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PRELIM EXAMINATION 2022 23 ANSWER KEY Grade: 10 Subject : PHYSICS Date: 4/1/24 Max. Marks: 80 Dur. : 2 Hours Number of printed pages : 8 SECTION A : Answer all questions in this section QUESTION 1 15 (i) . Which of the following is true about resistors connected in parallel? (a) The p.d. across each resistor is equal (b) total resistance increases (c) An equal amount of current flows through each resistor (d) All of the above (ii). The electric meter records the consumption of (a) Charge (b) Current (c) Energy (d) Power (iii). When a body vibrates under the influence of an external periodic force, the vibrations are (a) Free vibrations (b) Forced vibrations (c) Resonant vibrations (d) None of the above (iv). 200 g of ice at 0 C needs _________ heat to melt. [Sp. latent heat of ice = 336000 J kg-1] (a) 6720 J (b) 67200 J (c) 672000 J (d) 67.2 J (v). In the circuit given alongside, the current through the resistor R is, (a) 0.01A (b) 0.02A (c) 0.04A (d) 0.05A (vi). The radiation with maximum penetrating power is: (a) (b) (c) X-radiation (d) (vii). Which one of the following statements is correct? (a) Live wire has zero potential (b) Potential of live and earth wire is always the same. (c) Fuse is connected in a neutral wire (d) Earth wire is used to prevent electric shock. (viii).Ram and Amit each having mass of 45kg reach the fourth floor of a building in time 4s and 5s respectively. The ratio of their power consumed is, (a) 4:5 (b) 5:4 (c) 1:1 (d) None of the above. 1 (ix). An object of mass 200 g moving with velocity 50 cm/s. What is its kinetic energy? (a) 2.1 105 erg (b) 2.0 105 erg (c) 2.8 105 erg (d) 2.5 105 erg (x). A machine in which V.R. is 4, has its M.A. nearly 4. If the friction in the machine increases : (a) V.R. and M.A. will remain the same. (c) M.A. will increase. (b) M.A. will decrease. (d) V.R. will increase and M.A. remains the same. (xi). A ray of light is normally incident on one face of an equilateral prism. What will be the angle of incidence at the second face of the prism? (a) 30 (b) 60 (c) 90 (d) 0 (xii). If the magnification produced by a lens is - 0.5, the correct statement is: (a) The lens is concave. (b) The image is virtual. (c) The image is magnified (d) The image is real and diminished formed by a convex lens. (xiii).Which color of light has the least wavelength? (a) Blue (b) Violet (c) Red (d) Orange (xiv). A body is pivoted and a force of 20 N is applied at a distance of 60 cm from the pivot, hence the moment of the force at that point is, (a) 12 N (b) 1200 Nm (c) 12 Nm (d) 120 N cm. (xv). Lenz s law gives, (a) the magnitude of induced current. (b) the magnitude of induced e.m.f (c) direction of induced current. (d) both (a) & (c) QUESTION 2 (i) Some hot water was added to three times its mass of cold water at 10 C and the resulting temperature was found to be 20 C. What was the temperature of the hot water? 3 Ans: Let the mass of hot water be m and temperature of hot water be .Mass of cold water = 3 m According to principle of mixtures Heat lost by hot water = Heat gained by cold water mxc( 20)=3m c(20 10) ---------------2 hence, 20=30; =50 C----------1 (ii) A pulley system has a velocity ratio of 4 and an efficiency of 90%. Calculate: a) Mechanical Advantage. b)The effort required to raise a load of 300N by the system. Ans: MA=(Eff x VR)/100 = (90 x 4)/100 = 3.6 ----------1 E = load/MA = 300/3.6 = 83.33N--------------------1 2 2 (iii)What is meant by Dynamic equilibrium? Give an example. 2 Ans: When a body remains in the same state of motion (translational or rotational). under the influence of the applied forces. the body is said to be in dynamic equilibrium. ---1 For example. a raindrop reaches the earth with a constant velocity is in dynamic equilibrium.------1 (iv) A ray of light enters a glass slab as shown in the figure.The critical angle of glass is 42 . (v) Draw a 3 pin plug identifying the terminals.Mention a safety precaution while handling it. Ans: 2 1. Hands should be completely dry. 2.Plug pins should fit in the socket tightly--------------1 (any 1) (vi) A radar sends a signal to an aircraft at a distance of 30 km away and receives back after 2 x 10-4 secs. What is the speed of the signal? 