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KENDRIYA VIDYALAYA SANGATHAN: HYDERABAD REGION ND
2 PRE- BOARD EXAMINATION (2015-16) (2015-16) CLASS: XII
MAX. MARKS: 70
SUBJECT: PHYSICS
TIME 3hrs
General Instructions:
(i)
All questions are are compulsory. compulsory.
(ii)
There are 26 questions in total. Questions 1 to 5 are very short answer type questions and carry one mark each.
(iii)
Questions No. 6 to 10 carry carry two marks each, questions 11 to 22 carry three marks each, question 23 is a value based question that carries four marks and questions 24 to 26 are long answer type type and carry five marks each.
(iv)
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 weightage. You have to attempt only one of the choices in such questions.
(v)
Use of calculators is not permitted. However, However, you you may use log tables tables if necessary. necessary.
(vi)
You may use the following values of physical constants wherever necessary: 8
c = 3 X 10 m/s -34
h = 6.63 X 10 Js -7
-1
µo = 4π X 10 T mA
9
2 -2
= 9 X 10 N m C
(1) Two identical balls having equal positive charge ‘q’ coulombs are suspended by two insulating strings of equal length. What would be the effect on the force when a plastic sheet is inserted between the two?
(1)
(2) Two identical heaters rated 220V, 1000W are placed in series with each other across a 220V line. What is their combined power?
(1)
(3) Why is the use of a.c voltage preferred over d.c voltage? Give two reasons.
(1)
(4) Two metals A and B have work functions 5eV and 8 eV respectively. Which metal has a lower threshold wavelength for photoelectric effect?
(1)
(5) What is the frequency of the output signal of (i) Half wave rectifier and (ii) full wave rectifier, if the frequency of the input signal is 50Hz.
(1)
6. An electric dipole is held in a uniform electric field. The dipole is aligned parallel to the o
field. Find the work done in rotating through the angle 180 ?
(2)
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7. A circular coil of ‘N’ turns and radius ‘R’ carries a current ‘I’. It is unwound and rewound to make another coil of radius
, current ‘I’ remaining the same. Calculate the ratio of
the magnitude of the magnetic moment of the new coil and the original coil? (OR) An alpha particle and a proton are moving in the plane of paper in a region where there is a magnetic field B directed normal to the plane of the paper. If the particles have equal momenta, what would be the ratio of radii of their trajectories in the field?(2) 8. Show that a pure inductor does not consume electric power when con nected across an a.c source.
(2)
9. Using Rutherford’s model of an atom, derive expression for the total energy of the electron in Hydrogen atom. What is the significance of total negative energy possessed by the electron?
(2)
10. A modulating signal is a square wave as shown in the fi gure below. The carrier wave is given by C(t) = 2 sin (4πt) volts.
m(t) in volts
1
2 t in sec
A. Sketch the amplitude modulated wave for the above message signal. B. What is the modulation Index?
(2)
11. Answer the following : (a) Why are the connections between the resistors in a metre bridge made of thick copper strips?
(1)
(b) Why is it generally preferred to obtain the balance point in the middle of the metre bridge wire? (c) Which material is used for the metre bridge wire and why?
(1) (1)
12. Define relaxation time of the free electrons drifting in a conductor. How is it related to the drift velocity of the free electrons? Use this relation to deduce the expression for the electrical resistivity of the material?
(3)
13. How will a dia, para and ferro magnetic materials behave when kept in a non -uniform external magnetic field? Give one example of each of these materials?
(3)
14. State Ampere’s circuital law. Use this to determine the magnetic field due to a straight infinitely long wire carrying current “I” at a distance “R” from the wire. Also draw the variation of the magnetic field with distance “r” for r>a and for r
diameter of the wire.
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(OR) State Biot- Savart Law. Use this to determine the magnetic field due to a circular coil carrying current at a point on the axial line of the coil. 15. Name the constituent radiation of the electromagnetic spectrum which: (a) Produce intense heating effect (b) Is absorbed by Ozone layer in the atmosphere (c) Is used for studying crystal structure. Write at least one more application of each.
(3)
16. In Young’s double slit experiment, the two slits 0.15mm apart are i lluminated by monochromatic light of wavelength 450 nm. The screen is 1.0 m away from the slits. nd
(a) Find the distance between the 2 bright and dark fringes from the central bright? (b) How will the fringe width pattern change, if the screen is moved away from the slits?
(3) 0
17. Two polaroids A and B are placed 90 to each other and the transmitted intensity is zero. What happens if another polaroid C is placed between these two bisecting the angle between them. Take the intensity of the unpolarised light I0. How will the intensity of the transmitted light vary on further rotating the third Polaroid.
(3)
18. Draw the ray diagram of compound microscope. Write the expression for its magnifying power if image is formed at normal adjustment.
(3)
19. Draw a graph showing the variation of the stopping potential with the frequency of incident radiation in relation to photoelectric effect. Deduce an expression for the slope of this graph using Einstein’s Photo-electric equation.
(3)
20. Write any two distinguishing features between conductors, semiconductors and insulators on the basis of energy band diagrams.
(3)
(21) Draw the circuit diagram for the common emitter amplifier using npn transistor. Prove that the output signal is out of phase with the input signal.
(3)
(22) Draw a schematic sketch showing the (i) ground wave (ii) Sky wave and (iii) Space wave modes of propagation. Write the frequency range of these modes of propagation?(3) (23) Mr.Raju, a daily wages worker, got affected by cancer. On knowing about it, all his co-workers started avoiding him, fearing that it was contagious. Mr.Raju felt very depressed. Mr.Rahul, a close friend, immediately took Mr.Raju to radiologist who examined him and said it was the beginning stage of cancer and it can be easily cured and he also certified that it is not a communicable disease. (i) What moral values did Mr.Rahul exhibit? (1) (ii) Name the radiation used to treat cancer? (1) A radioactive substance “X “has a half -life of 140 days. Initially it is 8 g. Find the (iii) time taken for this substance X to reduce to 1 g. (2)
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(24) State Gauss’ Law in electrostatics. Use this law to determine the electric field at a point due to a thin plane sheet of charge density ‘σ’. Calculate the field at the midpoint between two thin sheets of equal and opposite charge densities separated by a distance ‘d’? (5) (OR) Derive a formula for the capacitance of a parallel plate capacitor when a dielectric slab of thickness ‘t’ is introduced between the plates? Two parallel plate capacitors when connected in series give an equivalent capacitance of 3µF and when connected in parallel they give an equivalent of 16µF. Calculate the values of the individual capacitances of the capacitors? (25) (a) Derive an expression for the impedance of a series LCR- circuit connected across an a.c source. (b) Draw a graph showing the variation of the current through the circuit with the frequency of ac source. © What is the impedance of this circuit at resonance? (OR) a) Define self-inductance of a solenoid and write its SI unit. b) Derive an expression for the self-inductance a long air cored solenoid of length l, crosssectional area A and having the no. of turns N. (26) (a) What is interference of light? Write two essential conditions for sustained interference pattern to be produced on the screen. (b) Draw a graph showing the variation of the resultant intensity in the interference pattern against position x on the screen. © What is the effect on the interference pattern in Young’s double slit experiment when: (i) Screen is moved closer to the plane of the slits? (ii) Separation between the two slits is increased? Explain your answer in each case. (5) (OR) (a) What is diffraction of light? Draw a graph showing the variation of the intensity with angle in a single slit diffraction experiment. (b) Write one feature which distinguishes the observed pattern from the double slit interference pattern. (c) How would the diffraction pattern of single slit be affected when (i) The width of the slit is decreased? (ii) The monochromatic light is replaced by a source of white light?
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