## CBSE Sample Papers for Class 12 Physics Solved 2016 Set 14

Section A

1.When a charged particle moving with velocity v is subjected to magnetic field B,the force acting on it is non-zero. Would the particle gain any energy?

2. A rectangular coil of area A having number of turns N is rotated at f revolutions per second in a uniform magneitc field B, the field being perpendicular to the coil. Prove that the maximum emf induced in the coil is 2pie f NBA.

3. For circuits used for transporting electric power, a low power factor implies large  power loss in transmission. Explain.

4.What is the shortest wavelength present in the Paschen series of spectral lines?

5. Show graphically how the stopping potential for a given photosensitive surface  varies with the frequency of incident radiations

Section B

6.What is the name given to the curved, the tangent to which at any point gives the direction of magnetic field at that point? Can two such curves intersect each other? Justify your answer.

7.Calculate refractive index of material of an equilateral prism for which angle of  minimum deviation is 60 °?

8.If a point charge + q is taken first from A to C and then from C to B of a circle drawn with another point charge +q at the centre, in below figure, then along which path, more work will be done?

A point charge causes an electric flux of -1.0 xlO_3Nm2C_1 to pass through a spherical Gaussian surface of 10.0 cm radius centered on the charge.

• How much flux will pass through the surface, if the radius of the Gaussian surface is doubled?
• Find the value of the point charge.

9. Three dielectric slabs of dielectric constants KlrK2 and K3 have been put in between the plates of a capacitor as shown in figure. Determine the equivalent capacitance.

10. (i) What is the relation between critical angle and refractive index of a material? (ii) Does critical angle depend on the colour of light? Explain.

Section C

11. A transmitting antenna at the top of a tower has a height of 36 m and the height of the receiving antenna is 49 m. What is the maximum distance between them, for satisfactory communication in the LOS mode?  (Radius of the earth = 6400 km)

14. Draw a graph showing the variation of stopping potential with the frequency of incident radiation in relation to photoelectric effect.

• What does the slope of this graph represent?
• How can the value of work function of the material be determined from this graph? (3)

Or

The work function for the following metals is given, Na: 2.75 eV; K:2.30 eV; Mo:4.17 eV Ni:5.15 eV Which of these metals will not give photoelectric emission for a radiation of wavelength 3300 A from a He-Cd laser placed lm away from the photocell? What happens, if the laser is brought nearer and placed 50 cm away?

16. With proper circuit diagram, show the biasing of a p-n-p Explain the  movement of charge carriers through different parts of this transistor. Hence, show that Ie =IC + Ib.

18. State Lenz’s law

Two identical loops, one of copper and other of aluminium are rotated with the same speed, in a uniform magnetic field acting normal to the plane of the loops. State with reasons, for which of the coils

• induced emf,
• induced current, will be more.

19. (i) Why does the sky appear blue?

(ii) How do you account for the red colour of the sun during sunrise and sunset?

20.Draw a labelled diagram of Hertz’s experimental set up to produce electromagnetic waves. Explain the generation of electromagnetic waves using this set up.

22.  Draw a plot of variation of amplitude versus w for an amplitude modulated wave. Define modulation index. State its importance for effective amplitude modulation.

### Section D

23.There was a student suffering from an eye defect. In starting, for the long time he did not pay attention to his eye defect and therefore wasted that time, though he could have prepared for competitive examination, if he had given a thought for his eyesight correction. After few day he went to see an eye specialist who suggested him that he should wear spectacles to be fit and fine. He did the same, later he was in guilt and prayed his lost time to come back.

• What decision should be taken on any issue related to health? Give reasons.
• Name two kinds of common eyesight deffect in human eye.
• What focal length should the reading spectacles have for a person, for whom the  least distance of distinct vision is 50 cm.

Section E

24. (i) To cells of emf E1, E2 and internal resistances rt, r2, respectively are connected in series. Deduce expressions for the equivalent emf and equivalent internal resistance of the combination.

(ii) Define the term potential gradient and write its SI unit.

Or

(i) Two metallic wires of the same material and same length but different cross-sectional areas are joined together (a) in series, (b) in parallel, to source of emf. In which of the two wires will the drift velocity of electron be more in each of the two cases and why?

(ii) The resistivity of a wire is R ohm. Find its new resistance, if it is stretched to n times its original length.