# Electromagnetic Theory Questions and Answers – Conductors

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This set of Electromagnetic Theory Multiple Choice Questions & Answers (MCQs) focuses on “Conductors”.

1. Which of the following are conductors?
a) Ceramics
b) Plastics
c) Mercury
d) Rubber

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.

2. Find the range of band gap energy for conductors.
a) >6 eV
b) 0.2-0.4 eV
c) 0.4-2 eV
d) 2-6 eV

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

3. Conduction in metals is due to
a) Electrons only
b) Electrons and holes
c) Holes only
d) Applied electric field

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

4. Find the band gap energy when a light of wavelength 1240nm is incident on it.
a) 1eV
b) 2eV
c) 3eV
d) 4eV

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.

5. Alternating current measured in a transmission line will be
a) Peak value
b) Average value
c) RMS value
d) Zero

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.

6. The current in a metal at any frequency is due to
a) Conduction current
b) Displacement current
c) Both conduction and displacement current
d) Neither conduction nor displacement current

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.

7. For conductors, the free electrons will exist at
a) Valence band
b) Middle of valence and conduction band
c) Will not exist
d) Conduction band

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.

8. The current flowing through an insulating medium is called
a) Conduction
b) Convection
d) Susceptibility

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.

9. Find the conduction current density when conductivity of a material is 500 units and corresponding electric field is 2 units.
a) 500
b) 250
c) 1000
d) 2000

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

10. Calculate the convection current when electron density of 200 units is travelling at a speed of 12m/s.
a) 16.67
b) 2400
c) 2880
d) 0.06 