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KENDRIYA VIDYALAYA SANGATHAN, HYDERABAD REGION
Class-12 MODEL QUESTION PAPER - 14
SUBJECT: PHYSICS (THEORY)
Maximum Marks: 70 M Time Allowed: 3 hours
General Instructions:
(1) There are 33 questions in all. All questions are compulsory.
(2) This question paper has five sections: Section A, Section B, Section C, Section D and Section E.
(3) All the sections are compulsory.
(4) Section A contains sixteen questions, twelve MCQ and four Assertion Reasoning based of 1 mark
each, Section B contains five questions of two marks each, Section C contains seven questions of
three marks each, Section D contains two case study-based questions of four marks each and
Section E contains three long answer questions of five marks each.
(5) There is no overall choice. However, an internal choice has been provided in one question in
Section B, one question in Section C, one question in each CBQ in Section D and all three questions
in Section E. You have to attempt only one of the choices in such questions.
(6) Use of calculators is not allowed.
(7) You may use the following values of physical constants where ever necessary
i. c = 3 x 108 m/s ii. me = 9.1 x10-31 kg iii. e = 1.6 x 10-19 C
iv. µ0 = 4π x 10-7 Tm𝐴−1 v. h = 6.63 x10-34Js vi. ε0 = 8.854 x10-12𝐶2𝑁−1𝑚−2
vii. Avogadro’s number = 6.023 X 1023 per gram mole
SECTION - A
1. Two large vertical and parallel metal plates having a separation of 1 cm are connected to a dc voltage
source of potential difference X. A proton is released at rest midway between the two plates. It is
found to move at 450 to the vertical just after release. Then X is nearly
(a) 1x10-5 V (b) 1x 10-7V (c) 1×10-9 V (d) 1x 10-10 V
2. Two capacitors of capacitances C1 and C2 are connected in parallel. If a charge q is given to the
assembly, the charge gets shared. The ratio of the charge on the capacitor C 1 to the charge that on C2
is
(a)C1/ C2 (b)C2/C1 (c) C1C2 (d) 1/C1C2
3. The nature of parallel and anti-parallel currents are
(a) parallel currents repel and antiparallel currents attract.
(b) parallel currents attract and antiparallel cur-rents repel.
(c) both currents attract.
(d) both currents repel.
4. Susceptibility of a substance at 300 K is -0.00002. Its susceptibility at 600K is
(a) -0.00001 (b) -0.00004 (c) -0.00006 (d) -0.000025
5. A current l flows through a long straight conductor which is bent
into a circular loop of radius R in the middle as shown in the
figure. The magnitude of the net magnetic field at point O will be
(a) Zero (b)
(c) (d)
6. The objective of a telescope must be of large diameter in order to
(a) Remove chromatic aberration (b) remove spherical aberration & high magnification
(c) Gather light and for high resolution (d) increase its range of observation
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7. The variation of magnetic susceptibility with the temperature of a ferromagnetic material can be
plotted as
8. A square loop of wire, side length 10 cm is placed at angel of 45° with a magnetic field that changes
uniformly form 0.1 T to zero in 0.7s. The induced current in the loop (its resistance is 1Ω) is
(a) 1.0 mA (b) 2.5 mA (c) 3.5 mA (d) 4.0 mA
9. In an LCR circuit, capacitance is changed from C to 2C. For the resonant frequency to remain
unchanged, the inductance should be changed from L to
(a) 4L (b) 2L (c) L/2 (d) L/4
10. An electromagnetic wave of frequency 3 MHz passes from vacuum into a dielectric medium with
permittivity ε= 4. Then,
(a) Wavelength and frequency both remain unchanged.
(b) Wavelength is doubled and the frequency remains unchanged.
(c) Wavelength is doubled and the frequency becomes half.
(d) Wavelength is halved and the frequency remains unchanged.
11. The threshold frequency for photoelectric effect on sodium corresponds to a wavelength of 5000 Å.
Its work function is
(a) 4× 10-19 J (b) 1 J (c) 2 × 10-19 J (d) 3 x 10-19 J
12. If the binding energy per nucleon in 3Li7 and 2He4 nuclei are 5.60 MeV and 7.06 MeV respectively,
then in reaction 1H1+ 3Li7 → 2 2He4 energy of proton must be
(a) 28.24 MeV (b) 17.28 MeV (c)1.46 MeV (d) 39.2 MeV
For Questions 13 to 16, two statements are given –one labelled Assertion (A) and other labelled
Reason (R). Select the correct answer to these questions from the options as given below.
A) If both Assertion and Reason are true and Reason is correct explanation of Assertion.
B) If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C) If Assertion is true but Reason is false.
