Magnetostatics MCQ || Magnetostatics Questions and Answers

Answer.3. Energy must be spent, though no energy is required to maintain it.

Explanation:-

In order to establish a magnetic field around a coil, energy is required, though no energy is needed to maintain it. This energy is stored in the magnetic field and is not used up. When the current is decreased, the magnetic flux surrounding the coil is decreased, causing the stored energy to be returned to the circuit.

When the coil is connected to supply, the current increases from zero gradually and reaches the final value I =V/R after some time. During this change of current, an e.m.f is induced in L due to the change in magnetic flux linkages. This induced e.m.f opposes the rise of current. Electrical energy must be supplied to meet this opposition. This supplied energy is stored in the magnetic field.

Answer.1. 1 + M/H

Explanation:-

The magnetic flux density is given by

B = μ₀ (H + M)

Where, M = magnetization of the medium and H = magnetic field strength

μ_r is the relative permeability of a medium

⇒ μ₀μ_rH = μ₀ (H + M)

⇒ μ_rH = H + M

⇒ (μ_r – 1) H = M

⇒ (μ_r – 1) = M/H

μ_r  = 1 + M/H

Answer.1. Henry

Explanation:-

The SI unit of magnetic permeance is webers per ampere-turn that is H (henry). The permeance is analogous to the conductance in an electrical circuit. It is reciprocal of the reluctance (R) of the material in a magnetic circuit.

Answer.3. 14.4 V

Explanation:-

Given that, number of turns (N) = 360

Change in time (dt) = 0.01 s

Magnetic flux (ϕ) = 200 μWb

Since the flux is reversed, it changes from 200 μWb to -200 μWb, which is a change of 200 – (-200), i.e. 400 μWb

Change in magnetic flux (dϕ) = 400 μWb

Average induced emf is given by

$E = N\frac{{d\phi }}{{dt}}$

$E = 360 \times \frac{{400 \times {{10}^{ – 6}}}}{{0.01}} = 14.4\:V$

Answer.1. Ferromagnetic

Explanation:-

Ferromagnetic Substances: The substances which are strongly magnetized when placed in an external magnetic field in the same direction as the applied field are called ferromagnetic substances.

• These are characterized by parallel alignment of magnetic dipoles
• These substances are strongly attracted by a magnet
• It develops strong magnetization in the direction of the applied magnetic field
• By removing the magnetizing field, it does not lose its magnetization
• When placed in a non-uniform magnetic field, it tends to move from weaker to stronger regions of the magnetic field
• When placed in a uniform magnetic field, it aligns itself parallel to the direction of the magnetic field
• Magnetic susceptibility is much greater than 1
• Relative permeability is much greater than 1
• Magnetic permeability is much larger compared to free space

Answer.3. 2 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 (B_net) is given by

B_net = μ₀μ_rH

Where

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

μ_r is relative permeability = 1000 (given)

B_net = (4π  × 10^-7m × 1000 × 5000)/π

B_net = 2T

Answer.2. The e.m.f. of a generator

Explanation:-

Faraday’s first law of electromagnetic induction states that whenever a conductor is placed in a varying magnetic field, emf is induced which is called induced emf. If the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current.

Faraday’s second law of electromagnetic induction states that the magnitude of emf induced in the coil is equal to the rate of change of flux that linkages with the coil. The flux linkage of the coil is the product of the number of turns in the coil and flux associated with the coil.

These laws are related to the emf of a generator.

Answer.2. Change in flux

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

Answer.1. Diamagnetic material

Explanation:-

In paramagnetic materials, the magnetic field in the material is strengthened by induced magnetization.

In diamagnetic materials, the magnetic field in the material is weakened by induced magnetization. Ferromagnetic, ferrimagnetic materials possess permanent magnetization even without an external magnetic field.