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Subject: PHYSICS (Paper I) (Theory) Std : XII Date : Time : 3 hours Marks : 70 (Candidates are allowed additional 15 minutes for only reading the papers They must NOT start writing during this time) --------------------------------------------------------------------------------------------------------------------------------------Answer all questions in Part I and Ten questions from Part II, Choosing Four questions from section A, Three questions from section B and Three from section C. All working including rough work should be done on the same sheet as, and adjacent to the rest of the answer. The intended marks for questions or parts are given in brackets [ ]. A list of useful physical constants is given at the end of this paper. ----------------------------------------------------------------------------------------------------------------------------- ---------- PART I (All questions are compulsory) Question 1 A. Answer all questions by choosing the correct option A, B, C or D. i. Three charges 4q, Q and q are in a straight line in the positions 0, l/2 and l respectively. Resultant force at q will be zero if Q is equal to: (A) q (B) -2q (C) q/2 (D) 4q ii. Sensitivity of a moving coil galvanometer can be increased by (A) increasing number of turns (B) increasing magnetic field intensity (C) increasing area of coil (D) all of these [5] This Question paper consists of 6 printed sides. iii. When a charged particle is projected perpendicular to an uniform magnetic field, it describes a circular path in which: (A) its speed remains constant (B) its velocity remains constant (C) its momentum remains constant iv. v. (D) its kinetic energy increases. Which of the following transitions in a hydrogen atom emit the photon of highest frequency? (A) n =2 to n = 6 (B) n = 6 to n = 2 (C) n = 1 to n = 2 (D) n = 2 to n = 1 A beam of electron is used in Young s double slit experiment. The slit width is d. When the velocity of electron is increased, then (A) no interference is observed (B) fringe width increases (C) fringe width decreases (D) fringe width remains same B. Answer all questions briefly to the point: (i) [15] An electric dipole of dipole moment 20 X 10-6cm is enclosed by a closed surface. What is the net electric flux coming out of the surface? (ii) State the condition in which terminal voltage across a secondary cell is equal to its E.M.F. (iii) An e.m.f of 2V is induced in a coil, when current in it is changed from 0A to 10A in 0.40s. Find the coefficient of self inductance of the coil. (iv) Write two properties to select suitable materials for making permanent magnets? (v) In an AC circuit, R=4 , Z=5 , Vrms = 200V, Irms=1.5A. Calculate the average power consumed over a full cycle. (vi) What is the phase difference between electric field (E) and magnetic field (B) associated with a plane polarised light? (vii) A symmetric double convex lens of power 4D is cut in to two halves by a plane containing the principal axis, then what is the power of each half? (viii) What is meant by normal adjustment of a telescope? (ix) If the intensity ratio of maxima and minima in an interference pattern is 4:1.Find the ratio of slit widths? (x) State any one difference between interference of light and diffraction of light? (xi) What is the momentum of a photon of energy 42eV? (xii) What is the major difference between electromagnetic waves and matter waves? (xiii) Name the series of lines in the hydrogen spectrum which lie in the UV region? (xiv) For the working of a n-p-n transistor, what kinds of biasing are required at the emitter-base and collector-base junctions? (xv) What is the significance of average binding energy per nucleon of a nucleus? PART II (Answer ten questions in this part, choosing four questions from Section A, three questions from section B and three questions from Section C ) SECTION A (Answer any four questions) [20] Question 2: (a) Derive an expression for the electric field due to an electric dipole along the broad side on position. [3] (b) Three point charges Q1=25 C, Q2=50 C and Q3=100 C are kept at the corners A, B and C respectively of an equilateral triangle ABC having each side equal to 7.5m.Calculate the total electrostatic potential energy of the system. [2] Question 3: (a) Apply Kirchoff s Laws to determine the currents I 1 , I2 and Potential difference between the points B and E (VBE) from the circuit shown in figure below: [3] (b) Three capacitors each of capacitance(C) are connected in series and their equivalent capacitance is Cs. The same three capacitors are now connected in parallel and their equivalent capacitance is Cp. Find the ratio (Cp / Cs) [2] Question 4: (a) Define drift velocity. Derive an expression between drift velocity (vd) and current density( j) [3] (b) i) What is the value of magnetic susceptibility