Test: Conductors & Dielectrics


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20 Questions MCQ Test Electromagnetic Fields Theory (EMFT) | Test: Conductors & Dielectrics

Test: Conductors & Dielectrics for Electrical Engineering (EE) 2023 is part of Electromagnetic Fields Theory (EMFT) preparation. The Test: Conductors & Dielectrics questions and answers have been prepared according to the Electrical Engineering (EE) exam syllabus.The Test: Conductors & Dielectrics MCQs are made for Electrical Engineering (EE) 2023 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests for Test: Conductors & Dielectrics below.
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Test: Conductors & Dielectrics - Question 1

Which of the following are conductors?

Detailed Solution for Test: Conductors & Dielectrics - Question 1

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.

Test: Conductors & Dielectrics - Question 2

Find the range of band gap energy for conductors.

Detailed Solution for Test: Conductors & Dielectrics - Question 2

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

Test: Conductors & Dielectrics - Question 3

Conduction in metals is due to

Detailed Solution for Test: Conductors & Dielectrics - Question 3

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.

Test: Conductors & Dielectrics - Question 4

Alternating current measured in a transmission line will be

Detailed Solution for Test: Conductors & Dielectrics - Question 4

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.

Test: Conductors & Dielectrics - Question 5

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

Detailed Solution for Test: Conductors & Dielectrics - Question 5

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.

Test: Conductors & Dielectrics - Question 6

The current in a metal at any frequency is due to

Detailed Solution for Test: Conductors & Dielectrics - Question 6

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.

Test: Conductors & Dielectrics - Question 7

For conductors, the free electrons will exist at

Detailed Solution for Test: Conductors & Dielectrics - Question 7

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.

Test: Conductors & Dielectrics - Question 8

The current flowing through an insulating medium is called

Detailed Solution for Test: Conductors & Dielectrics - Question 8

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.

Test: Conductors & Dielectrics - Question 9

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

Detailed Solution for Test: Conductors & Dielectrics - Question 9

Answer: c
Explanation: The conduction current density is given by, J = σE
J = 500 X 2 = 1000 units.

Test: Conductors & Dielectrics - Question 10

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

Detailed Solution for Test: Conductors & Dielectrics - Question 10

Answer: b
Explanation: The convection current density is given by, J = ρeV
J = 200 X 12= 2400 units.

Test: Conductors & Dielectrics - Question 11

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

Detailed Solution for Test: Conductors & Dielectrics - Question 11

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.

Test: Conductors & Dielectrics - Question 12

Identify a good dielectric.

Detailed Solution for Test: Conductors & Dielectrics - Question 12

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.

Test: Conductors & Dielectrics - Question 13

A dielectric can be made a conductor by

Detailed Solution for Test: Conductors & Dielectrics - Question 13

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.

Test: Conductors & Dielectrics - Question 14

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

Detailed Solution for Test: Conductors & Dielectrics - Question 14

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.

Test: Conductors & Dielectrics - Question 15

For a dielectric which of the following properties hold good?

Detailed Solution for Test: Conductors & Dielectrics - Question 15

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.

Test: Conductors & Dielectrics - Question 16

The magnetic field which destroys the superconductivity is called

Detailed Solution for Test: Conductors & Dielectrics - Question 16

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.

Test: Conductors & Dielectrics - Question 17

The magnetic susceptibility in a superconductor will be

Detailed Solution for Test: Conductors & Dielectrics - Question 17

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

Test: Conductors & Dielectrics - Question 18

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

Detailed Solution for Test: Conductors & Dielectrics - Question 18

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.

Test: Conductors & Dielectrics - Question 19

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

Detailed Solution for Test: Conductors & Dielectrics - Question 19

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.

Test: Conductors & Dielectrics - Question 20

Find the velocity of an electron when its kinetic energy is equal to one electron volt (in 105m/s).
Given charge of an electron e = 1.6 x 10-19 and mass of an electron m = 9.1 x 10-31

Detailed Solution for Test: Conductors & Dielectrics - Question 20

Answer: c
Explanation: When the kinetic energy and one electron volt are equal, we can equate mv2/2 = eV. Put e and m in the equation to get velocity v = 5.9 x 105 m/s.

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