# Tricky Electromagnetic Theory Questions and Answers

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This set consists of Tricky Electromagnetic Theory Questions and Answers.

1. The tangential component of an electric field will be continuous in which boundary?
a) Conductor-Conductor
b) Conductor-Dielectric
c) Dielectric-Dielectric
d) Any boundary

Explanation: The tangential component of an electric field will be continuous in any type of boundary.

2. The depth of penetration of a wave in a lossy dielectric increases with increasing
a) Conductivity
b) Permeability
c) Wavelength
d) Permittivity

Explanation: The depth of penetration or skin depth is inversely proportional to the frequency. Thus it has to be directly proportional to the wavelength.

3. Magnetic vector potential is a vector
a) Whose curl is equal to the magnetic flux density
b) Whose curl is equal to the electric field intensity
c) Whose divergence is equal to electric potential
d) Which is equal to the vector product E x H

Explanation: The magnetic vector potential is A. The curl of A is a vector and it is equal to the magnetic flux density. It is given by Curl(A) = B.
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4. For the wave equation E = 10sin (wt-5z)ax, the wave propagation will be in the direction of
a) Y direction
b) Z direction
c) X direction
d) XY direction

Explanation: The equation represents the electric field component of the wave. Here, ax represents the electric wave direction and z represents the wave direction.

5. An electric field on a plane is described by its potential V = 20(r-1 + r-2), where r is the distance from the source. The field is due to
a) A monopole
b) A dipole
c) Both a monopole and a dipole

Explanation: In the potential given, one component is inversely proportional to r, which is due to a monopole and the other component is inversely proportional to r2, which is due to a dipole.

6. The electric field strength at a far-off point P due to a point charge, +q located at the origin O is 100 milliVolt/meter. The point charge is now enclosed by a perfectly conducting hollow metal sphere with its centre and the origin O. The electric field strength at the point, P is
a) Remains unchanged in its magnitude and direction
b) Remains unchanged in its magnitude but reverse in direction
c) Would be that due to a dipole formed by the charge, +q, at O and -q induced
d) Would be zero

Explanation: According to Gauss Law, the total displacement or electric flux through any closed surface surrounding charges is equal to the amount of charge enclosed. The total enclosed charge is -q + q = 0. Thus the flux and electric field strength will be zero.

7. The electric field of a uniform plane electromagnetic wve in free space, along the positive direction, is given by E = 10(ay + jaz)e-j25x. The frequency and polarization of the wave, respectively are
a) 1.2 GHz, left circular
b) 4 GHz, left circular
c) 1.2 GHz, right circular
d) 4 GHz, right circular

Explanation: The amplitudes of the components are the same. Hence it is circular polarization. The phase difference is +90 degree. Thus it is left hand circular polarisation. The frequency f = c/wavelength = 3 x 108 x 25/2 x 3.14 = 1.2 GHz.

8. Identify which one of the following will NOT satisfy the wave equation.
a) 50ej(wt – 3z)
b) sin(w(10z + 5t))
c) cos(y2 + 5t)
d) sin x cos t

Explanation: The wave cos(y2 + 5t) does not satisfy the general wave equation. Thus it is not an electromagnetic wave.

9. The intrinsic impedance of copper at high frequencies is
a) Purely resistive
b) Purely inductive
c) Complex with a capacitive component
d) Complex with a inductive component

Explanation: The intrinsic impedance is given by √(jwµ√ϭ + jw€). Since copper is a good conductor, ϭ << jw€, thus the intrinsic impedance will have +j component only. This refers to complex with an inductive component.

10. A loop is rotating about the y axis in a magnetic field B = Bocos(wt + α)at. The voltage in the loop is
a) Zero
b) Due to rotation only
c) Due to transformer action only
d) Due to both rotation and transformer action

Explanation: The voltage in the loop is due to two reactions – time varying magnetic field and voltage induced in a loop moving with velocity v in steady magnetic field. Thus the voltage in the loop is due to both rotation and transformer action.

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