D) If both Assertion and Reason are false.
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13. Assertion (A): The resistance of a conductor decreases with increase in cross sectional area.
Reason (R): On increasing the cross sectional area of a conductor, more current will flow
through the conductor
14. Assertion (A): The resistivity of a semiconductor increases with temperature.
Reason (R): The atoms of a semiconductor vibrate with larger amplitudes at higher temperatures
thereby increasing its resistivity.
15. Assertion (A): In photoelectron emission, the velocity of electron ejected from near the surface is
larger than that coming from interior of metal.
Reason (R): The velocity of ejected electron will be zero.
16. Assertion (A): The direction of induced e.m.f. is always such as to oppose the change that causes it.
Reason (R): The direction of induced e.m.f. is given by Lenz's Law.
SECTION - B
17. Name the parts of the electromagnetic spectrum which is
(i) suitable for radar systems used in aircraft navigation.
(ii) used to kill germs in water purifier.
18. Two similar bars, made from two different materials P and Q, are placed one by one, in a
non-uniform magnetic field. It is observed that
a) bar P tends to move from the weak to the strong field region.
b) bar Q tends to move from the strong to the weak field region.
(i) Identify the magnetic material used for making these two bars.
(ii) Show with the help of diagrams, the behavior of the field lines, due to an external magnetic
field, near each of these two.
19. A straight wire of length L is bent into a semi-circular loop. Use Biot-Savart’s law to deduce an
expression for the magnetic field at its center due to the current (i) passing through it.
20. Use the mirror equation to show that an object placed between F and 2F of a concave mirror
produces a real image beyond 2F.
21. Two material bars A and B of equal area of cross-section are connected in series to a dc supply. A is
made of usual resistance wire and B of an n-type semiconductor. In which bar is the drift speed of
free electrons greater? Why?
(OR)
Draw the energy band diagram of an n-type semiconductor. How does the energy gap of an intrinsic
semiconductor vary with increase in temperature?
SECTION - C
22. Two cells of emfs 1.5 V and 2.0 V having internal resistance 0.2 Ω and 0.3 Ω respectively are
connected in parallel. Calculate the emf and internal resistance of the equivalent cell?
23. A series LCR circuit with R=20 Ω, L =1.5 H and C = 35 µF is connected to a variable frequency 200V ac
supply. When the frequency of the supply equals the natural frequency of the circuit, what is the
average power transferred to the circuit in one complete cycle?
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24. a) State Faraday’s laws of electromagnetic induction.
b) A metallic rod of 1m length is rotated with a frequency of 50 rev/s, with one end hinged at the
centre and the other end at the circumference of a circular metallic ring of radius 1m, about an
axis passing through the centre and perpendicular to the plane of the ring. A constant uniform
magnetic field of 1T parallel to the axis is present everywhere. What is the emf between the
centre and the metallic ring?
25. Draw a graph between the frequency of incident radiation (ϒ) and the maximum kinetic energy of
the electrons emitted from the surface of a photo sensitive material. State clearly how this graph
can be used to find a) Planck’s constant and b) work function of the material.
(OR)
The work function of Cesium metal is 2.14eV. When light of frequency 6x 10 14Hz is incident on the
metal surface, photoemission of electrons occurs. What is
a) Maximum kinetic energy of the emitted electron
b) Stopping potential and
c) Maximum speed of the emitted photoelectrons
26. Draw a schematic arrangement of Geiger- Marsden experiment. Calculate the distance of closest
approach when a 7.7MeV α-particle approaches a gold nucleus (Z = 79).
27. Explain the formation of central maximum in Young’s single slit experiment. Write the expression for
the width of central maximum.
28. (a) The mass of a nucleus in its ground state is always less than the total mass of its constituents –
neutrons and protons. Explain.
(b) Plot a graph showing the variation of potential energy of a pair of nucleons as a function of their
separation.
SECTION - D
29. Case Study based question: Read the following paragraph and answer the questions.
Two sources of light which continuously emit light waves of same frequency (or wavelength) with
a zero or constant phase difference between them, are called coherent sources. Two independent
sources of light cannot act as coherent sources, they have to be derived from the same parent source.
In Young's double slit experiment, two identical narrow slits S1 and S2 are placed symmetrically with
respect to narrow slit S illuminated with monochromatic light. The interference pattern is obtained on
an observation screen placed at large distance D from S1 and S2.
