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CBSE XII Pre Boards 2015 : PHYSICS with Answers (KV Chennai)

10 pages, 42 questions, 19 questions with responses, 29 total responses,

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KENDRIYA VIDYALAYA SANGATHAN, CHENNAI REGION Class-XII Time allowed: 3 Hrs Common Pre-Board Examination-2014 - 15 Physics(Theory) Maximum Marks: 70 General Instructions : (i) (ii) (iii) (iv) (v) (vi) All questions are compulsory. There are 26 questions in total. Questions 1 to 5 are very short answer type questions and carry one mark each. Questions 6 to 10 carry two marks each, questions 11 to 22 carry three marks each, question 23 is value-based question carry four marks and questions 24 to 26 carry five marks each. There is no overall choice. However, an internal choice has been provided in one question of two marks, one question of three marks and all three questions of five marks each. You have to attempt only one of the choices in such questions Use of calculators is not permitted. However, you may use log tables if necessary. You may use the following values of physical constants wherever necessary. C = 3 X 108 m/s h =6.63 X 10-34 JS e = 1.6 X 10-19 C 0 X -7TmA-1 me= 9.01 X 10-31 Kg. 1. An electron and a proton, having equal momenta, enter a uniform magnetic field at right anglesto the field lines. What will be the ratio of their trajectories? (1) 2. If a wire is stretched to double its original length without loss of mass, how will the resistivity ofthe wire be influenced? (1) 3. If the potential difference across a capacitor is doubled, what happens to: (a) The charge on the capacitor and (b) The energy stored in the capacitors. (1) 4. Which of the following 120 V ac device costs more to operate: (a) One that draws an rms current of 10 A or (b) One that draws a Peak current of 12 A ? (1) 5. Calculate the threshold frequency of photon for photoelectric emission from a metal of workfunction 0.1 eV (1) 6. Give block diagram of basic units of all communication system. Explain briefly (2) 7. Define Self-inductance of a coil. Show that energy stored in an inductor of self inductance carrying current I is LI2 (2) -28. n identical capacitors when joined in series give an effective capacitance of C Units.What will be the capacitance if the capacitors are now placed in parallel combination? (2) 9. Show that the power due to the force exerted by the magnetic field on a moving charge is zero. Or A change of 0.2 mA in the base current causes a change of 5mA in the collector current for a common emitter amplifier. (i) Find the a.c. current gain of the transistor. (ii) If the input resistance is 2k , and its voltage gain is 75, calculate the load resistor used in the circuit. (2) 10. Draw a plot showing the variation of (i) Electric field (E) and (ii) Electric potential (V) with distance r due to a point charge Q. (2) 11.The binding energy per nucleon of deuteron (2H1) and helium nucleus (4He2) is known to be 1.1 MeV and 7 MeV respectively. If two deuteron nuclei react to form a single helium nucleus; calculate the energy released? (3) 12. Write Einstein s photoelectric equation. Explain the terms. (i) Threshold frequency and (ii) Stopping potential. (3) 13.A person with a normal near point (25cm) using a compound microscope with an objective of focal length 8 mm and an eye piece of focal length 2.5cm can bring a object placed 9 mm from the objective in sharp focus. What is the separation between the two lenses? How much is the magnifying power of the microscope? (3) 14. (i) define activity of a radioactive material and write its S.I unit. (ii) The sequence of step wise decay of a radioactive nucleus is D D1 -1 D2 If the atomic number and mass number of D2 are 71 and 176 respectively, what are their corresponding values for D? (3) 15. a) Define the term modulation index for an AM wave. What would be the modulation index for an AM wave for which the maximum amplitude is a while the minimum amplitude is b ? (Or) b) What is space wave propagation? Which two communication methods make use of this mode ofpropagation? If the sum of the heights of transmitting and receiving antenna in line of sight of communication is fixed at h, Show that the range is maximum when the two antenna have a height h/2 each. (3) -316.Two identical cells of emf 1.5V each joined in parallel, provide supply to an external circuit consisting of two resistors of 17 . Each joined in parallel. A very high resistance voltmeter reads the terminal voltage of the cells to be 1.4V. What is the internal resistance of each cell ? (3) 17. Write the principle of working of a potentiometer. Describe briefly, with the help of a circuit diagram, how a potentiometer is used to compare the emf of two cells? (3) 18. Give a labeled diagram and explain the principle and working of a van de Graff generator. (3) 19. Three light rays red (R), green (G), and blue (B) are incident on a right angled prism abc at face ab . The refractive indices of the material of the prim for red, green and blue wavelengths are 1.39, 1.44 and 1.47 respectively. Out of the three which colour ray will emerge out of face ac ? trace the path of these rays after passing through face ac ? (3) 20. An electromagnetic wave is travelling in a medium with a velocity V = V . The electric field oscillations of the electromagnetic wave are along the Yaxis. (3) (a) Identify the direction in which the magnetic field oscillations of the electromagnetic wave are taking place. (b) How are the magnitudes of the electric field and magnetic field in the electromagnetic wave related to each other? 