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CBSE XII Pre Boards 2015 : PHYSICS with Answers (KV Kolkata)

5 pages, 38 questions, 21 questions with responses, 31 total responses,

Kendriya Vidyalaya No.1 Salt Lake
Kendriya Vidyalaya (KV) No. 1, Salt Lake

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KENDRIYA VIDYALAYA SANGATHAN-KOLKATA REGION 3rd PRE-BOARD EXAMINATION: 2014-15 SUB: PHYSICS TIME ALLOWED: 3 HOURS M.M : 70 Please check that this question paper contains 26 questions. Please write down the Serial Number of the question before attempting it. 15 minutes time has been allotted to read this question paper. General Instructions: (i) (ii) (iii) (iv) (v) (vi) All questions are compulsory. There are 26 questions in total. Questions 1 to 5 carry one mark each, questions 6 to 10 carry two marks each, questions 11 to 22 carry three marks each and questions 24 to 26 carry five marks each. Question number 23 is a value based question and carries 4 marks. There is no overall choice. However, an internal choice has been provided in one question of two marks, one question of three marks and all three questions of five marks each. You have to attempt only one of the given choices in such questions. Use of calculators is not permitted. However, you may use the log tables, if necessary. You may use the following values of physical constants wherever necessary : c = 3 X 10 8 m/s h = 6.63 X 10 -34 Js e = 1.6 X 10 -19 C 0 X -7 0 = 8.85 x 10-12 N m2 C-1 T m A-1 0 = 9 X 109 N m2 C-2 Mass of electron me = 9.1 X 10-31 Kg Mass of proton mp = 1.67 X 10-27 Kg Mass of neutron mn = 1.675 X 10-27 Kg Q1. Can electrostatic potential at a point, be zero, while electric field at that point is not zero? Justify your answer. Q2. A conductor of length L is connected to a d.c. source of potential V. If the length of the conductor is tripled by gradually stretching it, keeping V constant, how will drift speed of electrons be affected? Q3. No force is exerted by a stationary charge when placed in a magnetic field. Why? (1) Q4. A concave mirror is held in water. What should be the change in focal length of the mirror? (1) Q5. An alpha particle and a proton are accelerated from rest through the same potential difference V. Find the ratio of de- Broglie wavelength associated with them. (1) Q6. A network of six identical capacitors, each of capacitance c is made in figure. Find the equivalent capacitance between the pointsA and B as shown in figure. A B (2) Q A of current. I ).Find the frequency and the rms value (2) Q8. State Malus law. Sketch the graph showing the variation of intensity of transmitted light on the angle of rotation between a polariser and analyser. (2) Q9. Draw a circuit diagram of a full wave rectifier. Explain the working principle. (2) Q10. Define the terms amplitude modulation and frequency modulation (2) Q11.(i) Distinguish between emf and terminal potential differenc of a cell. (3) (ii) A storage battery of emf 8 V and internal resistance 0.5 ohm is being charged by a 120 V d.c. supply using a series resistor of 15.5 ohm. What is the terminal voltage of the battery during charging? Q S K Use it to prove the wheat stone bridge principle. (3) Q13. A straight horizontal conducting rod of length 0.45 m and mass 60 g is suspended by two vertical wires at its ends. A current of 5 A is set up in the rod through the wires. What magnetic field should be set up normal to the conductor in order that the tension in the wires is zero? ( g = 9.8 m/s2) (3) OR Derive an expression for the magnetic dipole moment of an electron revolving around a nucleus in terms P (3) e) Q14. (i) Define mutual inductance of two coils. Write its SI unit. (3) (ii) How does the mutual inductance of a pair of coils change when a.Distance between the coils is increased and b.Number of turns in the coils is increased? Q15.State Faraday Lenz s laws of electromagnetic induction. Show that Lenz;s law does not violate the law of conservation of energy. (3) Q16. Draw the diagram showing the propagation of an electromagnetic wave in (3) x-direction and mark the electric and magnetic field vectors.. A capacitor, made of two parallel plates each of plate area A and separation d, is being charged by an external a.c. source. Write the expression for displacement current inside the capacitor . Q17.Draw the ray diagram of a refracting telescope in normal adjustment. Write any two advantages of reflecting telescope over refracting telescope. (3) Q18.State the conditions for total internal reflection to occur. (3) Calculate the speed of light in the medium wose critical angle is 45 0. Q19. Draw a graph showing the variation of stopping potential with the frequency of incident radiation in relation to photo electric effect. Find the expression for the slope of this graph using appropriate equation. (3) Q20.