 # CBSE [All India]-Set-2-2005

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• Q1

What should be the length of a dipole antenna for a carrier wave of frequency 6 x 108 Hz?

Marks:1

The length of the dipole antenna is given by the relation, /2 = c/2v

= (3 108)/(6 108)

= 0.5 m

• Q2

Show graphically how the stopping potential for a given photosensitive surface varies with the frequency of the incident radiation.

Marks:1

The required graph is given below: • Q3

In a series LCR circuit, the voltage across an inductor, capacitor and resistor are 20 V, 20 V and 40 V respectively. What is the phase difference between the applied voltage and the current in the circuit?

Marks:1

We have the relation, Hence the phase difference =0 .

• Q4

Under what condition does an electron moving through a magnetic field experience maximum force?

Marks:1

The maximum force is experienced when electron moves perpendicular to the direction of the magnetic field.

• Q5

How does the coulomb force between two point charges depend upon the dielectric constant of the intervening medium?

Marks:1

The coulomb force between two point charges is inversely proportional to the dielectric constant of the intervening medium.

• 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

A parallel capacitor is to be designed with a voltage rating 1 kV using a material of dielectric constant 3 and dielectric strength about 107 Vm-1 . For safety we would like the field never to exceed say, 10% of the dipole strength. What minimum area of the plats is required to have a capacitance of 50 pF?

Marks:2

Given that maximum permissible voltage=1kV=103V.

Also the maximum permissible value of electric field is given by,

=10% of 107V/m=106V/m.

The distance between the plates of the capacitor is given by, • Q8

A 4 capacitor is charged by a 200 V supply. The supply is then disconnected and the charged capacitor is connected to another uncharged 2 Fcapacitor. How much electrostatic energy of the first capacitor is lost in the process of attaining the steady situation?

Marks:2

Initial energy stored in the 4 F capacitor is given by, • Q9

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. 