# CBSE Board Class 12 Physics Previous Year Question Papers 2007

## CBSE Board Previous Year Question Papers 2007 for Class 12 Physics

General Instructions:
1. All questions are compulsory.
2. There is no overall choice. However, an internal choke has been pro vided in one
question of two marks, one question of three marks and all three questions of five
marks. You have to attempt only one of the chokes in such questions.
3. Question numbers 1 to 5 are very short answer type questions, carrying on 5 one mark each.   e
4. Question numbers 6 to 12  are short answer type questions, carrying t two marks each.   twotwo
5. Question numbers 1313 to 24 1313 24 are also short answer type questions, carryinghree marks  hree
each.
6. Question numbers 25 to 27  are long answer type questions, carrying five marks each.
7. Use of calculators is not permitted. However, you may use log tables, if necessary.
8. You may Use the following values of physical constants wherever necessary:

PHYSICS 2007

Q1. . An electric dipole of dipole moment 20, X 10 . .-6
Cm is enclosed by a closed surface. What is
the net flux coming out of the surface?
Q2. . An electron beam projected along +X-axis, experie . .  nces a force due to a magnetic field
along the +Y-axis. What is the direction of the magnetic field?
Q3. . The power factor of an A.C. circuit is 0.5. What  . .  will be the phase difference between
voltage and current in this circuit?
Q4.. Electrons are emitted from a photosensitive surfa .. ce when it is illuminated by green light
but electron emission does not take place by yellow light. Will the electrons be emitted when
the surface is illuminated by (i) red light, and (ii) blue light.
Q5. . What should be the length of the dipole antenna f . .  or a carrier wave of Frequency 3 X 1O8
Hz?
Q6. . Define 'electric line of force' and give its two  . .  important properties.
Q7. . (a) Why does the electric field inside a dielectr . .  ic decrease when it is placed in an external
electric field?
(b) A parallel plate capacitor with air between the plates has a capacitance of 8 PF. What will
be the capacitance if the distance between the plates he reduced by half and the space
between them is filled with a substance of dielectric constant K=6?
Or  There point charges of  are kept at the vertices, A, B and C
respectively of an equilateral triangle of side 20 cm as shown in the figure. What should be the
sign and magnitude of the charge to be placed at the mid-point (M) of side BC so that the
charge at A remains in equilibrium?
Q8. . Draw V - I graph for ohmic and non-ohmic material . .  s. Give one example for each.
Q9. . Define the terms 'Magnetic Dip' and 'Magnetic Dec . .  lination' with the help of relevant
diagrams.
Q10. . In the figure given below, a bar magnet moving to . .  wards the right or left induces an e.m.f.
in the coils (1) and (2). Find giving reason, the directions of the direction of the induced
currents through the resistors AB and CD when the magnet is moving (a) towards the right,
and (b) towards the left.
Q11. . (i) Draw the graphs showing variation of inductiv . .  e reactance and Capacitive reactance
with frequency of applied a.c. source.  (ii) Can the voltage drop across the inductor or the capacitor in a series LCR circuit be greater
Q12. . The image of a candle is formed by a convex lens  . .  on a screen. The lower half of the lens
is painted black to make it completely opaque. Draw the ray diagram to show the image
formation. How will this image be different from the one obtained when the lens is not painted
black?
Q13. . An electric dipole is held in a uniform electric  . .  field. (1) Using suitable diagram, show that
it does not undergo any translatory motion, and (ii) derive an expression for the torque acting
on it and specify its direction.
Q14. . A galvanometer with a coil of resistance 120 ohm  . .  shows full scale deflection for a current
of 2.5 MA. How will you convert the galvanometer into an ammeter of range 0 to 7.5 A?
Determine the net resistance of the ammeter. When an ammeter is put in a circuit, does it read
slightly less or more than the actual current in the original circuit? Justify your answer.
Q15. . Define the term 'resistively' and write its S. I. . .  Unit. Derive the expression for the
resistively of a conductor in terms of number density of free electrons and relaxation time
Or  State the principle of potentiometer. Draw a circuit diagram used to compare the e.m.f. of
two primary cells. Write the formula used. How can the sensitivity of a potentiometer be
increased?
Q16. . Explain, with the help of diagram, the principle  . .  and working of an a.c. Generator. Write
the expression for the e.m.f. generated in the coil in terms of its speed of rotation.