21.UsingBiot-Savarts law, derive the expression for the magnetic field in the vector form at a point on the axis of a circular current loop. (3) 22. Plot a graph to show the variation of the angle of deviation as a function of angle of incidence for light rays passing through a prism. Derive the relation for the refractive index of the prism in terms of the angle of minimum deviation and the angle of the prism. (3) -4- 23.Divya and her family visited Shanghai. There they wanted to visit a nearby town. They bought tickets for a railway train between the two cities. Divya s mother refused to board the train when she came to know that the train does not have wheels. This created a problem. It was necessary for the family to board the train and reach the other city. Seeing her family in quandary Divya talked to her mother by giving her complete details of how the train will move. This convinced her mother. The family boarded the train and reached their destination safely. (a) What is such train without wheels called? How does it move? (b) Which value do you think is shown by Divya? (4) 24. Derive an expression for the impedance of a series LCR circuit connected to an AC supply of variable frequency. Plot a graph showing variations of current with the frequency of the applied voltage. Explain briefly how the phenomenon of resonance in the circuit can be used in the tuning mechanism of a radio or a TV set? (5) Or Explain with the help of a labeled diagram, the principle, construction and working of an a.c generator. 25.State the assumptions and sign conventions in deriving the lens makers formula and also derive an expression for it. (5) Or (a)Howisawavefront different from a ray? (b) Draw the geometrical shape of the wave fronts when (i) Light diverges from a point source. (ii) Light emerges out of a convex lens, when a point source is placed at its focus (c) State Huygens principle. With the suitable diagram, prove law of reflection using Huygens principle. 26. (a)Draw a circuit diagram of a common emitter amplifier using p-n-p transistor. Show input and output voltages graphically. The current gain for common emitter amplifier is 59. If the emitter current is 6mA find (i) base current and (ii) collector current. (5) Or a) Distinguish between metals, insulators and semiconductors on the basis of their energy bands. b) Why are photodiodes used preferably in reverse basis condition? A Photodiode is fabricated from a semiconductor with band gap of 2.8 ev. Can it detect a wavelength using 6000nm? Justify. KENDRIYA VIDYALAYA SANGATHAN, CHENNAI REGION Class-XII Common Pre-Board Examination-2014 - 15 Physics(Theory)-Marking Scheme 1. As r = p mv = qB qB ( as qand B are have same values for both the particles Thus re :rp= 1:1 (1/2) (1/2) 2. No influence (1) 3. (a) Since Q = CV, It doubles (1/2) (b) Since UE = CV2, it quadruples (1/2) 4. (a) Which draw Irms = 10A, costs more to operate as it consumes more power. Since Irms for 12A is 12 = 8.5 A. 5. Threshold frequency = 2.42 x 1013 Hz 6. For correct block diagram For correct explanation 7. Definition Energy stored in the Inductor u = LI2 8. Let x be the capacitance of each capacitors. In series (1) 2 1 1 1 = + c x x (1) (1) (1) (1) (1) (1) 1 1 = n. x = nc c x In parallel y=ne + nc+ ----------------- +n (terms) =n2c 9. Power due to the force p = . p = F.V. cos900 = 0 (or) C Voltage gain Av RL= 6k (1) (1/2) (1/2) (1) (1) = (1) -210. For each graph (2 x 1 =2) 11. Binding energy per nucleon of 1H2 = 1.1 MeV Total B.E of two 1H2 = 2.2 MeV Total B.E of two deutron nuclei = 2 x 2.2 = 4.4 MeV B.E per nucleon of 2He4 = 7MeV Total B.E. of helium 2He4 = 4 x 7 = 28 MeV Energy released in fission = 28 4.4 = 23.6 MeV 12. K.E = h h (i) (ii) (1) (1) (1) (1) 0 Correct definition Correct definition (1) (1) 13. For the objective 1 1 1 = 0 0 0 1 (1/2) 0 = 7.2 cm (1/2) 1 1 = - (1/2) = 2.27 (1/2) Separation between objective and the eyepiece = V0 + (ue ) = 9.47 cm M.P = [1+ ] = 88 (1) -3- 14. For correct definition (1) For correct unit Mass number of D, A = 180 Atomic number of D, Z = 72 (1/2) (1 ) 15. For Correct Definition (1) (1) = = (1) + OR For correct Definition of Space wave propagation Used in TV, RADAR,Microwave communication For correct proof 16. r1 = ( ) =( 1.5 1.4 1.4 )8.5 r1 =0.61 r=2r1 = 2 x 0.61 = 1.22 17. For correct Principle For Correct diagram 1 2 = (1) (1) (1) 1 (1) (1) (1) (1) (1) (1) 2 18. For correct Principle (1) For Correct diagram (1) Working (1) -4- 19. The rays will emerge out of the face ac for which i< C where C is the critical angle (1/2) (1 ) 1 a g = herei=450 a g = 2 = 1.41 Thus the condition is (1/2) a g < 1.41 only Red Raywill emerge out. 20. (a) Magnetic field must oscillate along ie ( Z axis) because = ( x ) (b) 0 (1/2) = c (speed of light in vacuum ) (1 ) (1 ) 0 21.Biot Diagram For correct derivation (1) (1/2) (1 ) -522. For correct graph (1) For correct derivation (2) 23 (a) Such a train is called Maglev (Magnetic levitation) and explanation(3) (b) Concern for her family and good use of fundamentals learnt. (1) 24. Circuit Diagram (1/2) Phasor diagram For derivation of Impedance (1/2) (2) (1) (1) Resonance in the circuit (or) 25. For correct labelled diagram Principle Construction Workings Assumptions Sign conventions Diagram C (1) (1) (1) (2) (1/2) (1) (1/2) (3) (or) Correct answers of wave front and ray (i) Correct diagram (ii) Correct diagram Principle Diagram For proving (1/2) (1/2) (1/2) (1) (1) (1 ) -626. (a) Correct circuit diagram Input wave form Output wave form = Ic IB= , (2) (1/2) (1/2) IE = IB +IC Ib 1+ = 0.1 mA (1) Ic = 59 x 0.1 = 5.9 mA (1) (or) (a) Correct diagram Metals Insulators Semiconductors (b) Correct answer No it cannot detect [h has to be greater than Eg] (1) (1) (1) (1) (1)

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