Download 50 Questions of Class XII Physics - Kendriya Vidyalaya No.1 Harni

PHYSICS (THEORY) CLASS XII
K V NO.1, HARNI, VADODARA
SET OF 50 VERY SHORT ANSWER TYPE QUESTIONS WITH ANSWERS.
1. The sequence of bands marked on a carbon resistor is yellow, red, white and gold. What
is its resistance?
Ans: 42 x 109 ± 5%.
2. What is the ratio of electric field intensities at any two points between the plates of a
capacitor?
Ans: The ratio is one, as the electric field is the same at all points between the plates of
the capacitor.
3. The distance of the field point, on the equatorial plane of a small electric dipole, is
halved. By what factor will the electric field, due to the dipole change?
Ans: By a factor of 8.
4. Two charges q1 and q2, separated by a small distance, satisfy the equation q1 + q2 = 0.
What does it tell us about the charges?
Ans: It is a dipole consisting of equal and opposite charges.
5. An uncharged insulated conductor A is brought near a charge insulated conductor B.
What happens to the charge and potential of B?
Ans: The charge remains the same , but the potential decreases.
6. If the length of a wire conductor is doubled by stretching it, keeping the potential
difference across it constant, by what factor does the drift speed of the electrons
change?
Ans: The drift velocity becomes half if the length is doubled.
7. Two wires of equal length one of copper and the other of manganin have the same
resistance. Which wire is thicker?
Ans: Manganin.
8. If the permanent magnetic moment of the atoms of a substance is zero, then the
substance is called………………
Ans: Diamagnetic.
9. Two identical loops, one of copper and another of aluminium are rotated with the same
speed in the same magnetic field. In which case, the (a) induced e.m.f. and (b) induced
current will be more and why?
Ans: The induced e.m.f. will be same in both but the induced current will be more in the
copper loop as its resistance will be lesse as compared to that of aluminium loop.
10. The instantaneous current from an a.c. source is I = 6 sin 314t. What is the r.m.s. value
of the current?
Ans: 3(2)1/2 A.
11. When does a series LCR circuit have minimum impedance?
Ans: At resonance.
12. What are the dimensions of (LC)1/2.
Ans: Time
13. Why is the core of a transformer laminated?
Ans: To reduce the loss of energy due to eddy currents.
14. The voltage across each of the component of a series L-C-R circuit is 50V. What will be
the voltage across LC combination?
Ans: 0 V.
15. Which part of electromagnetic spectrum is used in operating a RADAR?
Ans: Microwaves.
16. Which part of electromagnetic spectrum has largest penetrating power?
Ans: Gamma rays.
17. Does size of a mirror affect the nature of the image?
Ans: No.
18. A concave mirror is held inside water. What would be the change in the focal length of
the mirror?
Ans: It will remain unchanged.
19. What is the critical angle for a material of refractive index (2)1/2?
Ans: µ = 1/(sin C); C = 45°.
20. Why should the objective lens of a compound microscope have small focal length?
Ans: M = - (L/fo)[1 + (D/fe)]. Hence if focal length of the objective lens of a compound
microscope is small, its magnifying power will be large.
21. What is phase difference between any two points on a wavefront?
Ans: Zero.
22. Name the shape of awavefront originating from a (i) point source and (ii) a line source.
Ans: (i) Spherical wave front (II) Cylindrical wave front.
23. What do you understand by a wavefront?
Ans: The locus of all the particles of the medium, which at ant instant are vibrating in the
same phase, is called the wavefront.
24. What will be the effect on the interference fringes, if red light is replaced by blue light?
Ans: The fringe width will decrease.
25. Is the law of conservation of energy obeyed by the interference?
Ans: Yes
26. The phase difference between two light waves from two slits of Young’s experiment is π
radian. What will be the nature of the central fringe in the fringe pattern?
Ans: it will be dark.
27. What should be the order of size of obstacle/aperture to observe diffraction?
Ans: The size of the obstacle should be of the order of the wavelength of the light used.
28. Which phenomenon establishes that light waves are transverse in nature?
Ans: Only transverse waves can be polarized. Since light wavesa can be polarized, they
must be transverse in nature.
29. What is the polarizing angle of a medium of refractive index √3.
Ans: 60°.
30. If intensity of incident radiation is doubled, what happens to the kinetic energy of the
electrons?
Ans: No effect.
31. If the maximum kinetic energy of electrons emitted by a photo cell is 5 eV, what is the
stopping potential?
Ans: 5V.
32. Are matter waves electromagnetic?
Ans: No
33. A small circular disc is placed in the path of light from a distant source. Will the centre of
shadow be bright or dark?
34. It will be bright.
35. Half life of a radioactive substance is 20 days. What percentage of a sample will decay
in 10 days : 25%, less than 25% or more than 25%?
Ans: More than 25%.
36. In what medium should two charges be placed so that force between them may become
one-third of that in vacuum?
Ans: Fm = Fo/3. Now, K = Fo/Fm
Hence, K=3 ( Medium having dielectric constant 3)
37. Which has more capacitance to store charges---hollow sphere or conducting sphere of
same charges?
Ans:Both the spheres will have the same capacitance.
38. Why do we prefer n-p-n transistor over p-n-p transistor?
Ans: Mobility of electrons is more than the mobility of holes.
39. Define the term demodulation.
Ans:
The process of retrieval of information from the carrier wave at the receiver end is called
demodulation.
40. GIve the reason why transmission of TV signals via sky waves is not possible.
Ans: TV signals are not reflected back by ionosphere.
41. Arrange the following in descending order of wavelength: X-ray, radiowave, infrared
light.
Ans: Radiowaves > Infrared light > X-rays.
42. Write the S.I unit for the activity of a radioactive nuclide.
Ans: Becquerel (Bq).
43. Which property of zener diode is used in regulating voltage?
Ans: For a large variation of current, voltage across zener diode does not change.
44. The radius of the first electron orbit of the hydrogen atom is 5.3 x 10-11 m. What is the
radius of the second orbit?
Ans:
rn  n2. 2.12 x 10-10 m.
45. What will be the ratio of the radii of two nuclei of mass numbers A1 and A2?
Ans: (R1/R2) = (A1/A2)1/3.
46. What fraction of tritium will remain undecayed after 25 years? Given, half life is 12.5
years.
Ans: N/No = (1/2)t/T. N/No = ¼.
47. Which type of biasing gives a semiconductor diode very high resistance?
Ans: Reverse biasing.
48. The truth table of a logic gate has the following form. Identify the gate.
A
0
1
0
1
B
0
0
1
1
Y
1
0
0
0
Ans: NOR gate.
49. Name the principle on which optical fibre works.
Ans: Total internal reflection.
50. In the block diagram of a simple modulator for obtaining an AM signal, shown in the
figure, identifythe boxes A and B. Write their functions.
Ans: A: Square law device, B: Bandpass filter centered at ωc.
(Prepared By: Dr. Mukti Sharma (PGT) physics, KV No.1, Harni, Vadodara).