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QUESTION: 1

Which of the following are conductors?

Solution:

Answer: c

Explanation: Normally, metals are said to be good conductors. Here mercury is the only metal (which is in liquid form). The other options are insulators.

QUESTION: 2

Find the range of band gap energy for conductors.

Solution:

Answer: b

Explanation: Conductors are materials with least band gap energy. The smallest range in this group is 0.2-0.4 eV.

QUESTION: 3

Conduction in metals is due to

Solution:

Answer: a

Explanation: Conduction in metals is only due to majority carriers, which are electrons. Electrons and holes are responsible for conduction in a semiconductor.

QUESTION: 4

Alternating current measured in a transmission line will be

Solution:

Answer: c

Explanation: The instantaneous current flowing in a transmission line, when measured using an ammeter, will give RMS current value. This value is 70.7% of the peak value. This is because, due to oscillations in AC, it is not possible to measure peak value. Hence to normalise, we consider current at any time in a line will be the RMS current.

QUESTION: 5

Find the band gap energy when a light of wavelength 1240nm is incident on it.

Solution:

Answer: a

Explanation: The band gap energy in electron volt when wavelength is given is, Eg = 1.24(μm)/λ = 1.24 x 10^{-6}/1240 x 10^{-9} = 1eV.

QUESTION: 6

The current in a metal at any frequency is due to

Solution:

Answer: a

Explanation: At any frequency, the current through the metal will be due to conduction current. Only at high frequencies and when medium is air, the conduction is due to displacement current. Thus in general the current in metal is due to conduction current, which depends on the mobility of the carriers.

QUESTION: 7

For conductors, the free electrons will exist at

Solution:

Answer: d

Explanation: In conductors, the free electrons exist in the conduction band. Since the band gap energy is very low, less energy is required to transport the free electrons to the conduction band, as they are readily available to conduct.

QUESTION: 8

The current flowing through an insulating medium is called

Solution:

Answer: b

Explanation: A beam of electrons in a vacuum tube is called convection current. It occurs when current flows through an insulating medium like liquid, vacuum etc.

QUESTION: 9

Find the conduction current density when conductivity of a material is 500 units and corresponding electric field is 2 units.

Solution:

Answer: c

Explanation: The conduction current density is given by, J = σE

J = 500 X 2 = 1000 units.

QUESTION: 10

Calculate the convection current when electron density of 200 units is travelling at a speed of 12m/s.

Solution:

Answer: b

Explanation: The convection current density is given by, J = ρeV

J = 200 X 12= 2400 units.

QUESTION: 11

A dielectric is always an insulator. But an insulator is not necessarily a dielectric. State True/False.

Solution:

Answer: a

Explanation: For a material to be dielectric, its permittivity should be very high. This is seen in insulators. For a material to be insulator, the condition is to have large band gap energy. However, this is not necessary for a dielectric.

QUESTION: 12

Identify a good dielectric.

Solution:

Answer: b

Explanation: Iron and magnesium are metals. Hence they need not be considered. Both ceramics and plastic are insulators. But dielectric constant is more for ceramics always. Hence ceramics is the best dielectric.

QUESTION: 13

A dielectric can be made a conductor by

Solution:

Answer: b

Explanation: On increasing the temperature, the free electrons in an insulator can be promoted from valence to conduction band. Gradually, it can act as a conductor through heating process. This condition is called dielectric breakdown, wherein the insulator loses its dielectric property and starts to conduct.

QUESTION: 14

Find the dielectric constant for a material with electric susceptibility of 4.

Solution:

Answer: b

Explanation: The electric susceptibility is given by χe = εr – 1. For a susceptibility of 4, the dielectric constant will be 5. It has no unit.

QUESTION: 15

For a dielectric which of the following properties hold good?

Solution:

Answer: b

Explanation: Superconductors are characterised by diamagnetism behaviour and zero resistivity, which true for a dielectric. They occur only at low temperature. Thus a dielectric can become a superconductor at low temperatures with very high dielectric breakdown voltage.

QUESTION: 16

The magnetic field which destroys the superconductivity is called

Solution:

Answer: d

Explanation: Critical field is that strong magnetic field which can destroy the superconductivity of a material. The temperature at which this occurs is called transition temperature.

QUESTION: 17

The magnetic susceptibility in a superconductor will be

Solution:

Answer: b

Explanation: Due to perfect diamagnetism in a superconductor, its magnetic susceptibility will be negative. This phenomenon is called Meissner effect.

QUESTION: 18

The superconducting materials will be independent of which of the following?

Solution:

Answer: b

Explanation: Superconducting materials depends only on the applied magnetic field, resultant magnetization at the temperature considered. It is independent of the applied electric field and the corresponding polarization.

QUESTION: 19

Find the mean free path of an electron travelling at a speed of 18m/s in 2 seconds.

Solution:

Answer: b

Explanation: The mean free path is defined as the average distance travelled by an electron before collision takes place. It is given by, d = v x τc, where v is the velocity and τc is the collision time. Thus d = 18 x 2 = 36m.

QUESTION: 20

Find the velocity of an electron when its kinetic energy is equal to one electron volt (in 10^{5}m/s).

Given charge of an electron e = 1.6 x 10^{-19} and mass of an electron m = 9.1 x 10^{-31}.

Solution:

Answer: c

Explanation: When the kinetic energy and one electron volt are equal, we can equate mv^{2}/2 = eV. Put e and m in the equation to get velocity v = 5.9 x 10^{5} m/s.

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