of aluminium, if its relative permeability is 1.000022? ii) State Curie s Law for paramagnetic substances. [2] Question 5: (a) With the help of a neat labelled diagram, prove that the magnitude of emf(e) is given by e=Blv where l is length of a metallic rod and v is the velocity with which it is pulled in a transverse magntic field B. [3] (b) A galvanometer has a deflection of 5 divisions per mA current. The resistance of the galvanometer is 40 . If a shunt of 2 is connected to the galvanometer and there are 50 divisions on the scale of the galvanometer, then what maximum current can this galvanometer measure? Question 6: (a) State the principle of working of a Moving coil galvanometer.Derive the formula I=k (b) [2] [3] Show graphically how a d.c current flowing through an LR circuit varies with time when the key is put i) ON ii)OFF. What is meant by time constant of an LR circuit. [2] Question 7: (a) In a L-R circuit,the potential difference across inductor(L) and resistor(R) are 200V and 150V respectively and the rms value of the current is 5A.Calculate: (i) the impendance of the circuit. (ii) phase angle between the voltage and the current. [3] (b) An electron is moving at 106m/s in a direction parallel to a current of 5A,flowing through an infinitely long straight wire, separated by a perpendicular distance of 10cm in air. Calculate the magnitude and direction of force experienced by the electron. [2] SECTION B (Answer any three questions) [15] Question 8: (a) With the help of a labelled diagram, show that the fringe separation ( ) in Young s double slit experiment is given by = D/d where the terms have their usual meanings. [3] (b) Determine the angular separation between central maximum and first order maximum of the diffraction pattern due to a single slit of width 0.25mm when light of wavelength 5890A0 is incident normally on it. [2] Question 9: (a) An equiconvex lens of focal length (f) and power(P) is cut in to two halves in thickness What are the focal length and power of each half? (b) A ray of light incident on an equilateral glass prism shows a minimum deviation of 300. Calculate the speed of light through glass prism. [3] [2] Question 10: (a) Obtain an expression for magnifying power of a compound microscope with necessary diagram when its final image is at the least distance of distinct vision. [3] (b) A point source of monochromatic light S is kept at the centre of the bottom of a cylinder. Radius of the circular base of the cylinder is 15 cm. The cylinder contains water(refractive index=4/3) to a height of 7.0 cm. Find the area of the water surface through which light emerges in the air.(take =22/7) [2] Question 11: (a) i) Define plane of polarization. ii) Green light is incident at the polarising angle on certain glass plate. The angle of refraction is 320 What are: i) the polarising angle ii) refractive index [3] (b) Show graphically how Intensity of light varies in Fraunhoffer diffraction. SECTION C (Answer any three questions) Question 12: (a) Show graphically the variation of photoelectric current in a phototube with: i) voltage across the tube ii)intensity of incident radiation ( > 0 ) [2] [15] [2] (b) De Broglie wavelength of electrons of kinetic energy E is . What will be its value if kinetic energy of electrons is made 4E ? [2] (c) Photons of energies twice and five times the work function of a metal fall successively on the metal surface. What is the ratio of the maximum velocities of photoelectrons emitted in the two cases(v1:v2) [1] Question 13: (a) State Bohr s postulates. Derive an expression for kinetic energy, potential energy and total energy of an electron in the n th Bohr s orbit. (b) The potential difference between the cathode and the target electrode in a Coolidge tube is changed from 20kV to 30 kV. What will be the fractional change in minimum wavelength of emitted X rays? [2] Question 14: (a) A neutron breaks in to a proton, an electron and an antineutrino. Calculate the energy released in this process in MeV. (mass of electron=9 X 10-31kg, mass of proton=1.6725 X 10-27kg, mass of neutron=1.6747 X 10 -27 kg) (b) [3] The half life of polonium is 140 days. In what time, will 15g of polonium be disintegrated out of its initial mass of 16g? [2] Question 15: (a) Draw a neat labelled diagram of a half wave rectifier and also sketch its input and output waveforms. (b) [3] Draw the symbolic diagram of NAND gate and prepare its truth table. USEFUL CONSTANTS Speed of light in air = 3 * 108 m/s. Electron Charge (e) = 1.6 10 19 C Mass of electron (me = 9.1 10 31 kg Planck s constant (h) = 6.6 10 34 J.s Electron volt (eV) = 1.6 10 19 J 1 a.m.u. = 931 MeV Rydberg Constant(R) = 1.097 10 7 / m 12 Permittivity of free space = 8.8 10 F / m Constant of proportionality = 10-7 H/m for Biot Savarts law [3] [2]
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