** cbse question paper for class 12, 2014 physics**

**General Instructions:**

(i) All questions are compulsory.

(ii) There are 30 questions in total. Question Nos. 1 to 8 are very short answer type questions and carry one mark each.

(iii) Questions Nos. 9 to 18 carry two marks each. Questions Nos. 19 to 27 carry three marks each and questions Nos. 28 to 30 carry five marks each.

(iv) There is no overall choice. However, an internal choice has been provided in one question of two marks, one question of three marks and all three questions of five marks each. You have to attempt only one of the choices in such questions.

(v) Use of calculators is not permitted. However, you may use log tables if necessary.

**Question: 2**The carrier wave is given by

C(*t*) = 2sin(8π*t*) volt.

The modulating signal is a square wave as shown. Find modulation index.

**Question: 3**“For any charge configuration, equipotential surface through a point is normal to the electric field.” Justify.

**Question: 4**Two spherical bobs, one metallic and the other of glass, of the same size are allowed to fall freely from the same height above the ground. Which of the two would reach earlier and why?

**Question: 6**A convex lens is placed in contact with a plane mirror. A point object at a distance of 20 cm on the axis of this combination has its image coinciding with itself. What is the focal length of the lens?

**Question: 7**Write the expression, in a vector form, for the Lorentz magnetic force F⃗ due to a charge moving with velocity V⃗ in a magnetic field B⃗ . What is the direction of the magnetic force?

**Question: 8**The figure given below shows the block diagram of a generalized communication system. Identify the element labelled ‘X’ and write its function.

**Question: 9**

**Question: 10**

*E*⃗ =5 ×103

*i*⌢ N/C, find the flux of this field through a square of 10 cm on a side whose plane is parallel to the y-z plane. What would be the flux through the same square if the plane makes a 30° angle with the x-axis ?

**Question: 11**

*λ*a. At the same angle of

*λ*a, we get a maximum for two narrow slits separated by a distance “a”. Explain.

**Question: 12**

**OR**

Identify the logic gates marked ‘P’ and ‘Q’ in the given circuit. Write the truth table for the combination.

**Question: 13**

**Question: 14**

**Question: 15**

**Question: 16**

**Question: 17**

(a) greater value of de-Broglie wavelength associated with it, and

(b) less momentum?

**Question: 18**

^{14}Hz is produced by a laser. The power emitted is 2.0 × 10

^{−3}W. Estimate the number of photons emitted per second on an average by the source.

(ii) Draw a plot showing the variation of photoelectric current versus the intensity of incident radiation on a given photosensitive surface.

**Question: 19**

Calculate the wavelengths of the first member of Lyman and first member of Balmer series.

**Question: 20**

Ultimately her parents agreed :

(1) In what way can the setting up of transmission tower by a mobile company in a residential colony prove to be injurious to health?

(2) By objecting to this move of her parents, what value did Sunita display?

(3) Estimate the range of e.m. waves which can be transmitted by an antenna of height 20 m. (Given radius of the earth = 6400 km)

**Question: 21**

**Question: 22**

(b) The total magnification produced by a compound microscope is 20. The magnification produced by the eye piece is 5. The microscope is focussed on a certain object. The distance between the objective and eyepiece is observed to be 14 cm. If least distance of distinct vision is 20 cm, calculate the focal length of the objective and the eye piece.

**Question: 23**

**Question: 24**

**OR**

_{1}and R

_{2 }when connected by a conducting wire acquire charges q

_{1}and q

_{2}respectively. Find the ratio of their surface charge densities in terms of their radii.

**Question: 25**

(b) Two long coaxial insulated solenoids, S

_{1}and S

_{2}of equal lengths are wound one over the other as shown in the figure. A steady current “I” flow thought the inner solenoid S

_{1}to the other end B, which is connected to the outer solenoid S

_{2}through which the same current “I” flows in the opposite direction so as to come out at end A. If n

_{1}and n

_{2}are the number of turns per unit length, find the magnitude and direction of the net magnetic field at a point (i) inside on the axis and (ii) outside the combined system.

**Question: 26**

(a) Name the em waves which are suitable for radar systems used in aircraft navigation. Write the range of frequency of these waves.

(b) If the earth did not have atmosphere, would its average surface temperature be higher or lower than what it is now? Explain.

(c) An em wave exerts pressure on the surface on which it is incident. Justify.

**Question: 27**

_{0}e

^{−λt}, for the law of radioactive decay.

^{+}decay of Na1122. Also write the basic nuclear process underlying this decay.

**Question: 28**

(ii) Light waves each of amplitude “a” and frequency “ω”, emanating from two coherent light sources superpose at a point. If the displacements due to these waves is given by y

_{1}= a cos ωt and y

_{2}= a cos(ωt + ϕ) where ϕ is the phase difference between the two, obtain the expression for the resultant intensity at the point.

(b) In Young’s double slit experiment, using monochromatic light of wavelength λ, the intensity of light at a point on the screen where path difference is λ, is K units. Find out the intensity of light at a point where path difference is λ/3.

**OR**

(a) How does one demonstrate, using a suitable diagram, that unpolarised light when passed through a Polaroid gets polarised?

(b) A beam of unpolarised light is incident on a glass-air interface. Show, using a suitable ray diagram, that light reflected from the interface is totally polarised, when μ = tan i_{B}, where μ is the refractive index of glass with respect to air and i_{B} is the Brewster’s angle.

**Question: 29**

(ii) Induced emf versus dI/dt

(iii) Magnetic potential energy stored versus the current.

**OR**

(ii) Alternating emf versus time generated by a loop of wire rotating in a magnetic field.

**Question: 30**

(b) Using the necessary circuit diagrams, show how the V-I characteristics of a p-n junction are obtained in

(i) Forward biasing

(ii) Reverse biasing

How are these characteristics made use of in rectification?

**OR**

(a) Differentiate between three segments of a transistor on the basis of their size and level of doping.

(b) How is a transistor biased to be in active state?

(c) With the help of necessary circuit diagram, describe briefly how n-p-n transistor in CE configuration amplifies a small sinusoidal input voltage. Write the expression for the ac current gain.