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# CBSE Class 12 Pre Board 2019 : Physics - Prelim 2 (St Xavier's Sr. Sec. School, Delhi)

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Pre Board Examination 2019 Std. 12 14-01-2019 Set 2 PHYSICS Max. Marks :70 Time : 3 hrs. INSTRUCTIONS: (i) (ii) (iii) (iv) (v) (vi) (vii) Q. Nos. 1 to 5 carry 1 mark each. Q. Nos. 6 to 12 carry 2 marks each. Q. Nos. 13 to 24 carry 3 marks each. Q. Nos. 25 to 27 carry 5 marks each. Use pencil for the diagrams and graphs. Answers should be to the point. Use log tables if necessary. SECTION A 1. The stopping potential in an experiment on photoelectric effect is 15 V. What is the maximum kinetic energy of the ejected photoelectron? (OR) A beam of electron if formed by accelerating electrons by potential V. Show graphically how the de-Broglie wavelength will vary with potential difference. Justify the shape of the graph. (1) 2. Identify the electromagnetic wave whose wavelength lie in the given range and write its -11 -14 one use10 m< <10 m. (OR) In which direction do the electric and magnetic field vectors oscillate in an electromagnetic wave propagating along x- direction? (1) 3. Write a relation for polarization vector P of a dielectric material in the presence of an external electric field E. 4. A convex lens of focal length f1 is kept in contact with a concave lens of focal length f2. Write the focal length of the combination. 5. Write two advantages of connecting cells in parallel. (1) (1) SECTION B 6. The inputs in the form train of pulses are given at A and B of the following combination of gates. Write the necessary supporting truth table and hence sketch the out wave form. (2) A A Y Y B B 7. A step down transformer converts voltage of 2200V to 220V in the transmission line. Number of turns in the primary coil is 5000. Efficiency of the transformer is 90% and its output power is 8kW. Determine (i) number of turns of the secondary coil (ii) input power (2) 8. Which mode of propagation of waves is suitable for television broadcasting and satellite communication and why? Draw suitable diagram depicting this mode of propagation. (2)

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