# CBSE [All India]-Set-3-2005

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• Q1

Two electric field lines never cross each other. Why?

Marks:1

Two electric field of lines never cross each other since it would imply two direction of electric field at the point of their intersection which is not possible practically.

• Q2

Under what conditions is the force acting on a charge moving through a uniform magnetic field minimum?

Marks:1

The force acting on charge is minimum or zero when charges move parallel or antiparallel to the direction of magnetic field.

• Q3

What is the power dissipated in an a.c. circuit in which voltage and current are given by,

Marks:1

From of the given equation it follows that phase difference between V and I is /2.

We have the relation,

• Q4

In an experiment on photoelectric effect, the following graphs were obtained between the photoelectric current ( I ) and the anode potential (V). Name the characteristic of the incident radiation that was kept constant in this experiment.

Marks:1

In the given experiment the frequency of incident radiation is kept constant.

• Q5

How does the effective power radiated by an antenna vary with wavelength?

Marks:1

• Q6

Define electric field intensity. Write its S. l . unit. Write the magnitude and direction of electric field intensity due to an electric dipole of length 2a at the mid- point of the line joining the two charges.

Marks:2

The electric feld intensity at any point may be defined as the force experienced by a unit positive charge placed at that point.

Mathematically,

• Q7

How does the resistivity of (i) a conductor and (ii) a semiconductor vary with temperature? Give reason for each case.

Marks:2

The mathematical relation for the resistivity of a material in terms of relaxation time, number density and mass and charge of charge carriers is given by relation,

= m/ne2

1. Resistivity of metal : Resistivity of metal will decrease with increase in temperature. This is because free electrons collide more frequently with positive metal ions, which results in decrease of relaxation time. This ultimately increases resistivity of metal.

2. Resistivity of semiconductors  : In this case the number density (n) of free electron increases exponentially, with temperature, which results in decrease of resistivity.

• Q8

Write two characteristic properties to distinguish between diamagnetic and paramagnetic materials.

Marks:2

The characteristic properties to distinguish between diamagnetic and paramagnetic materials are given in the table below:

• Q9

A right-angled crown glass prism with critical angle 41o is placed before an object PQ, in two positions as shown in the figures (i) and (ii). Trace the paths of the rays from P and Q passing through the prisms in the two cases

Marks:2

The paths of the rays from P and Q passing through the prisms in the two cases is given below,

• Q10

Mention the factors on which the resonant frequency of a series LCR circuit depends. Plot a graph showing variation of impedance of a series LCR circuit with the frequency of the applied a.c. source.

Marks:2