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CBSE Class 12 Pre Board 2020 : Physics (Kendriya Vidyalaya (KV) Agra)

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KENDRIYA VIDYALAYA SANGATHAN AGRA REGION PRE- BOARD EXAMINATION 2019-20 CLASS - XII SUBJECT PHYSICS(THEORY) TIME 3:00 Hrs M.M.:70 SET H GENERAL INSTRUCTIONS (i) All questions are compulsory. there are 37 questions in all. (ii) this quetion paper has four section A, B, C and D. (iii) Section A contains 20 questions for one mark each,Section B contains 7 questions for two marks each, Section C contains 7 questions for thre marks each andSection D contains 3 questions for five marks each. there is no over all choice. However, internal choices have been provided in two (iv) questions of one marks, two questions of two marks, one question of three marks and each question of five marks. (v) Use of calculators is not permitted. (vi) You may use the following values of physical constants wherever necessary: c = 3x108 m/s, h = 6.626x10-34Js e = 1.602 x10-19 C m0 = 4 px10- 7TmA-1 1/4peo= 9 109 Nm2C 2 Mass of neutron mn=1.672x10-27 kg Mass of proton mp=1.672x10-27 kg Mass of electron me = 9 1 10-31 kg SECTION (A) 1 The unit J/C is the unit of a)electric field 2 (b) electric potential energy (c) electric potential (d) electric dipole moment Following graph shows the variation of photoelectric current with anode potential for two light beams of same wavelength but different intensity. Find the correct relation: 1 (a) I1>I2 (b) I1 =I2 (c) I1<I2 (d) I1 I2 Page 1 of 6 3 Electric potential energy of electric dipole situated at 900 with the electric field is a) PE b) +PE c) zero 1 d) +2PE 1 4 Kirchhoff s II law for the electric network is based on: a)Law of conservation of charge b)Law of conservation of energy c)Law of conservation of angular momentum d)Law of conservation of mass 5 If both the number of turns and core length of an inductor is doubled keeping other factors constant, 1 then its self-inductance will be(a) Unaffected (b) doubled (c) halved (d ) quadrupled 6 The direction of induced current is such that it opposes the cause that has produced it. 1 This is the law of (a) Lenz (b) Faraday (c)Kirchhoff (d) Fleming 7 The figure shows variation of R, XL and XC with frequency f in a series L, C, R circuit. Then for what 1 frequency point, the circuit is inductive. (c) (a) A (b) B C (d) A and B 8 If , , represents wavelength of X-Rays, microwaves & visible rays then (a) > > (b) > > (c) > > (d) > > 9 The angle between pass axis of polariser and analyser is 450 ,the percentage of polarised light passing through analyser is (relative to light incident on the polariser) (a) 25% (b) Page 2 of 6 50% 1 (c) 75% (d) 100% 10 Which of the following is correct for Malus Law (a) I =I02cos2 (c) I =I02sin2 1 (b) I =I0 cos2 (d) I =I0tan-1 11 Direction of electric dipole moment is ____________________. 12 A cell of e.m.f. E and resistance r is connected across an external resistance R. the potential difference across the terminals of a cell for r = R is _______________ 1 13 The alloys which are used for making resistances have very low Temperature coefficient of resistance and high 1 . 1 14 The value of the magnetic susceptibility for a paramagnetic substance is____________ 15 In EM waves transport both__________and _____ 1 16 On which factors the maximum kinetic energy of emitted photoelectrons depends? 1 17W 1 hat is the value of total angular momentum of the electron in the nth allowed orbits? 18 Write the nuclear reaction equation for alpha decay of 226 . 88Ra 19 What is a Zener diode? 1 1 1 20 The resistance of p-n junction when reverse biased is high or low. SECTION (B) 21 A parallel plate capacitor of plate area A and separation d is filled with dielectrics of dielectric constants K1and K2 as shown in the figure. Find the net capacitance of the capacitor. 2 22 What do you mean by wattless current ? Give two examples of such a circuit. 2 23 Identify the electromagnetic waves whose wavelength vary as and also write one use for each. 2 (i) 10-12 m < <10-8 m (ii) 10-3 m < < 10-1 m 24 You are given the following three lenses. Which two lenses will you use as an eyepiece and as an Page 3 of 6 objective to construct an astronomical telescope? Give reason. 2 25 Monochromatic light of wavelength 589 nm is incident from air on a water surface. If for water is 1.33, 2 find the wavelength, frequency and speed of the refracted light. 