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Preboard Examination Class:XII PHYSICS (Three hours) Maximum Marks:70 This paper is divided into four Sections A, B, C and D. Answer all questions. Section A consists of one question having sub-parts of one mark each. Section B consists of seven questions of two marks each. Section C consists of nine questions of three marks each, and Section D consists of three questions of five marks each. Internal choices have been provided in two questions each in Section B,Section C and Section D. The intended marks for questions are given in brackets [ ]. Section A (Compulsory) (A)Answer all questions in this part (7 1 = 7) Question 1 (a) The spatial distribution of the electric field due to two charges (A, B) is shown in figure. Which one of the following statements is correct? A. B. C. D. A is +ve and B is -Ve and |A| > |B| A is -ve and B is +Ve and |A| = |B| Both are positive but A > B Both are negative but A > B. (b) The electrostatic potential energy of two point charges 1 C each, placed 1 m apart in air is: A. 9 103 J B. 9 109 J C. 9 10 3 J D. 9 10 3 eV (c) Identical piece of Ge and Cu are taken and cooled, then A. Resistivity of both increases B. Resistivity of both decreases C. Resistivity of Cu increases and Ge decreases D. Resistivity of Cu decreases and Ge increases. (d) A charged particle moves with velocity v in a uniform magnetic field B. The magnetic force experienced by the particle is A. always zero B. never zero and v are perpendicular D. zero, if B and v are parallel C. zero, if B . (e) Two lenses of power 25D and +15D are in contact with each other.The focal length of the combination is.. A. 10 cm B. -20 cm C. -10 cm D. +20 cm (f) In the circuit of figure the bulb will become suddenly bright if.... A. Contact made or broken B. contact is made D. would not become bright at all. C. contact is broken (g) Assertion: Gauss s law is not much useful in calculating electrostatic field when the system has some symmetry. Reason: :Gauss s law is based on the inverse square dependence on distance contained in the coulomb s law. A. Both Assertion and Reason are correct, and Reason is the correct explanation for Assertion. B. Both Assertion and Reason are correct, but Reason is not the correct explanation for Assertion. C. Assertion is true and Reason is false. D. Assertion is false and Reason is true. (B)Answer all questions given below briefly and to the point:[7] (a) State the condition for a balanced Wheatstone bridge. (b) Write an expression for Vector form of Ohm s law? Page 2 (c) A galvanometer is first converted into a voltmeter of range (0-3V) and then into a voltmeter of range (0-6V).In which case the resistance would be higher one? (d) With what type of source of light are spherical wavefronts associated? (e) What is meant by a microscope in normal use? (f) What type of semiconductor is obtained when a crystal of silicon is doped with a pentravalent element? (g) Define multiplication factor of a fissionable mass in nuclear fission. Section B (14 marks) Question 2 [2] (a) What are eddy currents? (b) Write any one applications of eddy currents. Question 3 [2] Name and define the unit used for measuring the coefficient of mutual inductance. State the relation of this unit with the units of magnetic flux and electric current. Question 4 [2] (a) Two heating elements of resistance R1 and R2 when operated at a constant supply of voltage V, consume powers P1 and P2 respectively. Deduce the expressions for the power of their combination when they are, in turn, connected in (i) series and (ii) parallel across the same voltage supply. OR (b) The potential difference applied across a given resistor is altered so that the heat produced per second increases by a factor of 9. By what factor does the applied potential difference change? Question 5 [2] (a) How are electromagnetic waves produced? (b) State how vectors E,B and c are oriented in an electromagnetic wave. Page 3 Question 6 [2] (a) State Bohr s quantization condition for defining stationary orbits. (b) Atomic radius of first orbit of hydrogen atom is 0.053nm . What is the radius of its fifth orbit? Question 7 [2] Mention any two properties of an LCR circuit at resonance. Question 8 [2] (a) Compare the focal lengths of the two lenses shown below if the radius of curvature of the curved surface is the same in both lenses. OR (b) i. Write the expression for equivalent focal length of two thin convex lenses placed in contact with each other. ii. What is the magnification of combination given in part (i) Page 4 Section C (27 marks) Question 9 [3] Derive the expression for equivalent emf and resistance in two cells of emf E1 and internal resistance r1 , emf E2 and internal resistance r2 connected in Series. Question 10 [3] Obtain an expression for intensity of electric field at a point due to a long straight charged wire using Gauss Law. Question 11 [3] Answer the following questions : (a) Draw the ray diagram of a compound microscope,when final image lies at infinity. (b) Write an expression for magnifying power of an astronomical telescope, when final image lies at infinity.