# Electromagnetic Theory MCQ || Electromagnetic Theory Questions and Answers

Ques.1. Find H = ___________ A/m at the center of a circular coil of diameter 1 m and carrying a current of 2 A.

1. 0.6366
2. 0.1636
3. 6.366
4. 2

Explanation

The magnetic field intensity (H) of a circular coil is given by

H = I/2R

Where I is the current flow through the coil

R is the radius of the circular coil

Given

Current (I) = 2 A

Diameter = 1 m

Magnetic field intensity = H = 2/2 × 0.5

H = 2 A/m

Ques.2. What is the SI unit for magnetic reluctance?

1. Tesla
2. Henry
3. Tesla-1
4. Henry-1

Explanation

The SI unit for magnetic reluctance is Henry-1

Magnetic Units:

 QUANTITY UNIT Flux density (B) Tesla (T) Magnetic flux (ϕ ) Weber (Wb) Magnetic Field Intensity (H) Ampere – turns/meter (AT/m) Magnetomotive force (Fm) Ampere-turn (At) Permeability (μ) Webers/ampere-turns – meter (Wb/Atm) Reluctance (R) Ampere – turns/Weber (At/Wb) Magnetic reluctance Henry-1

Ques.3. Which of the following materials is used for the generation of ultrasonic waves by using magnetostriction effect?

1. Paramagnetic material
2. Ferromagnetic material
3. Diamagnetic materials
4. Both paramagnetic and diamagnetic material

Explanation

Ferromagnetic substance or material is used for the generation of ultrasonic waves by using the magnetostriction effect.

Magnetostriction Effect:

• When a magnetic field is applied parallel to the length of a ferromagnetic rod made of a material such as iron or nickel, a small elongation or contraction occurs in its length. This is known as magnetostriction.
• The change in length depends on the intensity of the applied magnetic field and the nature of the ferromagnetic material.
• The change in length is independent of the direction of the field.

Ques.4. In Lenz’s law the induced emf ‘e’ opposes the-

1. Flux
2. Change in flux
3. Both the flux and change flux
4. None of the options

Explanation

In Lenz’s law, the induced emf ‘e’ opposes the change (increase or decrease) in flux.

Lenz’s law: When a voltage is generated by a change in magnetic flux according to Faraday’s law, the polarity of the induced voltage is such that it produces a current whose magnetic field opposes the change which produces it.

The induced emf is given by the rate of change of magnetic flux linked with the circuit i.e.

e = −dφ/dt

Where dΦ = change in magnetic flux and e = induced e.m.f.

Ques.5. According to Gauss’s law, the electric field due to an infinitely long thin charged wire varies as:

1. 1/r
2. 1/r2
3. r
4. r2

Explanation

Electric field due to an infinitely long straight conductor is

$E = \frac{\lambda }{{2\pi {_o}r}}{\rm{\;}}$

Where

λ = linear charge density

r = radius of the cylinder

εo = permittivity of free space.

From the above equation, it is clear that the electric field of an infinitely long straight wire is proportional to 1/r. Hence option 1 is correct.

Ques.6. According to Gauss’s law, if E is _________ the charge density in the ideal conductor is zero.

1. Positive
2. Negative
3. Zero
4. Unity

Explanation

The electric field and charge density in a perfect conductor are both zero, and thus the conductor is an equipotential and is electroneutral. If E is zero, the charge density in the ideal conductor is zero.

Ques.7. The force per unit length between two stationary parallel wires carrying (steady) currents _____.

1. Is inversely proportional to the separation of wires
2. Is proportional to the magnitude of each current
3. Satisfies Newton’s third law
4. All of the above

Explanation

The force between two current-carrying parallel conductors:

• Two current-carrying conductors attract each other when the current is in the same direction and repel each other when the currents are in the opposite direction.
• Force exerted on one wire due to currents through two wires is inversely proportional to the distance between them. Thus the force becomes half if the distance between the wires is doubled.
• The attractive force between two parallel current-carrying wires is  proportional to the product of the two currents.

Force per unit length on conductor

$\frac{F}{l}=~\frac{{{\mu }_{0}}}{2\pi }\frac{{{i}_{1}}{{i}_{2}}}{d}$

It satisfies newton’s third law.

Ques.8. What is analogous to electric field in magnetic circuits?

1. Magnetic Field
2. Magnetic flux density
3. Magnetomotive force
4. Reluctance

Explanation

Magnetic Field is analogous to the electric field in magnetic circuits

Magnetic circuits are analogous to electric circuits. Some of the quantities are given below.

 Magnetic Circuit Electric Circuit Flux Current Permeance Conductance Reluctance Resistance Permeability Conductivity Flux density Current density Magnetic field Electric field Magnetomotive force Electromotive force

Ques.9. Which of the following effect proves the wave nature of light?

1. Photoelectric effect
2. Compton effect
3. Pair production
4. Polarization

Explanation

Polarization of light occurs when light is reflected, refracted, and scattered.  The direction of vibration of particles is a property associated with waves. Since light shows the vibration of the electric and magnetic field through polarization, the wave nature of light is concluded from polarization.

Ques.10. In the magnetic circuit shown below, what is the flux density produced if the relative permeability of the core material under the given condition is 1000?

1. 1 T
2. 3 T
3. 2 T
4. 4 T

Explanation

Given

N = 100
I = 5 A
L = 2πr = 2π × 5 × 10-2 m

Magnetic Field Strength H = NI/L

H = (100 × 5)/2π × 5 × 10-2

H = 5000/π

The net Magnetic Field Density (Bnet) is given by

Bnet = μ0μrH

Where

μ0 is absolute permeability. = 4π  × 10-7m

μr is relative permeability = 1000 (given)

Bnet = (4π  × 10-7m × 1000 × 5000)/π

Bnet = 2T

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