## Saturday, 26 July 2014

### CLASS 12 PHYSICS TEST PRACTICE 4

Practice Test Paper 4

Physics (Theory) – XII

Time allowed: 3 hours                                                                                                 Maximum marks: 70  Q. 1. An electrostatic field line cannot be discontinuous. Why? 1 Q. 2. The force experienced by a particle of charge q moving with a velocity in a magnetic field is given by = q( x ). Which pairs out of these vectors are always at right angles to each other? 1 Q. 3. Why does the electric field inside a dielectric decrease when it is placed in an external electric field? 1 Q. 4. Electrons are emitted from a photosensitive surface when it is illuminated by green light but electron emission does not take place by yellow light. Will the electrons be emitted when the surface is illuminated by (a) red light, and (b) blue light? 1 Q. 5. Name the process by which exact reproduction of a document at a distant place can be received. 1 Q. 6. Define the term ‘temperature coefficient of resistivity’. Show graphically the variation of resistivity with temperature for nichrome. 2 Q. 7. Define ‘electric line of force’ and give its two important properties. 2 Q. 8. The image of a candle is formed by a convex lens on a screen. The lower half of the lens is painted black to make it completely opaque. Draw the ray diagram to show the image formation. How will this image be different from the one obtained when the lens is not painted black? 2 Q. 9. Write two characteristic properties to distinguish between diamagnetic and paramagnetic materials. 2 Q. 10. A circular coil of radius 8 cm and 20 turns rotates about its vertical diameter with an angular speed of 50 s-1 in a uniform horizontal magnetic field of magnitude 3x10-2T. Find the maximum and average value of the emf induced in the coil. 2 Q. 11. Write two conditions necessary for total internal reflection to take place. 2 Q. 12. Draw a labelled circuit diagram to show the use of zener diode as a voltage regulator. 2 Q. 13. A galvanometer with a coil of resistance 120 ohm shows full scale deflection for a current of 2.5 mA. How will you convert the galvanometer into an ammeter of range 0 to 7.5 A? Determine the net resistance of the ammeter. When an ammeter is put in a circuit, does it read slightly less or more than the actual current in the original circuit? Justify your answer. 3 Q. 14. Give reasons for the following: (a)    Long distance radio broadcasts use short-wave bands. (b)   The small ozone layer on top of the stratosphere is crucial for human survival. (c)    Satellites are used for long distance TV transmission. 3 Q. 15. Deduce an expression for the capacitance of a parallel plate capacitor with air as the medium between the plates. 3 Q. 16. With the help of a circuit diagram explain in brief the use of a potentiometer for comparison of emf of two cells. 3 Q. 17. For a common emitter transistor amplifier, the audio signal voltage across the collector resistance of 2 kΩ is 2 V. If the current amplification factor of the transistor is 100, calculate (a)    input signal voltage, (b)   base current, and (c)    power gain. Given that the value of the base resistance is 1 kΩ. 3 Q. 18. Using Huygen’s principle, explain the refraction of a plane wavefront at a plane surface. Hence prove Snell’s law. 3 Q. 19. Write the nuclear reactions for the following: (a)    a-deacy of (b)   b- - decay of (c)    b+ - decay of 3 Q. 20. A converging lens has a focal length of 20 cm in air. It is made of material of refractive index 1.6. If it is immersed in a liquid of refractive index 1.3, what will be its new focal length? How does the nature of the lens change if this lens is immersed in a liquid of refractive index 1.8? 3 Q. 21. Write two applications each of (a)    microwaves, (b)   infra-red waves, and (c)    radio waves. 3 Q. 22. Mention the significance of Davisson-Germer experiment. An a-particle and a proton are accelerated from rest through the same potential difference V. Find the ratio of de-Broglie wavelengths associated with them. 3 Q. 23. (a)    Draw a labelled ray diagram to show the formation of an image by a compound microscope. Write the expression for its magnifying power. (b)   How does the resolving power of a compound microscope change, when   (i)                 refractive index of the medium between the object and the objective lens increases, and (ii)               wavelength of the radiation used is increased? 3 Q. 24. Distinguish between analog and digital communication. Write any two modulation techniques employed for the digital data. Describe briefly one of the techniques used. 3 Q. 25. A dielectric slab of thickness ‘t’ is kept in between the plates, each of area ‘A’, of a parallel plate capacitor separated by a distance ‘d’. Derive an expression for the capacitance of this capacitor for t<