbers and x, y are the atomic numbers. How can the products R and S be represented i.e., what are the new values of a, b, x and y after the emission of an alpha particle and a beta-particle from P and Q respectively? 26) The nucleus 24 11Na emits a particle to change into magnesium Mg. Write down the symbolic equation for this process. What are the numbers 24 and 11 called and what do they signify for the nucleus Na? 27) i) A thorium isotope 233 90Th undergoes an decay and changes into radium. What is the atomic number and mass number of the radium produced? ii) If radium undergoes a further disintegration and emits two particles, represent this reaction in the form of an equation. 28) A nucleus 234 90Th decays to 206 82 Pb . Find the number of alpha and beta particles emitted? MISCELLANEOUS EXERCISE: 1) What are the free (or conduction) electrons in a metal? Why do they not leave the metal surface? How can they may be made to leave the metal surface? 2) The minimum amount of energy required to emit electrons from a metal surface is called the: a) ionisation energy b) dissociation energy c) cohesive energy d) work function. 3) State four ways of providing energy to a metal to emit electrons from its surface. 4) Explain the term thermionic emission. Mention its one use. 5) What do you mean by thermionic emission? What are the requisites of a metal to be used as an electron emitter? 6) Why is the cathode ray tube evacuated to a low pressure? 7) How can the brightness of the pattern on the screen of cathode ray tube be changed? 8) A TV tube requires an accelerating voltage of the order of: a) 150 volt b) 1000 volt c) 20 kV d) 500 kV 9) Name three constituents of an atom and state their masses and charges. 10) What do you mean by (i) the atomic number, and (ii) the mass number of a nucleus? 11) What are isobars? Give one example. Name the atoms of a substance having same atomic number, but different mass numbers. Give one example of such a substance. How do the structures of such atoms differ? 12) Is it possible to deflect -radiations in a way similar to and -particles, using the electric or magnetic field? Give reason. 13) A radioactive substance emits radiations: 98 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 12: Thermionic Emission and Radioactivity 99 a) , and simultaneously b) in the order , and one by one c) at one time a and 13 and then d) and , or and 14) In -emission from a radioactive substance, an electron is ejected. This electron comes from: a) the outermost orbit of atom b) the inner orbits of atom c) the surface of substance d) the nucleus of atom 15) Compare the penetrating powers of , and radiations? 16) How are -radiations produced? Mention two common properties of the gamma radiations and visible light. 17) Which of the following radiations is least penetrating (a) -particles (b) -particles c) X-rays d) -radiations 18) Which of the following radiations suffers maximum deflection in a magnetic field: a) -particles (b) -particles c) X-rays 19) Radioactivity is a nuclear phenomenon . Comment on this statement. d) -radiations 20) What kind of change takes place in an atom when a -particle is emitted? 21) How does the position of an element change in the periodic table, when it gives out an alpha particle? 22) What happens to the position of an element in the periodic table when it emits a (i) -particle and (ii) -radiation? Give reasons for your answer. 23) A nucleus 233 88 Ra emits an alpha particle. Represent the change by an equation. Identify the new nucleus formed the following: 84 Po, 85 At, 86 Rn, and 87 Fr, 24) The mass number (A) of an element is not changed when it emits ________________ 25) The atomic number of a radioactive element is not changed when it emits __________ 26) A radioactive nucleus resulting nucleus P QY A Z X first emits a beta particle and then an alpha particle to give the What will be the values of P and Q in terms of A and Z? 27) Why are the alpha particles not used in radio therapy? 28) Why do we usually use isotopes emitting gamma radiations as radioactive tracers in medical science? 29) When does the nucleus of an atom become radioactive? 30) What are radio isotopes? Give one example of a radio isotope. State one use of radio isotopes. 31) Which of the following is the radio isotope in each pair? a) 12 6C , 14 6C b) 30 15 P , 32 15 P c) 39 19 K , 40 19 K Give reason for your answer. 