# Electromagnetic Theory Questions and Answers – Maxwell Law in Time Varying Fields

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This set of Electromagnetic Theory online quiz focuses on “Maxwell Law in Time Varying Fields”.

1. Find the curl of E when B is given as 15t.
a) 15
b) -15
c) 7.5
d) -7.5

Explanation: From Maxwell first law, we get Curl of E as the negative derivative of B with respect to time. Thus Curl(E) = -dB/dt. On substituting B= 15t and differentiating, Curl(E) = -15 units.

2. The charge build up in a capacitor is due to
a) Conduction current density
b) Displacement current density
c) Polarisation
d) Magnetization

Explanation: The capacitor consists of a dielectric placed between two conducting plates, subjected to a field. The current due to a dielectric is always due to the displacement current density.

3. The surface integral of which parameter is zero?
a) E
b) D
c) B
d) H

Explanation: The divergence of the magnetic flux density is always zero. By Stokes theorem, the surface integral of B is same as the volume integral of the divergence of B. Thus the surface integral of B is also zero.
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4. Harmonic electromagnetic fields refer to fields varying sinusoidally with respect to time. State True/False.
a) True
b) False

Explanation: Fields that varying sinusoidally with respect to time are called as harmonic fields. An example for harmonic fields is A sin wt.

5. When electric potential is null, then the electric field intensity will be
a) 0
b) 1
c) dA/dt
d) –dA/dt

Explanation: The electric field intensity is given by E = -Grad(V)- dA/dt, where V is the electric potential and A is the magnetic vector potential. When V is zero, then E = -dA/dt.

6. The gradient of the magnetic vector potential can be expressed as
a) –με dV/dt
b) +με dE/dt
c) –με dA/dt
d) +με dB/dt

Explanation: The gradient of A is the ratio of the negative gradient of electric potential to the speed of light c. We can write c = 1/√(με). Thus grad(A) = -με dV/dt is the required expression.

7. Find the time constant of a capacitor with capacitance of 2 microfarad having an internal resistance of 4 megaohm.
a) 2
b) 0.5
c) 8
d) 0.25

Explanation: The time constant of capacitor is given by T = RC, where R = 4×106 and C = 2×10-6. Thus T = 4×106 x2x10-6 = 8 seconds.

8. Which components exist in an electromagnetic wave?
a) Only E
b) Only H
c) Both E and H
d) Neither E or H

Explanation: In an electromagnetic wave, the electric and magnetic components coexist. They propagate perpendicular to each other and to the direction of propagation in space.

9. The propagation of the electromagnetic waves can be illustrated by
b) Ampere law
c) Flemming rule
d) Coulomb law

Explanation: By Flemming’s rule, when the thumb and the middle finger represent the inputs (say current and field respectively), then the fore finger represents the output (force, in this case). The EM propagation can be illustrated by this rule.

10. Which one of the following laws will not contribute to the Maxwell’s equations?
a) Gauss law
c) Ampere law
d) Curie Weiss law

Explanation: The Gauss law, Faraday law and the Ampere law are directly used to find the parameters E, H, D, B. Thus it contributes to the Maxwell equations. The Curie Weiss law pertains to the property of any magnetic material. Thus it is not related to the Maxwell equation.

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