(i) In a Young’s double slit experiment, the separation between the two slits is 0.9 mm and the fringes
are observed one metre away. If it produces the second dark fringe at a distance of 1 mm from the
central fringe, the wavelength of monochromatic source of light used is
(a) 500 nm (b) 600 nm (c) 450 nm (d) 400 nm
ii) In a Young’s double-slit experiment the fringe width is 0.2 mm. If the wavelength of light used is
increased by 10% and the separation between the slits is also increased by 10%, the fringe width will
be
(a) 0.20 mm (b) 0.401 mm (c) 0.242 mm (d) 0.165 mm
iii) In an interference experiment, third bright fringe is obtained at a point on the screen with a light of
700 nm. What should be the wavelength of the light source in order obtain 5th bright fringe at the
same point
(a) 500 nm (b) 630 nm (c) 750 nm (d) 420 nm
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iv) In a Young’s double slit experiment, the source illuminating the slits is changed from blue to violet.
The width of the fringes
(a) Increases (b) Decreases (c) Becomes unequal (d) Remains constant
(OR)
v) In Young’s double slit experiment, the separation between the slits is halved and the distance
between the slits and screen is doubled. The fringe width will
(a) be halved (b) be doubled (c) be quadrupled (d) remain unchanged
30. Case Study based question: Read the following paragraph and answer the questions.
P-N junction diode: P-N junction is a semiconductor diode. It is obtained by bringing p-type
semiconductor in close contact with n- type semiconductor. A thin layer is developed at the p- n junction
which is devoid of any charge carrier but has immobile ions. It is called depletion layer. At the junction a
potential barrier appears, which does not allow the movement of majority charge carriers across the
junction in the absence of any biasing of the junction. P-N junction offers low resistance when forward
biased and high resistance when reverse biased.
(i) How many junction(s) do a diode consist?
a) 0 b) 1 c) 2 d) 3
(ii) If the positive terminal of the battery is connected to the anode of the diode, then it is known as
a) Forward biased b) Reverse biased
c) Equilibrium d) Schottky barrier
(iii) During reverse bias, a small current develops known as
a) Forward current b) Reverse current
c) Reverse saturation current d) Active current
(iv) If the voltage of the potential barrier is V0. A voltage V is applied to the input, at what moment will
the barrier disappear?
a) V< V0 b) V= V0 c) V> V0 d) V<< V0
(OR)
v) Which of the following statements is incorrect?
(a) The resistance of intrinsic semiconductors decrease with increase of temperature.
(b) Doping pure Si with trivalent impurities give p-type semiconductors.
(c) The majority carriers in n-type semiconductors are holes.
(d) A p-n junction can act as a semiconductor diode
SECTION - E
31. a) Derive an expression for the electric field ‘E’ due to a dipole of length ‘2a’ at a point situated
at a distance ‘r’, from the center of the dipole, on the axial line.
b) Draw a graph of E versus ‘r’ for r>>a.
c) Four equal point charges each 16µC are placed on the four corners of a square of side 0.2m.
Calculate the force on any one of the charges.
(OR)
a) Using Gauss’ law, deduce the expression for the electric field due to a uniformly charged
spherical conducting shell of radius ‘R’ at a point i) outside and ii) inside the shell.
b) Two charges of magnitude -2Q and +Q are located at point (a,0) and (4a, 0) respectively. Find the
electric flux due to these charges through a sphere of radius 3a with its centre at the origin.
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32. (a) Plot a graph to show variation of the angle of deviation as a function of angle of incidence for
light passing through a prism.
(b) Derive an expression for refractive index of the prism in terms of angle of minimum deviation
and angle of prism.
(c) A ray of light incident normally on one face of a right isosceles prism is totally reflected as shown
in fig. What must be the minimum value of refractive index of glass? Give relevant calculations.
(OR)
a) Define a wave front.
b) Use Huygen’s geometrical construction to show the propagation of plane wavefront from a rarer
medium to a denser medium. Hence derive Snell’s law of refraction.
c) What is the effect on the interference fringes in Young’s double slit experiment, if the separation
between the two slits is decreased? Justify your answer.
33. (a) Plot a graph showing the variation of resistance of a conducting wire as a function of its
radius, keeping the length of the wire and its temperature as constant.
(b) Prove that the current density of a metallic conductor is directly proportional to the drift
speed of electrons.
(c) The number density of free electrons in a copper conductor is 8.5×10 28m3. How long does an
electron take to drift from one end of a wire 3 m long, to its another end? The area of cross
section of the wire is 2.0 ×10–6 m2 and it is carrying a current of 3.0 A.
(OR)
(a) Use Kirchhoff ’s laws to determine the value of current I1
in the given electrical circuit.
(b) Draw a circuit diagram showing balancing of Wheatstone
bridge. Use Kirchhoff’s laws to obtain the balance
in terms of the resistances of four arms of Wheatstone
bridge.
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