32) Which of the following is most harmful for the human being: a) particles b) particles c) rays 33) Why should a radioactive substance not be touched by hand? 34) The material used for safety from nuclear radiations is: a) copper b) platinum c) iron d) light rays d) lead PREVIOUS YEAR BOARD QUESTIONS: 1) i) What is the value of the speed of gamma radiations in air or vacuum? ii) Name a material which exhibits fluorescence when cathode rays fall on it. Volume 2 of 2 Universal Tutorials X ICSE Physics [2012] 99 100 2) Give any Iwo important sou urces of back kground radi iations. [2012] 3) i) Draw a sim mplified labelle ed diagram of o a hot catho ode ray tube e. ii) Name a com [2012] mmon device e where a ho ot cathode ra ay tube is use ed. 4) A certain nucle eus X ha a mass m number r 14 and atomic number 6. The nucle eus X changes to 14Y7 aft ter the loss of o a particle. i) Name the particle p emitted. ii) Represent the t change in the form of an equation n. iii) A radioactive substanc ce is oxidize ed. What cha ange would you expect to take place in the [2012] nature of its s radioactivity y ? Give a re eason for you ur answer. 5) Fil ll in the blank ks in the follo owing senten nces with app propriate words: i) During the emission of a beta partic cle, the _____ ___ number remains the same. ii) The minimu um amount of o energy re equired to em mit an electro on from a m metal surface e is called [2010] _______. 6) A mixture of ra [2010] adioactive substances giv ves off three types of radiations. i) Name the radiation r whic ch travel with h the speed of o light. ii) Name the radiation r whic ch has the highest ionizin ng power. 7) i) State two properties p wh hich a substa ance should possess p whe en used as a thermionic emitter. e ii) When an alpha a particle e gains two electrons e it becomes b neu utral and bec comes an at tom of an [2010] element wh hich is a rare gas. What is s the name of o this are ga as? 8) i) Define radio oactivity. ii) What happe ens inside th he nucleus th hat causes th he emission of o beta partic cle? iii) Express the [2010] e above chan nge in the form of an equ uation. 9) i) Name a dev vice which is s commonly used to conv vert an electr rical signal in nto a visual signal. s ii) The nucleus 202X84 emit ts an alpha particle p and forms f the nucleus Y. Rep present this change c in the form of an equation. iii) What chang ges will take e place in the e mass num mber and ato omic number r of the nucle eus Y if it [2010] emits gamm ma radiations s? 10) Give two important precaut tions that should be taken while hand dling radioac ctive materials? (2009) 11) i) What is the e name give en to atoms of o a substan nce which ha ave the sam me atomic nu umber but different ma ass numbers s? ii) What is the (2009) e difference in n the atomic structures of o such atoms s? 12) a) i) When doe (2008) es the nuclea ar of an atom m become ra adioactive? ii) How is the t radioacti ivity of an el lement affec cted when it undergoes a chemical change c to form a che emical compo ound? iii) Name the t product of nuclear fi ission which is utilized to t bring about further fossil on of 235 92 U . b) ) i) Mention one use and d one harmfu ul effect of rad dioactivity. e source of background b r radiation. ii) Give one c) ) i) The abov ve diagram shows s an ele ectron gun of f a hot cathod de ray tube, a) Name e the parts X and Y. b) A 6 V d.c. sou urce and a 1000 V d.c. source are a available e. Show how w these sour rce should be e connected to the term minals of X an nd Y so as to o obtain a foc cused beam of fast mov ving electrons. ii) Give one e use of a ca athode ray tu ube. 13) i) A particular r type of high energy inv visible electro omagnetic ra ays help us to study the structure (2007) of crystals. Name these e rays and giv ve another im mportant use e of these ray ys. ii) ) How does the t speed of light in glass s change on increasing th he waveleng gth of light? 