# Electromagnetic Theory Questions and Answers – Skin Effect

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

1. The skin effect is a phenomenon observed in
a) Insulators
b) Dielectrics
c) Conductors
d) Semiconductors

Explanation: The skin of the conductor allows a certain amount of electromagnetic power to pass through it. This phenomenon is called the skin effect. This is the reason why, electromagnetic waves cannot travel inside a conductor.

2. The skin depth is measured in
a) Meter
b) Millimetre
c) Centimetre
d) Micrometer

Explanation: The depth to which the electromagnetic waves pass through the conductor is very small. It is measured in μm.

3. The skin depth is calculated from the amplitude of the wave. State true/false
a) True
b) False

Explanation: The skin depth is the measure of the depth to which the amplitude of an EM wave will reduce to 36.8% of its initial value. Thus it can be calculated if the initial amplitude is known.

4. The attenuation constant is 0.5 units. The skin depth will be
a) 0.5
b) 0.25
c) 2
d) 4

Explanation: The skin depth is the reciprocal of the attenuation constant. Thus δ = 1/α. On substituting for α = 0.5, we get δ = 1/0.5 = 2 units.

5. Calculate the skin depth of a conductor, having a conductivity of 200 units. The wave frequency is 10 GHz in air.
a) 355.8
b) 3.558
c) 35.58
d) 0.3558

Explanation: The skin depth is calculated by δ = 1/√(πfμσ), where f is the frequency, μ is the permeability and σ is the conductivity. For the given data, f = 10 x 109, μ = 4π x 10-7 in air and σ = 200, we get δ = 355.8 μm.

6. The effective skin resistance of a material with conductivity 120 and skin depth of 2μm is
a) 4.16 kilo ohm
b) 4.16 mega ohm
c) 41.6 kilo ohm
d) 41.6 mega ohm

Explanation: The effective skin resistance is given by Rs = 1/δσ, where δ is the skin depth and σ is the conductivity. For the given data, δ = 2 x 10-6 and σ = 120, we get Rs = 1/(120x2x10-6) = 4.16 kilo ohm.

7. The skin depth is used to find which parameter?
a) DC resistance
b) AC resistance
c) Permittivity
d) Potential

Explanation: Since the skin depth varies for different frequencies, it can be used to calculate the varying AC resistance for a material.

8. The relation between the skin depth and frequency is given by
a) Skin depth α f
b) Skin depth α 1/f
c) Skin depth α √f
d) Skin depth α 1/√f

Explanation: The skin depth is given by δ = 1/√(πfμσ). Thus the relation between the skin depth and the frequency is, Skin depth α 1/√f.

9. A perfect dielectric acts as a
a) Perfect transmitter
b) Perfect reflector

Explanation: A perfect dielectric acts as a perfect transmitter. In other words, a wave incident on a perfect dielectric will transmit completely through it.

10. A perfect conductor acts as a
a) Perfect transmitter
b) Perfect reflector

Explanation: A perfect conductor acts as a perfect reflector. In other words, a wave incident on a perfect conductor will be totally reflected back into the same medium. There will be no skin effect.

11. The resultant electric field of two components in the x and y direction having amplitudes 6 and 8 respectively is
a) 100
b) 36
c) 64
d) 10

Explanation: The resultant electric field of two components is given by E = √(Ex2 + Ey2). For the given data, the electric field will be E = √(62+82) = 10 units.

12. The skin depth of the wave having a frequency of 3MHz and a velocity of 12 m/s is
a) 2
b) 3
c) 4
d) 6

Explanation: The velocity of a wave is the product of the frequency and the skin depth. Thus v = f.δ. To get δ, put v = 12 and f = 3MHz, we get δ = 12/(3×106) = 4 μm.

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