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Class 12 ISC Notes 2018 : Physics Definitions etc (Little Flower School (LFS), Gorakhpur)

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Ishan Pathak
Little Flower School (LFS), Gorakhpur
6th-12th Physics, Chemistry, Biology, Biotech

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Modern Physics Photoelectric effect: Emission of electrons from a surface when a light is incident on it. Discovered by Frank Hertz Work Function: Minimum energy, associated with the surface, which when supplied to it through photons, results in just emission of photoelectrons. Threshold Frequency: Minimum frequency of light which when incident on the surface results in emission of photoelectrons Threshold Wavelength: Maximum wavelength of light which when incident on the surface results in emission of photoelectrons Stopping Potential: Minimum negative potential of the plate for which Photoelectric current I becomes zero Compton Effect: When a monochromatic beam of X rays is scattered by a metallic target. The scattered X Rays contain radiation not only of the same Wavelength as incident on the substance but also radiations of longer wavelengths. Laws of PE Effect: For a given PE surface, PE current is directly proportional to the intensity of incident light and is independent of its frequency KE of photoelectrons emitted is directly proportional to the frequency and independent of intensity of incident light Instantaneous effect ie the moment a photon falls on a surface, immediately an electron is emitted without a delay One to One Effect. A photon always causes emission of an electron De-Broglie Hypothesis: Postulated that material particles like electrons, photons may exhibit wave aspect. Accordingly a moving material particles behaves as waves and the wavelength associated with that particles is Matter waves: The waves associated with material particles are called matter waves Line Spectrum: The spectrum containing distinct lines over dark background is called the line spectrum Conclusions of Rutherford: Atom is hollow Atom consists of positively charged nucleus of very small size at its centre Nearly whole mass of atom is concentrated in nucleus Amount of positive charge in nuclei of different metals is different Drawback of Rutherford: Couldn t explain line spectrum of atoms Couldn t explain stability of atoms Postulates of Bohr Electrons revolve around nucleus in certain privileged circular orbits. As long as an electron remain in one of these orbits, its energy remains constant. Necessary centripetal force required by electron for its circular notion is provided by Electrostatic force of attraction between electron and nucleus Angular momentum of electron is always integral multiple of h/2pie Absorption Spectrum of Lyman: As atom can absorb energy from ground state only, so Balmer series cannot be found in Absorption spectrum of Hydrogen X-rays: When fast moving electrons strike a target of high melting point and high atomic weight, X rays are produced. Controls: The intensity of incident electron determines the intensity of X rays i.e. greater is the number of electrons striking the target, more intense are the x rays produced. The intensity of x rays may be increased by increasing the filament current The potential difference across the filament and target determines the energy and hence the penetrating power of x-rays. The penetrating power of X rays may be controlled by varying the potential difference across the tube. Hard and Soft X Rays: X-rays upto 4A wavelength have high penetrating power and are called hard X rays whereas those of wavelength greater than 4A have low penetrating power and are known as Soft X Rays Continuous Spectrum: It consists of radiations of all possible wavelengths within a definite wavelength range having a definite short wavelength limit. These are produced due to deceleration of electrons near heavy nucleus. Independent of material of target Characteristic Spectrum: It consists of radiations of definite wavelengths superimposed on continuous spectrum. Characteristic of the material of target. Moseleys Law: The square root of frequency of a particular spectral line in the X ray spectrum of an element is closely proportional to the atomic number of the element emitting it. Isotopes: Atoms of same element , having same atomic number but different mass number Isobars: Atoms of different elements, having same mass number but different atomic number Isotones: Nuclei having same number of neutrons Mass Defect: It was observed that the mass of a nucleus is always less that the mass of constituent nucleons. This difference of mass is called the mass defect. Binding Energy: Refers to the energy required to separate the constituent particles of a nucleus by a large distance. Signifies stability a directly proportional manner. Radioactivity: Some heavy nuclei are unstable and spontaneously decay into other elements by emission of certain radiations. Rutherford Soddy Law: The