100 Universal Tu utorials X IC CSE Physics s Volu ume 2 of 2 Chapter 12: 1 Thermion nic Emission and Radioact tivity 101 14) a) Name the three t main parts of a hot cathode ray y tube. Mention one impo ortant functio on of each (2007) of the three e main parts. b) ) i) State the e principle on n which the fu unctioning of f a nuclear re eactor is bas sed. ii) Name a material that t can be used d as fuel in a nuclear rea actor iii) How is th he activity in a nuclear re eaction contro olled? c) ) i) What happens to the e atomic num mber of an ele ement when it emits: a) An alp pha particle b) A beta partic cle ii) Explain why w alpha and beta pa articles are deflected d in an a electric or a magnetic c field but gamma rays r are not deflected in such a field. 15) a) i) Define th (2006) hermionic em mission ii) Mention one use of th hermionic em mission iii) Name a substance which w is a goo od thermionic emitter b) ) State three properties th hat are comm mon to and shown s by bot th beta rays and cathode e rays. 16) a) i) Mention two impor rtant precautions that should s be ta aken while handling ra adioactive materials, ii) State on ne use of radio isotopes b) ) i) Draw a labeled diagram of a hot cathode ray y tube. ii) Why are e materials of low work fu unction proffe ered as therm mionic cathod de materials? 235 iii) Write an n equation to o show the fission of a nucleus of U with the e production n of three neutrons s. 17) a) i) Copy an nd complete the alongside diagram by showing and labeling g the paths of alpha, beta and d gamma rad diations in an n electric field d. ii) Name th he radiations s which have the least penetrating pow wer b) ) i) Give one e difference between b a ch hemical chan nge and a nu uclear change ii) How is a cathode ray y tube used to t convert an n electrical signal into a v visual signal? ? Volume 2 of 2 Universal Tu utorials X IC CSE Physics s 101 102 2014 Board Paper Section I (40 marks) Attempt all questions. Question 1) a) A force is applied on (i) a non-rigid body and (ii) a rigid body. How does the effect of the force differ in the above two cases? [2] b) A metallic ball is hanging by a string from a fixed support. Draw a neat labelled diagram showing the forces acting on the ball and the string. [2] c) (i) What is the weight of a body placed at the centre of the earth? (ii) What is the principle of an ideal machine? [2] d) Is it possible to have an accelerated motion with a constant speed? Explain. [2] e) (i) When does a force do work? (ii) What is the work done by the moon when it revolves around the earth? [2] Question 2) a) Calculate the change in the Kinetic energy of a moving body if its velocity is reduced to 1 /3rd of the initial velocity. [2] b) State the energy changes in the following devices while in use: i) A loud speaker. ii) A glowing electric bulb. [2] c) i) What is nuclear energy? ii) Name the process used for producing electricity using nuclear energy. [2] d) State one important advantage and disadvantage each of using nuclear energy for producing electricity. [2] e) i) The conversion of part of the energy into an undesirable form is called _______. ii) For a given height h, _________ the length 1 of the inclined plane, lesser will be the effort required. [2] Question 3) a) Draw the diagram given below and clearly show the path taken by the emergent ray. 45 [2] b) i) What is consumed using different electrical appliances, for which electricity bills are paid? ii) Name a common device that uses electromagnets. [2] c) i) A ray of light passes from water to air. How does the speed of light change? ii) Which colour of light travels fastest in any medium except air? [2] d) Name the factors affecting the critical angle for the pair of media. [2] e) i) Name a prism required for obtaining a spectrum of Ultraviolet light. ii) Name the radiations which can be detected by a thermopile. [2] Question 4) a) Why is the colour red used as a sign of danger? [2] b) i) What are mechanical waves? ii) Name one property of waves that do not change when the wave passes from one medium to another. 