## Saturday, 26 July 2014

### CLASS 12 PHYSICS FOR PRACTICE1

Practice Test Paper 1

Physics (Theory) – XII

Time allowed: 3 hours                                                                                                 Maximum marks: 70  Q. 1. An electric dipole of dipole moment 20x10-6 C.m is enclosed by a closed surface. What is the net flux coming out of the surface? 1 Q. 2. A bulb and a capacitor are connected in series to an a.c. source of variable frequency. How will the brightness of the bulb change on increasing the frequency of the a.c. source? Give reason. 1 Q. 3. Write one condition under which an electric charge does not experience a force in a magnetic field. 1 Q. 4. Name the type of biasing of a p-n junction diode so that the junction offers very high resistance. 1 Q. 5. What should be the length of the dipole antenna for a carrier wave of frequency 3x108 Hz? 1 Q. 6. Draw V-I graph for ohmic and non-ohmic materials. Give one example for each. 2 Q. 7. How does the resistivity of (a) a conductor and (b) a semiconductor vary with temperature? Give reason for each case. 2 Q. 8. State the condition under which the phenomenon of resonance occurs in a series LCR circuit. Plot a graph showing variation of current with frequency of a.c. source in a series LCR circuit. 2 Q. 9. Explain the term (a) pulse-amplitude modulation (PAM) and (b) pulse-code modulation (PCM). 2 Q. 10. An electron and a proton have the same kinetic energy. Which of the two has a greater wavelength? Explain. 2 Q. 11. Explain, with the help of a circuit diagram, how the thickness of depletion layer in a p-n junction diode changes when it is forward biased. 2 Q. 12. An electron and a proton are moving in the same direction and possess same kinetic energy. Find the ratio of de Broglie wavelengths associated with these particles. 2 Q. 13. Define the term ‘resistivity’ and write its S.I. unit. Derive the expression for the resistivity of a conductor in terms of number density of free electrons and relaxation time. 3 Q. 14. A figure divided into squares, each of size 1 mm2 is being viewed at a distance of 9 cm through a magnifying lens of focal length 10 cm, held close to the eye. (a)    Draw a ray diagram showing the formation of the image. (b)   What is the magnification produced by the lens? How much is the area of each square in the virtual image? What is the angular magnification of the lens? 3 Q. 15. Two cells of emf 1.5 V and 2 V and internal resistance 1 ohm and 2 ohm respectively are connected in parallel to pass a current in the same direction through an external resistance of 5 ohm. (a)    Draw the circuit diagram. Using Kirchhoff’s laws, calculate the current through each branch of the circuit and potential difference across the 5 ohm resistor. 3 Q. 16. Deduce an expression for the capacitance of a parallel plate capacitor with air as the medium between the plates. 3 Q. 17. Define the term ‘resistivity’ and ‘conductivity’ and state their S.I. units. Draw a graph showing the variation of resistivity with temperature for a typical semiconductor. 3 Q. 18. A system has two charges qA = 2.5x10-7 C and qB =-2.5x10-7 C located at points A (0,0,-15cm) and B(0,0,+15cm) respectively. Calculate the electric dipole moment of the system. What is its direction? 3 Q. 19. How will a dia-, para- and a ferro-magnetic material behave when kept in a non-uniform external magnetic field? Give one example of each of these materials. 3 Q. 20. The sequence of stepwise decays of a radioactive nucleus is     D D1 D2 D3 If the nucleon number and atomic number for D2 are 176 and 71 respectively, what are the corresponding values of D and D3? Justify your answer in each case. 3 Q. 21. Name the radiations of electromagnetic spectrum which are used in (a)    Warfare to look through fog. (b)   Radar and geostationary satellites. (c)    Studying the structure and properties of atoms and molecules. 3 Q. 22. What is an optical fibre? Explain with the help of a diagram, how light can travel through it and remain confined to the fibre. 3 Q. 23. A radioactive sample contains 2.2 mg of pure , which has half-life period of 1224 seconds. Calculate (a) the number of atoms present initially, and (b) the activity when 5 mg of the sample will be left. 3 Q. 24. Draw a schematic diagram of a single optical fibre structure. Explain briefly how an optical fibre is fabricated. Describe in brief, the mechanism of propagation of light signal through an optical fibre. 3 Q. 25. Define the term electric field intensity. Write its SI unit. Derive an expression for the electric field intensity at a point on the axis of an electric dipole. Or State Gauss theorem in electrostatics. Use it to obtain an expression for the electric field intensity at a point near a uniformly charged infinite plate sheet. 5 Q. 26. Draw a labelled diagram of a compound microscope when the image is formed at infinity. Deduce the expression for its magnifying power. How can the magnifying power be increased? Or Draw a labelled diagram to show the image formation in an astronomical telescope. Write an expression for its magnifying power. 5 Q. 27. (a) Distinguish between metals, insulators and semiconductors on the basis of their energy bands. (b) A semiconductor has equal electron and hole concentration of 6x108 /m3. On doping with certain impurity, electron concentration increases to 9x1012/m3. (i)                 Identify the new semiconductor obtained after doping (ii)               Calculate the new hole concentration Or (a)    Draw a circuit diagram of a p-n-p transistor as an amplifier in a common emitter configuration. (b)   Calculate the voltage gain for a transistor having a current gain 40, collector resistance 6 kilo-ohm and input resistance 1 kilo-ohm. 5