rate of decay of atoms at any instant is directly proportional to the number of atoms or amount of substance present at that instant. Half Life: Time in which one half of the radioactive substance is disintegrated. Activity: Rate of decay of any substance per second. Pair Annhilation: When an electron and a positron come near each other, they are completely annihilated and two gamma photos moving in opposite direction are produced. Conversion of energy into mass. Pair Production: When a gamma ray photon of energy equal to or greater than 1.02 MeV passes near a heavy nucleus, a pair of an electron and positron is produced. Conversion of mass into energy. Fission: The splitting of heavy nucleus into two or more fragments of comparable masses, with an enormous release of energy. The sum of masses before reaction is greater than the sum of masses after reactuin, the difference in mass being release in form of fission energy UC chain reaction: In this process, the number of fissions in a given interval on the average goes on increasing and the system will have explosive tendency. Controlled Chain Reaction: In this process, the number of fissions in a given interval is maintained constant by absorbing a desired number of neutrons. Fuel: The fissionable material which plays the key role in the operation of a reactor Moderator: It slows down the neutrons to thermal energies through elastic collisions between its nuclei and fission neutrons. Heavy water, Graphite Control Rods: To maintain steady rate of fission, the neutron absorbing material known as control rods are used. Made up of cadmium or boron steel. Coolant: To remove undesirable amount of heat produced. Fusion: The phenomenon of combination of two or more light nuclei to form a heavy nucleus with release of enormous amounts of energy. The sum of masses before fusion is greater than sum of masses after fusion. Neutrons ideal: Neutrons are neutral particles, so they can easily penetrate the nucleus to form unstable compound nucleus which can easily undergo fission. Heavy Water: Available in huge quantity in form of a liquid and its mass is nearest to the mass of neutron. According to the principle of mechanics, for maximum momentum transfer, the mass if incident particles must be equal to mass of target particle. Besides they have negligible absorption affinity for neutrons. They simply slow them down. Mass Energy Relation: Mass can be converted into energy if it travels with the speed of light Difficulties of Fusion: The nuclei to be fused are positively charged, they would repel each other strongly. Hence they must be brought together not only by very high pressure but also with high KE. For this, a temperature of order of 10^8 is required. 1 Unified mass unit: It is defined as one twelfth of the mass of an C12,6 atom. Decay Constant: Defined as the reciprocal of the time during which the number of atoms in a radioactive substance reduces to 36.8% of their initial number Binding Energy Curve: The curve has almost a flat maximum roughly from mass number 50-80. So the nuclei having mass number between 50-8- are most stable and Fe has the maximum stability with BE per nucleon For nuclei having mass no above 80, the average BE per nucleon decreases slowly and drops to 7.6Mev for U235 For nuclei having mass number below 50, Avg BE per nucleon decreases and below 20 it decreases sharply. So the nuclei having mass no below 20 are comparatively less stable. If two or more very light nuclei such as H2, be combined to form a heavier nucleus, the BE/nucleon will increase by a much greater amount than in the fission process. This will result in much larger release of energy. If a very heavy nucleus be split into two lighter nuclei near the flat maximum of the curve, the BE/nucleon will increase by about 1MeV. Hence, energy will be released in the process. Coolidge tube is used for production of X-rays. It has a hard glass bulb having a high vacuum of about 10^-6 Hg. Two tubes are connected to it. One of the tubes has a Filament F through which current is passed by means of a battery B. The filament, on being heated, emits thermionic electrons whose number per second depends upon the temperature of the filament. Around the filament is a Mb cylinder which kept at a negative potential relative to filament. It concentrates the electrons in the form of a beam Just in front of F is a copper block whose front end is at 45 defrees. On this surface is fixed a piece of T of some metal of high atomic weight and melting point known as target. While tube enxlosed in a lead envelope. When an alternating potential difference of 20k volt is applied between F and T by means of a step up transformer, then electrons emitting from F strikes T at a very high speed and X rays are emmited. This happens only during one half of cycle ie when target is positive wrt to F. Dirong the other half, T becomes negative wrt filament, electrons are repelled by the target. Hence it acts as its own rectifier. Wave Optics Electromagnetic waves: