Magnetic Forces and Materials Questions and Answers

Answer: A

Correct answer: 25 W

Each cycle dissipates the loop area per unit volume, so the power is that area multiplied by the frequency and the volume: \[ 500 \times 50 \times 10^{-3} = 25\ \mathrm{W}. \] Hysteresis loss is therefore proportional to frequency, which distinguishes it from eddy current loss - proportional to the square of frequency - and lets the two be separated experimentally.

Answer: B

Eddy current loss varies as the square of the lamination thickness, so halving it quarters the loss. The limit is set by manufacture and by the space the extra insulation occupies, which reduces the effective iron area of the core.

Answer: C

The relation \( \tan\theta_2 = \mu_r\tan\theta_1 \) gives a tangent of a thousand, whose angle is 89.94 degrees. Flux entering iron at any appreciable angle therefore runs almost parallel to the surface within it, which is the basis of magnetic shielding.

Answer: D

The force is always perpendicular to the velocity, so it contributes nothing to the work integral and the kinetic energy is constant. Only the electric part of the Lorentz force, or a changing magnetic field through the induced electric field, can alter a particle's energy.

Answer: A

The induced moments oppose the applied field, so the material is repelled from the region of strongest field. The effect is tiny but real, and it is what allows a frog or a drop of water to be levitated in a sufficiently strong field gradient.

Answer: B

Hysteresis loss rises with the first power of frequency, since one loop area is dissipated per cycle, while eddy current loss rises with the square, since both the induced voltage and the resulting current grow with frequency. Separating them by measurement at two frequencies exploits this difference.

Answer: C

A permanent magnet wants a broad loop, with a large coercivity so that stray fields cannot demagnetise it and a large remanence so that it holds a useful flux. It is the transformer core, cycled continuously, that needs the narrow loop.

Answer: D

In a uniform field the currents on opposite sides run in opposite directions through the same field, so the forces cancel as a resultant while forming a couple. In a non-uniform field they no longer cancel and a net force appears.

Answer: A

Continuity of the tangential intensity means the tangential flux density falls by the permeability ratio on leaving the iron, while the normal flux density is unchanged, so the emerging flux is dominated by its normal component. The reason is exactly what produces the effect.

Answer: B

Lamination reduces the eddy current loss, which depends on the area available to circulating currents. Hysteresis loss is a property of the material and its flux excursion and is quite unaffected by how the core is cut up.