# Electromagnetic Theory Questions and Answers – Lossy and Lossless Dielectrics

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This set of Electromagnetic Theory Multiple Choice Questions & Answers (MCQs) focuses on “Lossy and Lossless Dielectrics”.

1. For a dielectric, the condition to be satisfied is
a) σ/ωε > 1
b) σ/ωε < 1
c) σ = ωε
d) ωε = 1

Explanation: In a dielectric, the conductivity will be very less. Thus the loss tangent will be less than unity. This implies σ/ωε < 1 is true.

2. For a perfect dielectric, which parameter will be zero?
a) Conductivity
b) Frequency
c) Permittivity
d) Permeability

Explanation: The conductivity will be minimum for a dielectric. For a perfect dielectric, the conductivity will be zero.

3. Calculate the phase constant of a wave with frequency 12 rad/s and velocity 3×108 m/s(in 10-8 order)
a) 0.5
b) 72
c) 4
d) 36

Explanation: The phase constant is given by β = ω√(με), where ω is the frequency in rad/s and 1/√(με) is the velocity of wave. On substituting √(με) = 3×108 and ω = 12, we get β = 12/(3×108) = 4 x 10-8m/s.

4. For a lossless dielectric, the attenuation will be
a) 1
b) 0
c) -1
d) Infinity

Explanation: The attenuation is the loss of power of the wave during its propagation. In a lossless dielectric, the loss of power will not occur. Thus the attenuation will be zero.

5. Calculate the velocity of a wave with frequency 2 x109 rad/s and phase constant of 4 x 108 units.
a) 0.5
b) 5
c) 0.2
d) 2

Explanation: The velocity of a wave is the ratio of the frequency to the phase constant. Thus V = ω/β. On substituting the given values, we get V = 2 x109/ 4 x 108 = 5 units.

6. Which of the following is the correct relation between wavelength and the phase constant of a wave?
a) Phase constant = 2π/wavelength
b) Phase constant = 2π x wavelength
c) Phase constant = 1/(2π x wavelength)
d) Phase constant = wavelength/2π

Explanation: The phase constant is the ratio of 2π to the wavelength λ. Thus β = 2π/λ is the correct relation.

7. In lossy dielectric, the phase difference between the electric field E and the magnetic field H is
a) 90
b) 60
c) 45
d) 0

Explanation: In a lossy dielectric, the E and H component will be in phase. This implies that the phase difference between E and H will be 0.

8. The intrinsic impedance is the ratio of square root of
a) Permittivity to permeability
b) Permeability to permittivity
c) Phase constant to wavelength
d) Wavelength to phase constant

Explanation: The intrinsic impedance is the impedance of a particular material. It is the ratio of square root of the permeability to permittivity. For air, the intrinsic impedance is 377 ohm or 120π.

9. Calculate the skin depth of a material with attenuation constant of 2 units.
a) 2
b) 1
c) 0.5
d) 4

Explanation: The skin depth of a material is the reciprocal of the attenuation constant. Thus δ = 1/α. On substituting for α = 2, we get δ = ½ = 0.5 units.

10. Calculate the phase constant of a wave with skin depth of 2.5 units.
a) 5/2
b) 5
c) 2
d) 2/5

Explanation: The skin depth is the reciprocal of the phase constant and the attenuation constant too. Thus δ = 1/β. On substituting for δ = 2.5, we get β = 1/δ = 1/2.5 = 2/5 units.

11. An example for lossless propagation is
a) Dielectric waveguide propagation
b) Conductor propagation
c) Cavity resonator propagation
d) It is not possible

Explanation: There are many techniques employed to achieve zero attenuation or maximum propagation. But it is not achievable practically. Thus lossless propagation is not possible practically.

12. Skin depth phenomenon is found in which materials?
a) Insulators
b) Dielectrics
c) Conductors
d) Semiconductors

Explanation: Skin depth is found in pure conductors. It the property of the conductor to allow a small amount of electromagnetic energy into its skin, but not completely. This is the reason why EM waves cannot travel inside a good conductor.

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