The waves propagating in space through electric and magnetic fields varying in space and time. Characteristics: Speed of light Neutral Transverse In EM waves the electric and magnetic fields are mutually perpendicular to each other and also perpendicular to the waves of propagation Reflection, Refraction, Interference, Diffraction and Polarisation Ratio of magnitudes of electric and magnetic field vectors in free space is constant equal to c Huygens Principle: HP is a geometrical construction, which is used to determine the new position of a wavefront. Every particle of the medium situated on the primary wavefront act as a source of secondary wavelets sending out fresh waves in all direction. The secondary wavelets travel in the medium with speed of Primary Wavefront The envelope of secondary wavelets in the forward direction at any instant, shows the new position of the wavefront at that instance Coherent Sources: Those sources which can produce light waves continuously of same wavelength and frequency, which are either in the same phase or have a constant phase difference Interference: The phenomenon of redistribution of light intensity due to superposition of two light waves. Fringe Width: The distance between any two consecutive bright fringes or any two consecutive dark fringes is called the fringe width. Diffraction: The phenomenon of bending of light round the sharp edges or corners of the obstacles or apertures and speeding into the regions of geometrical shadow is called diffraction. The size of object should be of the order of wavelength. Fraunhofer Diffraction: In this, both the source and screen should be at infinite distance from the diffracting obstacle. Interference vs Diffraction Interference is due to superposition of light waves coming from 2 coherent sources while Diffraction is due to the superposition of secondary wavelets originating from the different parts of same wavefront In Interference the fringes may or may not be equally spaced but in diffraction the fringes are unequally spaced In Interference, all maxima are of equal intensity while in Diffraction the central maximum is brightest and the intensity of subsequent maxima on either side goes on decreasing In interference the dark fringes are perfectly dark but in Diffraction they are not In interference the bands are larger in number but in Diffraction they are few. Similarities Both establish wave nature, cannot tell transverse nature and energy is redistributed in both. Polarisation: The phenomenon of asymmetry of vibrations about the direction of propagation of light is called polarization Unpolarised Light: The light having vibrations of electric field vector in all possible directions perpendicular to the direction of wave propagation. Polarised Light: The light having vibrations of electric field vector in only one direction perpendicular to the direction of wave propagation Plane of Vibration: The plane containing vibration of electric vector and direction of propagation of light. Plane of Polarization: Plane perpendicular to the plane of vibration and contains direction of propagation of wave (not electric) is known as plane of polarization. Brewsters Law: If unpolarised light falls on a transparent surface of RI n at certain angle i p called polarizing angle then reflected light is plane polarized. Under this condition n=tanip Polaroid: It is a device to produce and analyse plane polarized light. It is a thin and large sheet of crystalline polarizing material. When a beam of light passes through the Polaroid, only the vibrations parallel to the transmission plane get transmitted. Used in sun glasses to protect the eyes from glare, Photography as filters, 3D Motion pictures Analysis: For a given light beam is allowed to fall on an analyzer and the Polaroid us gradually rotated about the direction of incident ray. In full rotation 360 degrees: If light beam shows no variation in intensity, then given beam is unpolarised If light beam shows variation in intensity but the minimum intensity is non zero, then the beam is partially polarized If light beam shows variation in intensity and intensity becomes twice maximum and twice zero in complete rotation, then the light is plane polarized. Alternating Current Circuits RMS: The rms value of an AC is defined as the square root of average of i 2 during a complete cycle, where is the instantaneous value of the alternating current. Impedance: The hindrance offered by a circuit to the flower of AC is called impedance. Z Reactance: The hindrance offered by inductance and capacitance in AC Circuit is called reactance. Xl Xc AC: Changes in magnitude and direction periodically Choke Coil: A coil made of Cu having high inductance and negligible resistance. IT is used in AC circuit to control the current. The power loss is negligible. Wattless Current: When pure inductor or pure capacitor is connected to AC source, the current flows in the circuit but with no power loss. Resonance: At resonance, phase difference between current