﻿ State the underlying principle of a potentiometer. Write two fac

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## CBSE Class 12 Physics Solved Question Paper 2015

#### Short Answer Type

1.

Draw a graph to show variation of capacitive-reactance with frequency in an a.c. circuit.

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2.

The field lines of a negative point charge are as shown in the figure. Does the kinetic energy of a small negative charge increase or decrease in going from B to A?

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3.

Distinguish between emf and terminal voltage of a cell.

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4.

In a meter bridge shown in the figure, the balance point is found to be 40 cm from end A. If a resistance of 10  is connected in series with R, balance point is obtained 60 cm from A. Calculate the values of R and S.

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5.

A series LCR circuit is connected across an a.c. source of variable angular frequency ‘’. Plot a graph showing variation of current 'i' as a function of ‘’ for two resistances R1 and R2 (R1 > R2).

Answer the following questions using this graph:

(a) In which case is the resonance sharper and why?

(b) In which case in the power dissipation more and why?
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6.

State Lenz's law. Illustrate, by giving an example, how this law helps in predicting the direction of the current in a loop in the presence of a changing magnetic flux.

In a given coil of self-inductance of 5 mH, current changes from 4 A to 1 A in 30 ms. Calculate the emf induced in the coil.

OR

In what way is Gauss's law in magnetism different from that used in electrostatics? Explain briefly.

The Earth's magnetic field at the Equator is approximately 0.4 G. Estimate the Earth's magnetic dipole moment. Given: Radius of the Earth = 6400 km.

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7.

How are electromagnetic waves produced? What is the source of energy of these waves? Draw a schematic sketch of the electromagnetic waves propagating along the + x-axis.

Indicate the directions of the electric and magnetic fields. Write the relation between the velocity of propagation and the magnitudes of electric and magnetic fields.
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8.

Find the equivalent capacitance of the network shown in the figure, when each capacitor is of 1  F. When the ends X and Y are connected to a 6 V battery, find out

(i) the charge and

(ii) the energy stored in the network

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# 9.State the underlying principle of a potentiometer. Write two factors by which current sensitivity of a potentiometer can be increased. Why is a potentiometer preferred over a voltmeter for measuring the emf of a cell?

Underlying principle of potentiometer: Potential difference across a uniform wire is directly proportional to the length of the part across which the potential is measured when, a steady current flow through the wire.

Sensitivity of potentiometer can be increased by:

(i) Increasing the length of the wire

(ii) Decreasing the current in the wire using a rheostat

Potentiometer is preferred over voltmeter because potentiometer used the null method. During the balanced condition of potentiometer, no current is drawn by the galvanometer. Voltmeter measures the voltage across the terminals of a cell when the cell is in closed circuit, that is, when current is flowing through the cell.

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10.

What is the function of a 'Repeater' used in communication system?

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