Test: Electromagnetic Fields Theory- 3 - Electrical Engineering (EE) MCQ

A lever maintains the same level of work required to lift something as if it weren't used.

W = Fd

The amount of force required is decreased, but the distance that you need to move is increased while the work remains constant.

The amount of Power going into a transformer is the same amount that is coming out (not accounting for losses)

Power = Voltage × Current

P = VI

You can increase the voltage of a transformer, but the current will vary inversely as the power remains unchanged.

The relation between magnetic field intensity (H) and magnetic flux density (B) is

B = μ H Wb/m²

Where, μ is the magnetic permeability

Given that, E = 120 V

Number of turns (N) = 200

We know that, 

The relation between magnetic field intensity (H) and magnetic flux density (B) is

B = μ H

The Curie temperature or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, to be replaced by induced magnetism.

The reluctance is inversely proportional to the relative permeability. Hence material with lesser permeability will offer more reluctance.

Here core A has lesser permeability and hence offer more reluctance.

Those substances which are weekly magnetised when placed in an external magnetic field in the same direction as the applied field are called paramagnetic substances. They tend to move from weaker to the stronger part of the field. The magnetism exhibited by these substances is called Para magnetism.

Examples: Aluminium, platinum, sodium, calcium

The inductance of a solenoid is given by,

Energy stored in an inductor is given by,

Permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. It is the degree of magnetization that a material obtains in response to an applied magnetic field. Hence permeability should be high to rapid reversal of magnetism.

Hysteresis loss is caused by the magnetization and demagnetization of the core as current flows in the forward and reverse directions. Hence it should be low.

Reluctance of a magnetic coil is given by,

The current flowing in an electric circuit is due to the existence of electromotive force similarly magnetomotive force (MMF) is required to drive the magnetic flux in the magnetic circuit. The magnetic pressure, which sets up the magnetic flux in a magnetic circuit is called Magnetomotive Force. The SI unit of MMF is ampere-turn (AT)

Given that, Number of turns (N) = 180

Length (l) = 0.5 m

Current (I) = 4 A

Magnetic field strength (H) is given by,

Temporary magnets are made from soft metals and only retain their magnetism while near a permanent magnetic field or electronic current. Common temporary magnets include nails and paperclips, which can be picked up or moved by a strong magnet

Temporary magnets are used in telephones, loud speakers, generators and electric motors

The ability of a coil to retain some of its magnetism within the core after the magnetisation process has stopped is called retentivity, while the amount of flux density still remaining in the core is called residual magnetism

The magnetic field strength (H) of a short solenoid is given by,

Where N is the number turns

I is the current in ampere

L is the length

From the above expression we can observe that the magnetic field strength is proportional to ampere turns and inversely proportional to length.

The electric field intensity due to an infinite plane sheet of charge is given by,

Thus, the field is uniform and does not depend on the distance from the plane sheet of charge.

Permeance is the reciprocal of reluctance. In a magnetic circuit, permeance is a measure of the quantity of magnetic flux for a number of current-turns.

The SI unit of magnetic permeance is webers per ampere-turn that is H (henry).

We know that, B = μH

Magnetic field density is inversely proportional to length. Hence magnetic flux density increases with decrease in distance.

Ferrites are usually ferrimagnetic ceramic compounds derived from iron oxides. Like most of the other ceramics, ferrites are hard, brittle, and poor conductors of electricity.