2 Ans: Given: Distance of aircraft =30 km = 30,000 m time taken = 2 10 4 second Speed of signal (V)=2d/t = 60000/2 10 4 -----------1 = 30000 104 m/s = 3 108 m/s. ----------1 Therefore, the speed of the signal is 3 108 m/s. (vii) Illustrate the deflection of Becquerel rays in an electric field. 2 Ans: QUESTION 3 (i) In the adjoining figure, a current carrying conductor AB is placed between two magnets. What happens to the wire if the direction of current is from A to B? Name the rule applied to deduce the above observation. 3 2 Ans: Due to the force,the wire will deflect in inward direction perpendicular to plane of paper. ---1 Fleming s Left hand Rule-------1 (ii) State the 2 conditions under which work done by a force is zero. 2 Ans: (i) When there is no displacement (S=0) and, -----------1 (ii) When the displacement is normal to the direction of the force ( =90 ).----------1 (iii) An object is placed at a distance of 24 cm in front of a convex lens of focal length 8 cm. Calculate a). The distance of the image from the lens; b). The magnification of the image 2 Ans: a)Given u= 24cm, f = +8cm, v = ? Using the lens formula, 1/f = 1/v 1/u 1/v = 1/f + 1/u 1/v =1/8 + 1/( 24) = (3 1) /24 = 1/12 -------------1 or v = +12cm -----------------1 b) m = v/u = 12/-24 = -0.5.--------------1 (iv) Find the effective resistance across P and Q in the network shown below: 2 Ans:Rs = 7 + 3 = 10 1/Rp = 1/10 + 1/10 , Rp = 5 Req = 5 + Rp + 5 = 15 (v) Name a radio isotope used in the medical field. State the use of the emitted radiations. 2 Ans : cobalt -60.--------1 Used to treat cancer ( Radiotherapy)--------1 SECTION B : Answer any FOUR questions in this section QUESTION 4 P Q (i) State the Principle of Moments.Mention an instrument that works on this principle. 2 Ans: At equilibrium, the total clockwise moment about a point equals the total anticlockwise moment about the same point.-------1 Beam balance --------1 (ii) Ways to increase the speed : Ans: i) increasing the strength of current. ii) increasing no. of turns in the coil. iii) increasing area of the coil. iv) increasing the strength of magnetic field. (iii) State the principle and transformation of energy in a D.C. Motor. 2 3 Ans: Principle:When kept in a magnetic field, a current-carrying conductor experiences a force and develops a tendency to move.--------1 Transformation: Electrical energy into mechanical energy.-------1 4 (vi) A nucleus Am emits an particle and changes to Np which emits a particle and then radiations to change to 244 Pu 94 .Show the nuclear changes in the form of equations stating the atomic no. and mass no. of each nucleus. - 248 Am 95 3 - 244 Np 93 244 Pu 94 244 Pu 94 . QUESTION 5 (i) An electric motor of rating 3HP - 220V is in use. Find the safe current of the fuse wire connected to the motor. Ans: P = 3 x 746 = 2238W 2 V = 220V , I = P/V = 2238/220 --------1 I = 10.17 A -----1 (ii) What is the effect of pressure on, a). the melting point of ice. b). the boiling point of water. 2 Ans: a. the melting point of ice decreases with the increase in pressure.----1 b. the boiling point of water increases with increase in pressure.--------1 (iii) a). Define Refractive index. b).What is the cause of the refraction of light? 3 Ans; The refractive index is the measure of bending of a light ray when passing from one medium to another. OR It can also be defined as the ratio of the velocity of a light ray in air Or vacuum to the velocity of light in a substance, n = c/v. OR It is the The ratio of the sine of the angle of i to the sine of the angle of refraction is a constant, for a given pair of media -----2 b.The cause of the refraction of light is that light travels at different speeds in different media. This change in the speed of light when it moves from one medium to another causes it to bend.----1 (iv) Three resistors are connected to a 24 V battery as shown. 3 Calculate, a). the equivalent resistance of the circuit. b).the total current in the circuit. c).the potential difference across the 4 resistor. Ans: a. Rp= 6 x 3/6 + 3 = 2 , Req = 4 + 2 = 6 -------1 b. I = /Req = 24/6 = 4 A ---------1 c. V(across 4 ) = 4 x 4 = 16V--------1 QUESTION 6 (i) How is the frequency of a stretched string related to its, a).length b). tension. Ans: the frequency of the stretched wire is, a)inversely proportional to the length ----1 5 2 And b.directly proportional to the tension of the string.