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CBSE Class 12 Physics Solved Question Paper 2014

Short Answer Type

21.

Draw a circuit diagram of n-p-n transistor amplifier in CE configuration. Under what condition does the transistor act as an amplifier?

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22.

(a) Using the phenomenon of polarization, show how transverse nature of light can be demonstrated.

(b) Two Polaroid P₁ and P₂ are placed with their pass axes perpendicular to each other. Unpolarised light of intensity Io is incident on P₁. A third Polaroid P₃ is kept in between P₁ and P₂such that its pass axis makes an angle of 30^o with that of P₁. Determine the intensity of light transmitted through P₁, P₂ and P₃.

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23.

A convex lens of focal length 20 cm is placed coaxially with a convex mirror of radius of curvature 20 cm. The two are kept at 15 cm apart. A point object lies 60 cm in front of the convex lens. Draw a ray diagram to show the formation of the image by the combination. Determine the nature and position of the image formed.

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24.

Write any two distinguishing features between conductors, semiconductors and insulators on the basis of energy band diagrams.

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25.

For the past some time, Aarti had been observing some erratic body movement, unsteadiness and lack of coordination in the activities of her sister Radha, who also used to complain of severe headache occasionally. Aarti suggested to her parents to get a medical check-up of Radha. The doctor thoroughly examined Radha and diagnosed that she has a brain tumor.

(a) What, according to you, are the values displayed by Aarti?

(b) How can radioisotopes help a doctor to diagnose brain tumor?

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26.

Write two basic modes of communication. Explain the process of amplitude modulation. Draw a schematic sketch showing how amplitude modulated signal is obtained by superposing a modulating signal over a sinusoidal carrier wave.

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27.

An electron microscope uses electrons accelerated by a voltage of 50 kV. Determine the de-Broglie wavelength associated with the electrons. Taking other factors, such as numerical aperture etc. to be same, how does the resolving power of an electron microscope compare with that of an optical microscope which uses yellow light?

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Long Answer Type

28.
Draw a labelled diagram of Van de Graff generator. State its working principle to show how by introducing a small charged sphere into a larger sphere, a large amount of charge can be transferred to the outer sphere. State the use of this machine and also point out its limitations.

Van De Graff generator: Van de Graff generator is capable of producing very high potential of the order of 10 MV.

The underlying principle of Van de Graff generator is that a charge given to a hollow conductor is transferred to outer surface and is distributed uniformly over it.

Construction: It consists of a large hollow metallic sphere S mounted on two insulating columns A and B and an endless belt of rubber or silk is made to run on two pulleys P₁ and P₂ by the means of an electric motor. C₁ and C₂ are two sharp metallic spikes in the form of combs. The lower comb C₁ is connected to the positive terminal of a very high voltage source and the upper comb C2 is connected to the inner surface of metallic sphere S.

Working:

i. When comb C₁ is given very high potential, then it produces ions in its vicinity, due to action of sharp points.

ii. The positive ions, so produced, get sprayed on the belt due to the repulsion between positive ions and comb C₁.

iii. These positive ions are carried upward by the moving belt. The pointed end of C₂ just touches the belt.

iv. The comb C₂ collects positive charge from the belt which immediately moves to the outer surface of sphere S.

v. As the belt goes on revolving, it continues to take (+) charge upward, which is collected by comb C₂ and transferred to outer surface of sphere S. Thus the outer surface of metallic sphere S gains positive charge continuously and its potential rises to a very high value.

vi. When the potential of a metallic sphere gains very high value, the dielectric strength of surrounding air breaks down and its charge begins to leak, to the surrounding air.

vii. The maximum potential is reached when the rate of leakage of charge becomes equal to the rate of charge transferred to the sphere.

To prevent leakage of charge from the sphere, the generator is completely enclosed in an earthed connected steel tank which is filled with air under high pressure.

Uses:

1. Used to accelerate charged particles such as electrons, protons, ions etc, used for nuclear disintegration.

Limitations:

1. Generator allows only one route for the movement of charge as it is a series combination.

2. It can accelerate only charged particles and not the uncharged particles.

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29.

(a) Deduce the expression for the torque acting on a dipole of dipole moment p in the presence of a uniform electric field E⃗.

(b) Consider two hollow concentric spheres, S₁ and S₂, enclosing charges 2Q and 4Q respectively as shown in the figure.

(i) Find out the ratio of the electric flux through them.

(ii) How will the electric flux through the sphere S₁ change if a medium of dielectric constant 'εr' is introduced in the space inside S₁ in place of air ?

Deduce the necessary expression.

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30.

(a) Deduce an expression for the frequency of revolution of a charged particle in a magnetic field and show that it is independent of velocity or energy of the particle.

(b) Draw a schematic sketch of a cyclotron. Explain, giving the essential details of its construction, how it is used to accelerate the charged particles.

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