102 Universal Tutorials X ICSE Physics Volume 2 of 2 Chapter 12: Thermionic Emission and Radioactivity 103 3 c) Find the equivalent resistance between points A and B A 3 3 4 5 B 6 d) 50 g of metal piece at 27 C requires 2400 J of heat energy so as to attain a temperature of 327 C. Calculate the specific heat capacity of the metal. [2] e) An electron emitter must have ___________ work function and ________melting point. [2] Question 5) a) i) A man having a box on his head, climbs up a slope and another man having an identical box walks the same distance on a levelled road. Who does more work against the force of gravity and why? [4] 5N ii) Two forces each of 5N act vertically upwards and 50 cm downwards respectively on the two ends of a uniform metre rule which is placed at its mid-point as shown in the diagram. Determine the magnitude of the resultant moment 5N of these forces about the midpoint. b) i) A body is thrown vertically upwards. Its velocity keeps on decreasing. What happens to its kinetic energy as its velocity becomes zero? ii) Draw a diagram to show how a single pulley can be used so as to have its ideal M.A 2. [3] c) Derive a relationship between mechanical advantage, velocity ratio and efficiency of a machine. [3] Question 6 a) i) Light passes through a rectangular glass slab and through a triangular glass prism. In what way does the direction of the two emergent beams differ and why? ii) Ranbir claims to have obtained an image twice the size of the object with a concave lens. Is he correct? Give a reason for your answer. [4] b) A lens forms an erect, magnified and virtual image of an object. i) Name the lens. ii) Draw a labelled ray diagram to show the image formation. [3] c) i) Define the power of a lens. ii) The lens mentioned in 6(b) above is of focal length 25cm. Calculate the power of the lens. [3] Question 7) a) The adjacent diagram shows three different modes of vibrations P, Q and R of the same string. P i) Which vibration will produce a louder sound and why? ii) The sound of which string will have maximum shrillness? Q iii) State the ratio of wavelengths of P and R. [4] R b) A type of electromagnetic wave has wavelength 50 . i) Name the wave. ii) What is the speed of the wave in vacuum? iii) State one use of this type of wave. [3] c) i) State one important property of waves used for echo depth sounding. ii) A radar sends a signal to an aircraft at a distance of 30 km away and receives it back after [3] 2 10 4 second. What is the speed of the signal? Volume 2 of 2 Universal Tutorials X ICSE Physics 103 104 Question 8) a) Two resistors of 4 and 6 are connected in parallel to a cell to draw 0.5A current from the cell. i) Draw a labelled circuit diagram showing the above arrangement. ii) Calculate the current in each resistor. [4] b) i) What is an Ohmic resistor? ii) Two copper wires are of the same length, but one is thicker than the other. (1) Which wire will have more resistance? (2) Which wire will have more specific resistance [3] c) i) Two sets A and B, of three bulbs each, are glowing in two separate rooms. When one of the bulbs in set A is fused, the other two bulbs also cease to glow. But in set B, when one bulb fuses, the other two bulbs continue to glow. Explain why this phenomenon occurs. ii) Why do we prefer arrangements of Set B for house circuiting? [3] Question 9) a) Heat energy is supplied at a constant rate to 100g of ice at 0 C. The ice is converted into water at 0 C in 2 minutes. How much time will be required to raise the temperature of water from 0 C to 20 C? [Given: sp. heat capacity of water = 4.2 J g-1 C-1 sp. latent heat of ice 336 [4] J g-1]. -1 b) Specific heat capacity of substance A is 3.8 J g K whereas the Specific heat capacity of [3] Substance B is 0.4 J g-1 K.-1. i) Which of the two is a good conductor .of heat? ii) How is one led to the above conclusion? iii) If substances A and B are liquids then which one would be more useful in car radiators? c) i) State any two measures to minimize the impact of global warming. ii) What is the Greenhouse effect? Question 10) a) i) Name two factors on which the magnitude of an induced e.m.f. in the secondary coil depends. ii) In the following diagram an arrow shows the motion of the coil towards the bar magnet. (1) State in which direction the current flows, A to B or B to A? (2) Name the law used to come to the conclusion. [4] Magnet N A S B 24 b) A nucleus 11Na emits a beta particle to change into Magnesium (Mg) i) Write the symbolic equation for the process. ii) What are numbers 24 and 11 called? iii) What is the general name of 24 12 Mg with respect to 24 11Na ? c) In a cathode ray tube state: i) The purpose of covering cathode by thorium and carbon. ii) The purpose of the fluorescent screen. iii) How is it possible to increase the rate of emission of electrons? 104 Universal Tutorials X ICSE Physics [3] [3] Volume 2 of 2

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