Q17. . Give reasons for the following:
(ii) The small ozone layer on top of the stratosphere is crucial for human survival.
(iii) Satellites are used for long distance TV transmission.
Q18. . A figure divided into squares, each of size 1 mm . .
is being viewed at a distance of 9cm
through a magnifying lens of local length 10cm, held close to the eye.  (i) Draw a ray diagram showing the formation of the image.
(ii) What is the magnification produced by the lens? How much is the area of each square in
the virtual image?
(iii) What is the angular magnification of the lens?
Q19. . Ultraviolet light of wavelength 2271  . .  from a 100 W mercury source radiates a photo cell
made of molybdenum metal. If the stopping potential is 1.3 V, estimate the work function of the
metal. How would the photo cell respond to high intensity (10 5 Wm -2 ) red light of wavelength
6328  produced by a He Ne laser? Plot a graph showing the variation of photoelectric current
with anode potential for two light beams of same wavelength but different intensity.
Q20. . (a) Draw a graph showing the variation of potenti . .  al energy of a p of nucleons as a
function of their separation. Indicate the regions in which nuclear force is
(i) attractive, and (ii) repulsive.
(b) Write two characteristic features of nuclear force which distinguish it from the coulomb
force.
Q21. . (a) Show that the decay rate 'R' of a sample of a . .  radionuclide is related to the number of
radioactive nuclei 'N' at the same instant by the expression
(b) The half-life of  U against a  - decay is 1.5 x 1017 s. What is the activity of a sample of U having 25 x 1020
atoms?
Q22. . 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. In the following circuits which one of the two
diodes is forward biased and which is reverse based?
Q23. . Distinguish between, analog and digital communica . .  tion. Write any two modulation
techniques employed for the digital data. Describe briefly one of the techniques used. 3
Q24. . Draw a schematic diagram of a single optical fibe . .  r structure. Explain briefly how an
optical fiber is fabricated. Describe in brief, the mechanism of propagation of light signal
through an optical fiber.
Q25. . (a) With the help of a labelled diagram, explain  . .  the principle and working of a moving coil
galvanometer.
(b) Two parallel coaxial circular coils of equal radius 'R' and equal number of turns 'N', carry
equal currents 'I' in the same direction and are separated by a distance '2R'. Find the
magnitude and direction of the net magnetic field produced at the mid-point of the line joining
their centres.
Or  (a) State Biot-Savart's law. Using this law, derive the expression for the magnetic field due
to a current carrying circular loop of radius 'R', at a point which is at a distance 'x' from its
centre along the axis of the loop.
(b) Two small identical circular loops, marked (1) and (2), carrying equal currents, are placed
with the geometrical axes perpendicular of each other as shown in the figure. Find the
magnitude and direction of the net magnetic field produced at the point O.
Q26. . . . (a) How is a wave front different from a ray? Draw the geometrical, shape of the wave
fronts when (i) light diverges from a point source, and (ii) light emerges out of a convex lens
when a point source is placed as its focus.
(b) State Huygens' Principle. With the help of a suitable diagram, prove Snell's law of refraction
using Huygens' Principle.
Or  (a) In Young's double slit experiment, deduce the conditions for (i) constructive and (ii)
destructive interference at a point on the screen. Draw a graph showing variation of the
resultant intensity in the interference pattern against position 'x' on the screen.
(b) Compare and contrast the pattern which is seen with two coherently illuminated narrow slits
in Young's experiment with that seen for a coherently illuminated single slit producing
diffraction.

Q27. . (a) . .  Distinguish between metals, insulators and semiconductors on the basis of their
energy bands.
(b) Why are photodiodes used preferably in reverse bias condition? A photodiode is fabricated
from a semiconductor with band gap of 2.8 EV. Can it detect a wavelength of 6000 nm?
Justify.
Or  (a) Explain briefly, with the help of circuit diagram, how V - I characteristics of a p-n
junction diode are obtained ii (i) forward bias, and (ii) reverse bias. Draw the shape of the
curves obtained.
(b) A semiconductor has equal electron and hole concentration of 6 x 108
/m3. On doping with certain impurity, electron concentration increases to 9 X1012 /m3.
(i) Identify the new semiconductor obtained after doping.
(ii) Calculate the new whole concentration.

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