Test: Electromagnetic Fields Theory- 1 - Electrical Engineering (EE) MCQ

In a perfect magnetic conductor the value of has infinite magnetic conductivity (inverse of magnetic reluctance). In a perfect magnetic conductor the normal electric field and tangential magnetic field is zero while tangential electric field and normal magnetic field is equal.

Ohm’s law in case of magnetic circuits can be stated as, the magnetic flux developed in the circuit is directly proportional to magnetomotive force. The relation is similar to Ohm’s law and can be formed by replacing electro motive force (emf) with mmf and electric current (i) with magnetic flux.

Flux = mmf/reluctance.

In a capacitor the value of capacitor is given by  hence with dielectric having high permittivity higher values of capacitance can be achieved with small capacitor size.

Given that, mmf = 50 Amp-turns

Magnetic flux = 25 Wb

Reluctance = mmf/flux = 50/25 = 2 Amp-turns/Wb

Permeance is a magnetic property, it is analogous to conductance. Reluctance is analogous to resistance.

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor. Hence the electrical field strength decreases.

Of the given material, only iron cobalt alloy cannot be used to make permanent magnets.

Given that, Magnetic flux density (B) = 4 Wb/sq. m.

Length (l) = 0.6 m

Velocity (v) = 50 m/s

Emf induced is, E=Blvsinθ

E=4×0.6×50×sin90=120

B-H curve is used to show the relationship between magnetic flux density (B) and magnetic field strength (H) for a particular material.

There is no hysteresis loss in air as air does not consists of magnetic material. So the B-H curve in the air should be a straight line.

According to the equation B = μH, this straight line passes through origin and has a slope of μ.

In paramagnetic materials, the magnetic field in the material is strengthened by the induced magnetization. Hence the magnetic susceptibility value is a small positive.

In diamagnetic materials, the magnetic field in the material is weakened by the induced magnetization. Hence the magnetic susceptibility value is a small negative.

Ferromagnetic, ferrimagnetic materials possess permanent magnetization even without external magnetic field. Hence these materials have large positive value of magnetic susceptibility.

Magnetic susceptibility is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field.

Magnetic susceptibility (X) of a material is the ratio of magnetization I (magnetic moment per unit volume) to the applied magnetizing field intensity H.

X = I/H

Given that, inductance (L) = 8 H

Rate of change of current = 0.5 A/sec

The magnetic field intensity at any point is defined as the force experienced by the point when placed under the influence of an external magnetic field.

Intensity of magnetization = pole strength/area of the pole of the magnet = 30/2 = 15 A/m

Reluctance in a magnetic circuits opposes the passage of magnetic flux. This is analogous to resistance in the electric circuit but rather than dissipating electric energy it stores magnetic energy. Permeance is the inverse of reluctance. In a magnetic circuit, permeance is a measure of the quantity of magnetic flux for a number of current-turns. This is analogous to electrical conductance in the electric circuit.

Inductance of a solenoid is given by,

Midway between two equal and similar charges, a third equal and similar charge is placed. As the charge is placed midway between two similar charges, the force acts on the third charge due to two charges will be same. Hence third charge will remain in stable equilibrium.

The given statement is, the line integral of the magnetic field intensity H around a closed path is equal to the total current linked by the contour. Ampere’s circuit law states that the line integral of the magnetic field around some closed loop is equal to the times the algebraic sum of the currents which pass through the loop.

The factor µo is called the permeability of free space and has the value 

4π × 10^-7 newtons/ampere² in SI units. Since B is measured in tesla (newtons per ampere-meter), the SI units for H must therefore be amperes per meter.