and applied voltage is zero and the impedance is minimum and current is maximum. Current Electricity Current: The charge flowing per second in an electric circuit is the measure of electric current. Current Density: The current density at a point in a conductor is the ratio of the current at that point in the conductor to the area of cross section of the conductor at that point. Resistance: It is the property of a conductor by virtue of which it opposes the flow of current through it. Ohms Law: If there is no change in the physical state of the conductor (such as temperature and pressure) then the current flowing through the conductor is directly proportional to the potential difference applied across the conductor. Mobility: Drift velocity of free electrons per unit electric field. 1 ohm: When a PD of 1V produces a current of 1A in a conductor, then resistance of conductor is said to be 1 ohm. Resistivity: Numerically equal to the resistance of a conductor of unit length and unit area of cross section. Drift Velocity: The average velocity of free electrons with which they move inside a conductor when a PD is applied across it. It is very small in comparison the thermal velocity. Relaxation time: Average time interval between two successive collisions between electrons. Mean free path: Average distance moved by a free electron between two successive collisions. Effect of Temperature: Metals: The resistance of a metal increases with increase of temperature. The reason is that with rise of temperature, the thermal speed of electrons increases which causes increased collisions of electrons with the lattice imperfections resulting in decrease of relaxation time. Since we know that relaxation time is inversely proportional to resistance. Hence the increase. Semiconductors: For semiconductors, the resistivity decreases rapidly with rise in temperature ie their temperature coefficient is negative. The reason is that for these substances the value of n is very small compared to metals, but with rise in temperature the number of free electrons, that is, the value of n goes on increasing. Therefore the resistance goes on decreasing. Although in these substances the relaxation time decreases, but the increase in the value of n is larger compared to decrease in relaxation time. Alloys: The resistivity of alloys also increases with rise in temperature, but this increase is much smaller compared to pure metals. On account of high and little temperature dependent resistivity, alloys are used to make wires for standard resistances, resistance boxes etc. Electrolytes: The resistivity of electrolytes also decreases with rise in temperature. The reason is that with rise in temperature the viscosity of electrolytes decreases so that ions get more freedom to move inside the electrolytes. Temperature Coefficient: TC of resistance of material of conductor is defined as change in resistance per unit rise of temperature. Power: The rate of doing work is called electric power ie work done per unit time Energy: The capacity to do electric work. Joules Heating Effect: States that when high amount of current is flowing through a conductor of resistance R for time t then the electric energy consumed will get transformed into thermal energy which is equal to I2RT. 1 KWH: The quantity of electric energy dissipated in 1 hour in a circuit when the electric power in the circuit is 1 kilowatt. Electric Cell: An electric cell is a source of electrical energy which maintains a continuous flow of charge in a circuit. EMF: The work done by the cell in forcing unit positive charge to flow in a whole circuit is called EMF. Terminal PD: The PD across terminals of a cell when it is in a closed circuit ie when the cell is in discharging position. Internal Resistance: The resistance offered by electrolyte of cell to the flow of current through it. DP to separation between electrodes, concentration of electrolyte IP to surface area of electrodes and temperature of electrolyte Junction Law: It states that at any junction in an electrical network, net amount of current entering the junction is equal to the net amount leaving the junction. Based on Conservation of Charge Voltage Law: It states that algebraic sum of all the PD in closed loop is zero. Back EMF: When the cell is giving current, the PD V across its plate is less than its EMF by a factor ir, or in other words the significant current from a cell can be drawn only when PD across its terminal is greater than ir. This product ir is called Back emf. Wheatstone bridge: Wheatstone devised a bridge arrangement of resistances by which the resistance of a given conductor can be determined. It is an arrangement of 4 resistors of comparable value. This arrangement is used to find the unknown resistance connected to it. Under balanced condition, Galvanometer shows Zero Deflection because PD across it is zero. The bridge is maximum sensitive when all the four resistances are of same order.

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