26 The ground state energy of hydrogen atom is 13.6 eV. What are the kinetic and potential energies of Electron in this state? 2 27 With the help of a circuit diagram explain the use of a Zener diode as a voltage stabilizer. 2 OR Define the term mean life of radioactive nuclei. How is the mean life of a given radioactive nucleus related to the decay constant and Half-life? SECTION (C) 28 Using Gauss s law, obtain the expression for electric field intensity at a point due to an infinitely large, plane sheet of charge of charge density C/m2. How the field is directed if the sheet is (i) positively charged 3 (ii) negatively charged 29 State the underlying principle of a cyclotron. Show that time period of ions in cyclotron is independent of both the speed of ion and radius of circular path. Also obtain an expression for maximum kinetic energy gained by the particle. 30 What is motional electromotive force (motional e.m.f.)? 3 A rod of length l is moved horizontally with a uniform velocity v in a direction perpendicular to its length through a region in which a uniform magnetic field B is acting vertically downward. Derive the expression for the emf induced across the ends of the rod. 31 (i) Draw a labelled ray diagram for the formation of image by a compound microscope in normal adjustment. (ii)Define magnifying power of a compound microscope in normal adjustment and give the formula for 3 it. OR Draw a labelled ray diagram of a astronomical telescope when image is formed at least distance of distinct vision. Define its magnifying power and deduce the expression for the magnifying power of the astronomical telescope. Page 4 of 6 32 Light of wavelength 2000 A0 falls on a metal surface of work function 4.2 eV. (i) 3 What is the m aximum kinetic energy (in eV) of the a. fastest and b. slowest electrons emitted from the surface? (ii) What will be the change in the energy of the emitted electrons if the intensity of light with same wavelength is doubled? (iii) If the same light falls on another surface of work function 6.5 eV, what will be the energy of emitted electrons? 33 State the law of radioactive decay. 3 (i) Write the mathematical expression for law of radioactive decay for a sample of radioactive nucleus. (ii) Plot a graph showing the number (N) of undecayed nuclei as a function of time (t) for a given radioactive sample having half-life T. 34 Draw a labelled circuit diagram of a junction diode as a full wave rectifier. Explain its underlying principle and working. Depict the input and output wave forms. 3 SECTION (D) 35 i) With the help of a circuit diagram, explain how a potentiometer is used to compare the emf s of two primary cells. Obtain the required expression used for comparing the emfs. ii) Write two possible causes for one sided deflection in a potentiometer experiment. 5 OR State the principle of potentiometer. In the following potentiometer circuit AB is a uniform wire of length 1 m and resistance 10 Ohm . Calculate the (i) potential gradient along the wire, and (ii) balance length AO (= l ). 36 Using Biot-Savart law, deduce the expression for the magnetic field at a point (x) on the axis of a circular current carrying loop of radius R. How is the direction of the magnetic field determined at this 5 point? OR Derive an expression for the force per unit length between the two infinitely long straight parallel current carrying conductors. Hence define one ampere of current. In which condition two wires will attract and repel. Page 5 of 6 37 Derive an expression for fringe width in Young s double slit experiment. 5 Two coherent sources have intensities in the ratio 25 : 16. Find the ratio of intensities of maxima to minima after interference of light occurs. OR When unpolarised light is incident on the boundary separating the two transparent media, explain with the help of a suitable diagram, the conditions under which the reflected light gets polarised. Hence derive the relation of Brewster s angle in terms of the relative refractive index of the two media. A narrow beam of unpolarised light of intensity IO is incident on a Polaroid P1. The light transmitted by it then incident on a second Polaroid P2 with its pass axis making an angle of 600 with relative to the pass axis of Polaroid P1. Find the intensity of light transmitted by P2 . END OF PAPER Page 6 of 6

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