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KENDRIYA VIDYALAYA SANGATHAN (RAIPUR REGION) FIRST PRE BOARD EXAMINATION 2012-2013 QUESTION PAPER PHYSICS – XII SET I Time – 3 hours MM – 70 General Instructions: a) All questions are compulsory. b) There are30 questions in total. Questions 1 to 8 are very short answer type questions and carry one mark each. c) Questions 9 to 18 carry two marks each, questions. 19 to 27 carry three marks each and questions 28 to 30 carry five marks each. d) There is no overall choice. However, an internal choice has been provided in one question of two marks, one question of three marks and three questions of five marks each. You have to attempt only one of the given choices in such questions. e) Use of calculators is not permitted. However, you may use log tables if necessary. f) You may use following physical constants whenever necessary. c= 3x108 m/s h=6.63x10-34Js e=1.6x10-19C μo=4x10-7TmA-1 1 = 9x109 Nm2C-1 4πEo 1. 2. 3. Two point charges placed at a distance r in air exert a force F on each other. At what distance will these charges experience the same force F in a medium of dielectric constant K ? Explain radial magnetic field. Fig. 1 shows a horizontal solenoid connected to a battery and a switch. A copper ring is placed on a frictionless track, the axis of the ring being along the axis of the solenoid. What happens to the ring, as the switch isclosed ? Figure 1 4. 5. Draw a sketch showing the incident, reflected and transmitted rays when light is incident at the polarising angle on a glass slab. What modification was made by Maxwell in Ampere circuital law? 6. An electron and alpha particle have the same de-Borglie wavelength associated with them. How are their kinetic energies related to each other? 7. The radioactivity of the sample is R1 at timet1 andR2 at time t2. The mean life of the sample is . What is the number of nuclei that have disintegrated in the time interval (t1 – t2)? 8. If the base region of a transistor is made large, as compared to a usual transistor, how does it affect (i) the collector current and (ii) current gain of this transistor? 9. A slab of material of dielectric constant has the same area as the plates of a parallel plate capacitor but has a thickness 3d / 4, where d is the separation of the plates. How is the capacitance changed when the slab is inserted between the plates? 10. State two properties of the material of wire used for suspension of coil in a moving coil galvanometer? 11. Meera lives in a town where the government is providing free electricity that is why most families in her locality waste electricity. They use traditional bulbs, use electric heaters for cooking, leave thier fans running even though no one is using the room. Meera one day attends a seminar on the conservation of electricity in her school. There she comes to know that there are many villages in her state without electricity. She convinces her father to replace traditional bulbs with CFL bulbs and conserve electricity and requests her father to convince his friends for the same. Slowly everyone in the locality takes inspiration from Meera’s family and start conserving electricity so that the surplus electricity could be used for lighting un electrified rural area. What according to you, are the values displayed by Meera and her family, friends and neighbours to help her uncle ? 12. How does the mutual inductance of a pair of coils change when 1) Distance between the coils is increased. 2) No. of turns of the coil is decreased. 13. 14. The electric potential as a function of distance x is shown in Fig. 3. Construct a graph of the electric field strength E. Fig. 3 An inductor 'L' of reactance XL, is connected in series with a bulb 'B' to an a.c. source as shown in Fig. 4 Fig. 4 Briefly explain how does the brightness of the bulb change, when (i) number of turns of the inductor is reduced and (ii) a capacitor of reactance XC=XL is included in series in the same circuit. A bulb B and a capacitor C are connected in series to the a.c. mains as shown in Fig. 5. 15. 16. 17. 18. 19. 20. 21. Figure 5 The bulb glows with some brightness. How will the glow of the bulb change when a dielectric slab is introduced between the plates of the capacitor? Give reasons in support of your answer. State one use and one method of detecting the rays beyond the visiblered end of the electromagnetic spectrum. Explain why does a convex lens behave as a converging lens when immersed in water (µ=1.33) and as a diverging lens, when immersed in carbon disulphide (µ=1.6). A radioactive isotope has a half life of T years. After how much time is its activity reduced to 25% of its original activity? Why are sky waves not used in transmission of T.V signals? State the Huygens principle .Prove Snell,s laws of refraction on the basis of Huygens principle. A parallel plate capacitor, each with plate area A and separation d, is charged to a potential difference V. The battery used to charge it is then disconnected. A dielectric slab of thickness d and dielectric constant is now placed between the plates. What change, if any, will take place in : (i) charge on plates, (ii) electric field intensity between the plates (iii) capacitance of the capacitor Justify your answer in each case. Or Define electric potential. Derive an expression for the electric potential at a distance r from a point charge q. Two convex lenses A and B of an astronomical telescope having focal lengths 5 cm and 20 cm respectively, are arranged as shown in the Fig. 6. (i) (ii) Fig.6 Which one of the two lenses you will select to use as the objective lens and why? What should be the change in the distance between the lenses to have the telescope in its normal adjustment position? (iii) 22. 23. 24. Calculate the magnifying power of the telescope in the normal adjustment position. You are given two circuits as shown in Fig. 7, which consist of NAND gates. Identify the logic operation carried out by the two circuits. Fig. 7 Show that the de-Broglie wavelength of electrons accelerated through a potential difference of V volts can be expressed as Derive an expression for the power dissipated in LCR series circuit,hence define power factor. 25. Calculate binding energy per nucleon of an alpha particle from the following data: Mass of free proton =1.00785 a.m.u. Mass of free neutron =1.008665 a.m.u. Mass of helium nucleus =4.002800 a.m.u. 1 a.m.u. =931.5MeV. 26. 27. 28. A transmitting antenna at the top of a tower has a height 32 m and that of the receiving antenna is 50 m. What is the maximum distance between them for satisfactory communication in LOS mode? Given radius of earth 6.4 x 106 m. Draw the input and output characteristic curve of a transistor in common emitter configuration. Explain the meaning of its cut off and saturation regions. For what practical purpose, do we use the transistor in these regions? State Kirchhoff's laws for an electrical network. Using Kirchhoff's laws, find the relation between the resistances of four arms of a Wheatstone bridge when the bridge is balanced. Draw a circuit diagram to determine the unknown resistance of a metallic conductor using a metre bridge. Or 29. Define the term potential gradient. Using this concept, explain the method for comparison of emfs of two primary cells using a potentiometer. Establish the relation used. Write two possible causes of potentiometer giving only one-sided deflection. Write an expression for the force experienced by a charged particle moving in a uniform magnetic field B. With the help of a diagram, explain the principle and working of a cyclotron. Show that cyclotron frequency does not depend on the speed of the particle. Or 30. Derive an expression for the torque acting on a loop of N turns, areaA, carrying current I, when held in a magnetic field B. With the help of a circuit diagram, show how a moving coil galvanometer can be converted into an ammeter of given range. Write the necessary mathematical formula. With the help of a ray diagram explain the phenomenon of total internal reflection. Obtain the relation between critical angle and the refractive index of the medium. Draw ray diagrams to show how a right angled isosceles prism can be used to (i) deviate the ray through 180°, (ii) deviate the ray through 90°, and (iii) invert the ray. Or With the help of a ray diagram, show the formation of image of a point object by refraction of light at a spherical surface separating two media of refractive indices n1 and n2 (n2>n1) respectively. Using this diagram, derive the relation. n2 n1 n2 n1 u R Write the sign conventions used. What happens to the focal length of convex lens when it is immersed in water? **********