**Class 12 – Physics – Sample Papers 2016 – Set 15**

### Section A

1.A carbon resistor is marked in a green, red and orange bands. What is the approximate resistance of the resistor?

2.The figure shows photoemissive characteristics of two photocathodes illuminated by the light emitted by a single source. Which photocathode has higher work function?

3.What is the cause of an electromagnetic wave?

4.Two circular loops, of radii r and 2r, have currents I and I/2 flowing through them in

clockwise and anti-clockwise sense, respectively. If their equivalent magnetic moments are and M_{2} respectively, then state the relation between and M_{2}.

5.The wires of the same material having lengths in the ratio 1 : 2 and radii 2 : 3 are connected in series with a battery. What is the ratio of potential difference across them?

**Section B**

6.What is dynamo effect? (2)

7.Why does a human does not feel the pressure of electromagnetic wave on his skin? Although electromagnetic wave come with speed of light (c).

8.The energy gap of silicon is 1.14 eV Find the maximum wavelength at which silicon starts energy absorption.

9.In communication system the range is r, if the height of antenna is doubled, then what will be the increase in range?

10. What do you understand by “sharpness of resonance” in a series L-C-R circuit? Write expression for Q-factor of the circuit.

Or

Show that the total induced charge simply depends upon the charge in magnetic flux and is independent of the time rate of change of flux.

**Section C**

11.Define relaxation time. What is the effect of increase in temperature on the relaxation time of electrons in metals? What is the effect on the drift velocity of electrons?

12.A sample of paramagnetic salt contains 2.0 xlO^{24} atomic dipoles each of dipole moment 1.5 x 10~^{23} J/T. The sample is placed under a homogeneous magnetic field of 0.64 T and cooled to a temperature of 4.2 K. The degree of magnetic saturation achieved is equal to 15%. What is the total dipole moment of the sample for a magnetic field of 0.98 T and a temperature of 2.8 K? (Assume Curie’s law).

14. In a double slit experiment, the angular width of the fringe is found to be 0.2 ° on a screen placed 1 m away. The wavelength of light used is 600 nm. What will be the angular width of the fringe, if the entire experimental apparatus is immersed in water? Take refractive index of water to be 4/3.

16. For a prism of refractive index 1.732, the angle of minimum deviation is equal to the angle of the prism. What is the angle of the prism?

Or

A Cassegrain telescope uses two mirrors as shown in figure below. Such a telescope is built with the mirrors 20 mm apart. If the radius of curvature of the large mirror is 220 mm and of the small mirror is 140 mm, then where will the final image of an object at infinity be?

17. A photosensitive metallic surface has work function hv_{0}. If photons of energy 2 hv_{0 }fall on this surface, the electrons come out with a maximum velocity of 4 x 10^{6} m/s. When the photon of energy is increased to 5 hv_{Q}, what will be the maximum velocity of photo-electron?

18. A proton and an alpha particle of the same velocity enters in d a region of uniform magnetic field acting in a plane perpendicular to the magnetic field. Deduce the ratio of the radii of the circular paths described by the particles. Explain, why the kinetic energy of the particle after emerging from the magnetic field remains unaltered.

19. What is space wave propagation? Which two communication methods make use of this mode of propagation? If the sum of the heights of transmitting and receiving antennas in line of sight of communication is fixed at h, show that the range is maximum when the two antenna have a height h/2 each.

22.In the Auger process, an atom makes a transition to a lower state without emitting a photon. The excess energy is transferred to an outer electron which may be ejected by the atom (this is called an Auger electron). Assuming the nucleus to be massive, calculate the kinetic energy of an n = 4 Auger electron emitted by chromium by absorbing the energy from a n = 2ton =1 transition.

**Section D**

23.One child found that a comb attracts the small pieces of paper after it get passed through hair. He wanted to know its reason that by what phenomenon it happens, but no one made him understand the same or did not answered satisfactorily. Next day he got ready for the school more early as he was eager to know the answer of the question which was disturbing him. As soon as he reached the school he asked the question from his teacher and got satisfied with the answer. He continued his day happily.

- Write about the child’s nature. What answer could be given by the teacher?
- Write basic properties of Electric charge.
- A polythene piece rubbed with the wool is found to have a negative charge of 3×10
^{7}C. Estimate the numbers of electrons transferred.

**Section E**

Or

What are eddy currents? How are these produced? In what sense are eddy current undesirable in a transformer and how are these reduced in such a device? An induced emf has no direction of its own. Comment.

26. (i) The inputs, A and B shown here, are used as the inputs for three different gates, G_{x}, G_{2}, and G_{3}. The outputs, obtained in the three cases, have the forms shown. Identify the three gates and write their truth tables.

(ii) The inputs, A and B, shown here, are used as the inputs for three different gates, G_{1}, G_{2} and G_{3}. One by one. The outputs, obtained in the three cases, have the forms shown. Identify the three gates and write their symbols.

Or

(i) The same input signal is applied to both the (input) terminals of a given logic gate. If the output is the

- same as the (common) input signal.
- inverted with respect to the (common) input signal.

Identify the logic gates involved in each case.

(ii) Write the truth tables for each of the combinations shown below. Also, identify the logic operations performed by them.

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