(No derivation required) Question 12 [3] (a) Two identical circular loops, one of copper and the other of aluminium are rotated about their diameters with the same angular speed in a magnetic field directed perpendicular to their axes of rotation. Compare (i) the emf induced, and (ii) the current in the two loops. Justify your answers. (b) Magnetic flux of 10 Wb is linked with a coil,when a current of 2 mA flows through it.Find the self inductance of the coil. Question 13 [3] (a) In a meter bridge with R and S in the gaps, the null point is found at 40 cm from A. If a resistance of 30 is connected in parallel with S, the null point occurs at 50 cm from A. Determine the values of R and S. Page 5 OR (b) i. In the potentiometer circuit given below, calculate the balancing length l. ii. Give reason, whether the circuit will work, if the driver cell of emf 5 V is replaced with a cell of 2 V, keeping all other factors constant Question 14 [3] (a) Derive an expression for the force experienced by a current carrying conductor placed in magnetic field.Under what conditions this force is (i)Zero and (ii) Maximum. OR (b) Obtain an expression for the magnetic moment of an electron moving with a speed v in a circular orbit radius r.State the rule to find its direction. Question 15 [3] Page 6 Draw a ray diagram for the formation of image of a point object by a thin double convex lens having radii of curvature R1 and R2 . Hence derive lens maker s formula. Question 16 [3] Young s double slit experiment is performed by a student.The examiner gives the following instructions to the student.State whether the responses of the student,in each case,are the correct or incorrect.Give a reason for your answer. (i) Examiner : The fringes on the screen are too crowded.Increase the distance between the fringes. Student : Increases the distance between the two slits ( thinking that distance between the fringes increases on increasing the distance between slits). (ii) Examiner : Bright fringes are not very bright.Increase their brightness. Student : Move the source of light towards the two slits. (iii) Examiner : Compare the distance between central and first maxima with the distance between second and third maxima. Student : Measures and states that the distance between central and first maxima is more than that between second and third maxima. Question 17 [3] i. What is meant by pair annihilation? ii. Give any one example where energy is converted to matter. iii. Why binding energy per nucleon found to be constant for nuclei in the range of mass number (A) lying between 30 and 170? Section D (15 Marks) Question 18 [5] (a) With reference to photoelectric effect, plot a labelled graph of stopping potential (Vo ) (vs) versus frequency (f ) of the incident radiation. (b) State how will you use this graph to determine the value of Planck s constant. Page 7 (c) Using Huygen s wave theory, prove that laws of reflection of light. OR (a) A coil having self inductance of 0.7H and resistance of 165 is connected to an AC source of 275V, 50Hz.Then calculate: [3] i. Reactance of the coil. ii. Impedance of the coil iii. Current flowing through the coil. (b) Draw a labelled graph showing variation of impedance of a series L-C-R circuit with frequency of the AC supply. [2] Question 19 (a) With reference to semi-conductors answer the following: [3] i. Name the diode that emits spontaneous radiation when forward biased. ii. Draw a labelled energy band diagram for a semiconductor. iii. Name the process that causes depletion region in a p-n junction. (b) Using Bohr s theory of hydrogen atom, derive the expression for the total energy of the electron in the stationary states of the atom [2] OR (a) Uranium undergoes nuclear fission as shown in the following nuclear reaction. [2] 92 U 235 + 0 n1 36 Kr90 + 56 Ba143 + 30 n1 Calculate the energy released during the fission reaction. (b) Draw a labelled graph of binding energy of a nucleus per nucleon versus its mass number: [2] (c) Write any one balanced equation to represent nuclear fusion. [1] Page 8 Question 20 [5] Read the passage given below and answer the questions that follow. The flow of charges in a particular direction constitutes the flow of Current. Quantitatively, electric current in a conductor across an area held perpendicular to the direction of flow of charge is defined as the amount of charge flowing across that area per unit time. (a) What amount of current flows if 1016 electrons flow in 1 ms(millisecond) through a definite cross section of a conductor? (b) What is the relation between drift velocity and mobility? (c) Define relaxation time for electron. (d) How is current density related to conductivity of a metal in an electric field? (e) Explain how current passing through the wire changes with different area of cross section of the wire. Useful Constants: Charge of proton=1.6 10 19 C Charge of electron= 1.6 10 19 C Permittivity of Vacuum ( 0 )=8.854 10 12 F m 1 Permeability of Vacuum ( 0 )=4 10 7 Hm 1 Speed of light =3 108 m/s Planck Constant = 6.626 10 34 Js 1 eV = 1.6 10 19 J Page 9
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