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CBSE Pre Board Class 12 2019 : Physics (Alagar Public School, Thoothukudi)

5 pages, 60 questions, 24 questions with responses, 27 total responses,

Raja Durai
Alagar Public School, Thoothukudi

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ALAGAR PUBLIC SCHOOL ACADEMIC YEAR 2 0 1 8 - 2 0 1 9 PREBOARD I EXAMINATION CLASS : XII DATE : 30.11.18 PHYSICS MARKS: 70 General Instruction : TIME: 3HRS All questions are compulsory. There are 27 questions in all. Section A contains 5 questions of one mark each, Section B contains 7 questions of two marks each, Section C contains 12 questions of three marks each, Section D contains 3 questions of five marks each. There is no overall choice. However, an internal choice has been provided in 1 question of 2 marks, 1 question of 3 marks and all the 3 questions of 5 marks weightage. You have to attempt only one of the choices in such questions. SECTION A 1. Where on the surface of Earth is the vertical component of earth magnetic field zero? 2. A bar magnet is moved in the direction indicated by the arrow between two coils PQ and CD. Predict the direction of the induced current in each coil. 3. Why soft iron is used in the fabrication of core of a transformer? 4. A radio can tune into any station in the 7.5MHz to 12MHz band. What is the corresponding wavelength band? 5. What is the Brewster angle for air to glass ( =1.5) transition? SECTION B 6. State the Ampere s circuital law. 7. Two identical loops, one of copper and the other of aluminium are rotated with the same speed, in s uniform magnetic field acting normal to the plane of the loops. State with reason, for which of the coils (i) induced emf (ii) induced current, will be more. 8. What do you mean displacement current ? Give its S.I. unit. (OR) Define the term wattless current . Class 12/PREBOARD 1 EXAM/2018-2019/PHYSICS Page 1 9. A resistor of 100 and and a capacitor of 100 F are connected in series to a 220 V, 50 Hz a.c. supply. i) Calculate the current in the circuit. ii) Calculate the (rms) voltage across the resistor and the capacitor. Do you find the algebraic sum of these voltages more than the source voltage? If yes, how do you resolve the paradox ? 10. Electromagnetic radiation with wavelength i) 1 are used to kill germs in water purifiers ii) 2 are used in T.V communication system iii) 3 are used in important role in maintaining the earth s warmth. iv) 4 are used for cooking purposes. Name the part of electromagnetic spectrum to which these radiation belong. 11. State two conditions for sustained interference of light. 12. What is zener diode? How is it represented symbolically? SECTION C 13. Use Biot-Savart law to derive the expression for the magnetic field on the axis of a current carrying circular loop of radius R. Draw the magnetic field lines due to a circular wire carrying current I. 14. a) Draw a labeled diagram of an ac generator and state its working principle. b) Derive the expression for maximum value of the induced emf. c) Show the variation of the emf generated versus time as the armature is rotated with respect to the direction of the magnetic field. 15. A source of ac voltage V = V o Sin t, is connected across a pure inductor of inductance L. Derive the expression for instantaneous current in the circuit. Show that average power dissipated in the circuit is zero. Establish phase relationship between the applied emf and current in the circuit. 16. a) Define the following using suitable diagrams : i) magnetic declination ii) angle of dip b) In what direction will a compass needle point when kept at the Class 12/PREBOARD 1 EXAM/2018-2019/PHYSICS Page 2 i) poles ii) equator 17. a) How are the following types of electromagnetic waves produced : i) Microwaves ii) Infrared waves Write two important uses of each of the above electromagnetic radiations. b) i) Name the electromagnetic waves that are widely used as a diagnostic tool in medicine. ii) The current in a circuit containing a capacitor is 0.15 A. What is the displacement current and where does it exist? 18. (i) Name the wave in electromagnetic spectrum is referred to as Heat waves . (ii) State the four Maxwell s equations and also write those name of the law mentioned. 