Q: The hysteresis loop of a material encloses an area of 100\text{ J}/\text{m}^3 per cycle. A 10^{-3}\text{ m}^3 core is magnetised at 50\text{ Hz}. The power loss is:

5\text{ W}. Power = \text{area} \times \text{volume} \times \text{frequency} = 100 \times 10^{-3} \times 50 = 5\text{ W}.

Question 1

Magnetic hysteresis is the phenomenon in which:

Accepted Answer

The magnetic induction lags behind the magnetising field. Because domain walls do not move reversibly, B depends on the previous magnetic history of the specimen.

Question 2

The retentivity of a magnetic material is the value of:

Accepted Answer

The magnetic induction remaining when the magnetising field is reduced to zero. Retentivity, or remanence, measures how much magnetisation survives after the field is switched off.

Question 3

The coercivity of a magnetic material is:

Accepted Answer

The reverse magnetising field needed to reduce the induction to zero. High coercivity means the magnetisation is hard to destroy, which is what a permanent magnet needs.

Question 4

The area enclosed by the hysteresis loop of a material represents:

Accepted Answer

The energy lost per unit volume per cycle of magnetisation. This energy appears as heat, which is why it matters so much in transformer cores.

Question 5

Soft iron is preferred for the cores of transformers and electromagnets because it has:

Accepted Answer

A narrow hysteresis loop with low retentivity and low coercivity. A narrow loop means little energy is wasted as heat in each magnetisation cycle, and the core demagnetises readily.

Question 6

Steel is preferred for making permanent magnets because it has:

Accepted Answer

High retentivity and high coercivity. High retentivity keeps the magnetisation strong and high coercivity makes it hard to destroy.

Question 7

The material chosen for a transformer core must have a hysteresis loop of:

Accepted Answer

Small area. The core is magnetised and demagnetised many times a second, so the loss per cycle must be as small as possible.

Question 8

The energy lost during a hysteresis cycle appears as:

Accepted Answer

Heat in the material. Irreversible domain wall motion dissipates the work done by the magnetising field.

Question 9

Saturation magnetisation of a ferromagnetic material corresponds to the state in which:

Accepted Answer

All the domains are aligned with the applied field. Once every domain is aligned, further increase of H produces almost no increase in magnetisation.

Question 10

The hysteresis loop of a material encloses an area of 100	ext{ J}/	ext{m}^3 per cycle. A 10^{-3}	ext{ m}^3 core is magnetised at 50	ext{ Hz}. The power loss is:

Accepted Answer

5	ext{ W}. Power = 	ext{area} 	imes 	ext{volume} 	imes 	ext{frequency} = 100 	imes 10^{-3} 	imes 50 = 5	ext{ W}.

Question 11

If the area of the hysteresis loop of a core is doubled, the energy lost per cycle:

Accepted Answer

Doubles. Energy loss per unit volume per cycle is exactly the loop area.

Question 12

If the frequency of magnetisation of a core is doubled, the rate of hysteresis energy loss:

Accepted Answer

Doubles. The loss per cycle is fixed, so twice as many cycles per second means twice the power loss.

Question 13

A core of volume 2 	imes 10^{-3}	ext{ m}^3 has a loop area of 200	ext{ J}/	ext{m}^3 and is magnetised at 50	ext{ Hz}. The power loss is:

Accepted Answer

20	ext{ W}. Power = 200 	imes 2 	imes 10^{-3} 	imes 50 = 20	ext{ W}.

Question 14

A specimen of volume 10^{-4}	ext{ m}^3 has a hysteresis loop area of 150	ext{ J}/	ext{m}^3. The energy lost in one cycle is:

Accepted Answer

0.015	ext{ J}. Energy per cycle = 150 	imes 10^{-4} = 0.015	ext{ J}.

Question 15

For the specimen of the previous type (0.015	ext{ J} per cycle), the energy lost in one minute at 50	ext{ Hz} is:

Accepted Answer

45	ext{ J}. Total = 0.015 	imes 50 	imes 60 = 45	ext{ J}.

Question 16

Of two materials, one has a tall narrow hysteresis loop and the other a wide loop. The one better suited to a transformer core is:

Accepted Answer

The narrow-loop material, because of its lower energy loss. Transformer cores reverse their magnetisation fifty times a second, so the loop area must be minimal.

Question 17

A material with very high coercivity is most suitable for making:

Accepted Answer

A permanent magnet. High coercivity resists demagnetisation by stray fields, which is exactly what a permanent magnet requires.

Question 18

On the hysteresis loop, the retentivity is read off as the value of B when:

Accepted Answer

H = 0. It is the intercept of the loop on the B-axis after the magnetising field has been removed.

Question 19

On the hysteresis loop, the coercivity is read off as the value of H when:

Accepted Answer

B = 0. It is the intercept on the H-axis, the reverse field just sufficient to demagnetise the specimen.

Question 20

Of two transformer cores, one has a loop area of 50	ext{ J}/	ext{m}^3 and the other 200	ext{ J}/	ext{m}^3. For equal volume and frequency, the ratio of their power losses is:

Accepted Answer

1 : 4. Power loss is proportional to the loop area, so the ratio is 50 : 200 = 1 : 4.

Hysteresis — MH-CET Physics MCQs with Solutions

Questions with solutions