(i) Find a relation between half-life and disintegration constant (3) (ii) A radioactive isotope has a half life of T years. How long will it take the activity to reduce to 3.125% of its original value? Q21.Using Bohrs postulates find the expression for radius of the nth orbit of a revolving of an atom. Hence write the expression for Bohrs radius. (3) Q22. What is space wave propagation? Which two communication methods make use of this mode of propagation? What does the term LOS communication mean? (3) Q23. While performing an experiment on determination of unknown resistance using a meter bridge, Rahul obtained deflection in the galvanometer in the same direction even after repeated adjustments in the circuit and thus could not get any results. In order to avoid getting noticed and scolded by the teacher, he pretended having performed the experiment and copied the readings obtained by another student. Answer the following questions based on above information: (i)Write two possible reasons for getting the deflection in the galvanometer in the same direction. (ii)Which two values is Rahul violating in copying the readings from another student? (4) Q24. Derive an expression for the energy stored in a parallel plate capacitor C, charged to a potential difference V. Hence derive an expression for the energy density of a capacitor. OR U G spherical shell of radius R at a point (i) Outside the spherical shell (ii) On the surface of the spherical shell (iii) Inside the spherical shell Draw a graph showing the variation of electric field with r, for r >R and r < R. (5) Q D I monochromatic ray of light passing through a prism of refracting angle A. Derive an expression for refractive index of material of a prism in terms of the angle of the prism (A) and angle of minimum deviation. OR Draw a ray diagram for formation of image of a point object by a thin double convex lens having radii of S curvature R1 and R2 H made and sign convention used. (5) Q26. Explain the principle, working and theory of a transformer by using its diagram. Also, explain the use of transformer in long distance transmission of electric power with the help of schematic diagram. OR Draw a labelled diagram of AC generator; explain its principle and working An armature coil consists of 20 turns of wire, each of circular area A=0.09 m 2 and total resistance 15 ohm. It rotates in a magnetic field of 0.5 Tesla H C maximum emf produced in the coil. (5) KENDRIYA VIDYALAYA SANGATHAN, KOLKATA REGION 2nd PRE BOARD EXAMINATION 2014-2015 CLASS-XII, SUBJECT: PHYSICS MARKING SCHEME TIME: 3 HOURS MAXIMUM MARKS:70 Question Answer key Value No. points 1 Yes, the electric potential due to electric dipole at any point on its + perpendicular bisector is zero whereas electric field intensity is not. 2 + Drift velocity =Vd = , drift velocity becomes one third 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 M B force =0 No change 1:2 2 The equivalent capacitance is the resultant of parallel combination two rows in which each row consists of 2C and C in series. So equivalent capacitance = 4C/3 F HZ R.m.s value of current = I0/(2)1/2 = 25(2)1/2 Statement of Malus law Sketching the graph Circuit diagram, working Correct definition of amplitude modulation and frequency modulation EMF and terminal pd = =7 A I= V = E + Ir , V =11.5 volt C K Proof BIL = Mg , BIL = Magnetic force, Mg = weight of rod B = 0.26 Tesla OR Expression of magnetic dipole moment of revolving electron = M = I 2 C I T T electron M = evr/2 Angular momentum of electron = L = mevr M M e Definition , SI unit Mutual inductance decreases, Mutual inductance increases Laws Correct answer with diagram Labelled Diagram Expression displacement current Labelled ray diagram advantages Two conditions Formula and answer 2.12 x 108 m/s + 1 1 1+1 1+1 1+1 1+1 1 + + 2 + 2 3 3 + 1+1 1 2 2 1 2 1 1 2 19 20 21 22 23 24 25 26 Graph of stopping potential with the frequency of incident radiation E Expression for the slope of the graph Relation Activity = = = = =(1/2)5 Total time =t = 5T, because number of half-lives = n = 5 Radius of nth orbit B It is the mode of propagation in which wave (above 40 MHZ) propagates in a straight line from the transmitting antenna to the receiving antenna. Two communication methods (i) Line of sight communication (ii) Satellite communication In line of sight communication, the space waves travel directly from transmitting antenna to receiving antenna. (i) Two possible reasons ii) Honesty, truthfulness, sincerity, desire to learn more and improve. Figure Correct derivation to get the expression for the energy stored in a parallel plate capacitor. Correct derivation to get expression for the energy density of a capacitor. OR Expression for the electric field due to a uniformly charged thin spherical shell of radius R at a point (i) Outside the shell (ii) On the surface of the shell (iii) Inside the shell Graph of electric field with r, for r > R and r < R Graph of angle of deviation with angle of incidence Ray diagram of refraction through a prism Correct derivation of the expression for refractive index of material of a prism in terms of the angle of prism and angle of minimum deviation. OR Correct ray diagram. D L ula. Assumptions and sign conventions. Diagram Principle Theory Working Correct explanation OR Diagram Principle Working M NBA Maximum emf = 270 volt 1 1.5 1.5 1.5 2 1 1 + 1 2 2 2.5 2 OR 1 1 1 1+1 1 1 3 OR 1 2 2 1 1.5 1 1 OR 1 1 1 1 1

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Additional Info : KV Sangathan, Kolkata Region PHYSICS Pre-Board
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