Subject

Physics

Class

CBSE Class 12

Pre Boards

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Sample Papers

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 Multiple Choice QuestionsLong Answer Type

31.

Describe the working principle of a moving coil galvanometer.

Why is it necessary to use
(i) a radial magnetic field and
(ii) a cylindrical soft iron core in a galvanometer?

Write the expression for the current sensitivity of the galvanometer.

Can a galvanometer as such be used for measuring the current? Explain.

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32.

(a) Define the term 'self-inductance' and write its S.I. unit.

(b) Obtain the expression for the mutual inductance of two long co-axial solenoids S1 and S2 wound one over the other, each of length L and radii r1 and r2 and n1 and n2 number of turns per unit length when a current I is set up in the outer solenoid S2.

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33. Draw a labelled diagram of AC generator. Derive the expression for the instantaneous value of the emf induced in the coil.
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34.

(a) Draw a labelled diagram of a step-up transformer. Obtain the ratio of secondary to primary voltage in terms of number of turn and currents in the two coils.

(b) A power transmission line feeds input power at 2200 V to a step-down transformer with its primary windings having 300 turns. Find the number of turns in the secondary to get the power output at 220 V.

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35.

Derive an expression for drift velocity of electrons in a conductor. Hence deduce Ohm's law.


Let an electric field E be applied the conductor. Acceleration of each electron is
><br>Velocity gained by the electron<br><br><img class=
Let the conductor contain n electrons per unit volume. The average value of time't', between their successive collisions, is the relaxation time, 't'.

Hence average drift velocity 

The amount of charge, crossing area A, in time Δt is
=neAvdΔt = IΔt

Substituting the value of vd, we get

But I = JA, where J is the current density

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36.

State the two Kirchhoff's laws. Explain briefly how these rules are justified.

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37.

The current is drawn from a cell of emf E and internal resistance r connected to the network of resistors each of resistance r as shown in the figure. Obtain the expression for (i) the current draw from the cell and (ii) the power consumed in the network.

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38.

(i) A ray of light incident on face AB of an equilateral glass prism shows the minimum deviation of 30°. Calculate the speed of light through the prism.


(ii) Find the angle of incidence at face AB so that the emergent ray grazes along the face AC.

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39. Distinguish between unpolarized light and linearly polarized light. How does one get the linearly polarised light with the help of a polar?
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