19. In YDSE, the slits are separated by 0.28mm and the screen is placed 1.4m away. The distance between the central bright fringe and the fourth bright fringe is measured to be 1.2cm. Determine the wavelength of light used in the experiment. 20. (i) Two independent sources of light cannot be coherent, why? Two coherent sources have intensities in the ratio 25:16. Find the ratios of the intensities of maxima and minima after interference. 21. (i) Draw a circuit diagram for a pn-junction diode in forward and reverse bias. Sketch the V-I graphs for them. (ii) What happens to the width of depletion layer of pn-junction when it is reverse biased? 22. State and define various logic gates. Draw the logic symbols and write the truth table for them. 23. Draw a block diagram of a simple modulation for obtaining an AM signal. Briefly explain how amplitude modulation is achieved? 24. Figure shows a metal rod PQ resting on the smooth rails AB and positioned between the poles of a permanent magnet. The rails, the rod and the magnetic field are in three mutual perpendicular directions. A galvanometer G connects the rails through a switch K. Length Class 12/PREBOARD 1 EXAM/2018-2019/PHYSICS Page 3 of the rod = 15 cm, B = 0.50 T, resistance of the closed loop containing the rod = 9.0 m . Assume the field to be uniform. i) Suppose K is open and the rod is moved with a speed of 12 cm/s in the direction shown. Give the polarity and magnitude of the induced emf. ii) What is the retarding force on the rod when K is closed ? iii) What is the induced emf in the moving rod if the magnetic field is parallel to the rails instead of being perpendicular? (OR) A metallic rod of length l is rotated with a frequency with one end hinged at the centre and the other end at the circumference of a circular metallic ring of radius l, about an axis passing through the centre and perpendicular to the plane of the ring. A constant and uniform magnetic field B parallel to the axis is present anywhere. i) Using Lorentz force, explain how emf is induced between the centre and the metallic ring and hence obtained the expression for it. ii) Calculate the current in the rod. SECTION D 25. a) Define self induction. Write its S.I unit. b) Derive an expression for the mutual inductance of two long co-axial solenoids of same length wound one over the other. c) A pair of adjacent coils has a mutual inductance of 1.5 H. If the current in one coil changes from 0 to 20 A in 0.5 s, what is the change of flux linkage with the other coil? (OR) a) Two long straight parallel conductors carry steady currents I 1 and I2 separated by a distance d. If the currents are flowing in the same direction, show how the magnetic field setup in one produces an attractive force on the other. Obtain an expression for this force. Hence define one ampere. b) A rectangular loop of wire size 2.5 cm X 4 cm carries steady current of 1 A. A straight wire carrying 2 A current is kept near the loop as shown. If the loop and the wire are coplanar, find the (i) torque acting on the loop and (ii) the magnitude and direction of the force on the loop due to the current carrying wire. Class 12/PREBOARD 1 EXAM/2018-2019/PHYSICS Page 4 2cm 26. (i) State Huygens postulate of wave theory. Sketch the wave front emerging from a linear source of light like a slit. (ii) Describe YDSE to produce interference pattern due to a monochromatic source of light. Deduce the expression for the fringe width. (OR) Explain the phenomenon of diffraction of light at a single slit. Show graphically the variation of intensity with angle in the diffraction pattern. 27. (i) What is meant by rectifier? (ii) With the help of circuit diagram, explain the working of half wave rectifier. (iii) Draw its input and output wave forms. (OR) Draw the circuit diagram for transfer characteristics of a base biased transistor in CE mode configuration. Explain briefly the meaning of cut off, active and saturation region. Referring to the diagram explain transistor as a switch. ******* ALL THE BEST ******* Class 12/PREBOARD 1 EXAM/2018-2019/PHYSICS Page 5

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Additional Info : Class 10 CBSE Pre Board 2019 : Physics (Alagar Public School, Thoothukudi)

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