---1 (ii) What are background radiations ? Name an external source which is a naturally occurring radioactive element. 2 Ans: Background radiations are the radioactive radiations such as alpha, beta and gamma to which we all are exposed even in the absence of an actual visible radioactive source.----1 External source radon -222----1 (iii) A lens forms an upright and magnified image of an object. a)Name the lens. b).Draw a ray diagram to show the position of image formed. c). What is the application of the lens in the above position? b. 3 Ans: a. Convex . c. Simple microscope?magnifying glass. (iv) An uniform half meter rule of mass 80g is pivoted at one end. Show with directions ,how to determine the minimum force required to keep this rule at equilibrium Ans: Acc. to principle, 80 x 25 = F x 50 ----1 F = 40gf (upwards) -----2 QUESTION 7 (i) A total internal reflecting prism is used in place of a plane mirror in a periscope. Give two suitable reasons. 2 Ans:1. A total reflecting prism gives us an image much brighter than that obtained by using a plane mirror. 2. When total internal reflection occurs from a prism, the entire incident light (100%) is reflected back into the denser medium, whereas in ordinary reflection from a plane mirror, some light is refracted and absorbed so the reflection is partial. 6 (ii) A type of electromagnetic wave has wavelength 200 . Name the wave and state one use of this type of wave. Ans:UV rays *Used for sterilizing air and surgical instruments.*To detect purity of gems eggs,ghee.*Ultraviolet radiations are used in producing vitamin D in the food of plants and animals. 2 iii). If the specific heat capacity of substance A is 4.7J/g K and of substance B is 2.5J/g K, then which substance can be used as a coolant in car radiators and which is a good conductor of heat? 3 Ans:A is used as a coolant----1 and B is a good conductor----1 iv) a). How can energy loss due to Hysteresis in the core of an transformer be reduced? Ans : Reduced by use of a soft iron core. b). The secondary coil of a transformer has 6 turns and the primary coil of 240 turns is connected to a 200V a.c. supply. What will be the output voltage? Ans Ns/Np = Es/Ep, 6/240 = Es/ 200, Es = 5V QUESTION 8 (i) A terrestrial telescope uses a lens in the eyepiece, a). Name the lens used. b). State the position of the object. Ans: a. Convex---1 c).What is the nature of the image formed? b. at 2F1 ---------1 (ii) a).What are Damped Vibrations? 3 c. Real, inverted, same size as the object b). Draw a displacement -time graph to show this motion. c).In this motion does the loss in energy depend on the shape and size of the vibrating body.3 Ans: a. The periodic vibrations of a body of decreasing amplitude in presence of a resistive force are called damped vibrations. -----1 --------1 c. Yes ---------1 iii) a).Name and state the use of the transformer used at the grid sub-station, during the transmission of power. b).Which coil of the above transformer is made thicker ? Why? 7 Ans: Step-up transformer. Use : to increase the alternating input emf. Primary coil is made thick to decrease resistance hence preventing loss of energy due to heating of the coils. QUESTION 9 (i) The figure shown alongside shows a current carrying solenoid. Copy the diagram to, 3 a). Show the polarity acquired by each face A and B of the solenoid. b). Draw the magnetic field lines with directions inside and around the coil Ans A- south and B - North . (ii) What is a fuse wire? State its characteristics and the principle it works on. 3 Ans: It is the device used to limit the current in an electric circuit.It safeguards the circuit and the appliances connected in the circuit from being damaged. It is a thin metal wire with low melting point and high resistivity.It works on the principle of the Heating effect of electric current. (iii) a). State the Work Energy Theorem. 4 b). Give an expression to relate KE and momentum. If a car and a truck have the same kinetic energy, which one will have greater momentum? Ans: a. According to the work-energy theorem, the work done by a force on a moving body is equal to the increase in its kinetic energy. b). K.E = p2/2m. c). Truck will have greater momentum 8
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