# Electromagnetic Theory Questions and Answers – Maxwell Law

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

1. The divergence of which quantity will be zero?
a) E
b) D
c) H
d) B

Explanation: The divergence of the magnetic flux density is always zero. This is because of the non existence of magnetic monopoles in a magnetic field.

2. Find the charge density when the electric flux density is given by 2x i + 3y j + 4z k.
a) 10
b) 9
c) 24
d) 0

Explanation: The charge density is the divergence of the electric flux density by Maxwell’s equation. Thus ρ = Div (D) and Div (D) = 2 + 3 + 4 = 9. We get ρ = 9 units.

3. Find the Maxwell equation derived from Faraday’s law.
a) Div(H) = J
b) Div(D) = I
c) Curl(E) = -dB/dt
d) Curl(B) = -dH/dt

Explanation: From the Faraday’s law and Lenz law, using Stoke’s theorem, we get Curl(E) = -dB/dt. This is the Maxwell’s first law of electromagnetics.
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4. Find the Maxwell law derived from Ampere law.
a) Div(I) = H
b) Div(H) = J
c) Curl(H) = J
d) Curl(B) = D

Explanation: From the current density definition and Ohm’s law, the Ampere circuital law Curl(H) = J can be derived. This is Maxwell’s second law of electromagnetics.

a) Transformer EMF
b) Back EMF
c) Generator EMF
d) Secondary EMF

Explanation: The stationary loop in a varying magnetic field results in an induced emf due to the change in the flux linkage of the loop. This emf is called as induced or transformer EMF.

6. In which of the following forms can Maxwell’s equation not be represented?
a) Static
b) Differential
c) Integral
d) Harmonic

Explanation: Maxwell equations can be represented in differential/point form and integral form alternatively. Sometimes, it can be represented by time varying fields called harmonic form.

7. The charge build up in the capacitor is due to which quantity?
a) Conduction current
b) Displacement current
c) Convection current
d) Direct current

Explanation: The charge in the capacitor is due to displacement current. It is the current in the presence of the dielectric placed between two parallel metal plates.

8. In metals which of the following equation will hold good?
a) Curl(H) = J
b) Curl(J) = dD/dt
c) Curl(H) = D
d) Curl(J) = dB/dt

Explanation: Generally, the Curl(H) is the sum of two currents- conduction and displacement. In case of metals, it constitutes conduction J and in case of dielectrics, it constitutes the displacement current dD/dt.

9. Find the flux enclosed by a material of flux density 12 units in an area of 80cm.
a) 9.6
b) 12/80
c) 80/12
d) 12/0.8

Explanation: The total flux in a material is the product of the flux density and the area. It is given by flux = 12 x 0.8= 9.6 units.

10. Find the electric flux density of a material with charge density 16 units in unit volume.
a) 1/16
b) 16t
c) 16
d) 162

Explanation: The electric flux density from Maxwell’s equation is given by D = ∫ ρ dv. On substituting ρ = 16 and ∫dv = 1, we get D = 16 units.

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