The larger area under the hysteresis curve (B-H) of a ferromagnetic material depicts

Correct — C, extent of lagging. Hysteresis means literally a lagging behind, and the B-H curve is drawn to show it: as the magnetising field H is cycled, the flux density B does not retrace its path but lags, so the material keeps some magnetisation (retentivity) when H returns to zero and needs a reverse field (coercivity) to be brought back to zero. A larger enclosed area means a larger retentivity and coercivity together, that is, a greater lag of B behind H — and a correspondingly larger energy loss per cycle, which is why transformer cores are made of soft magnetic materials with thin loops and permanent magnets of hard materials with fat ones.

• (a)maximum magnetization — Saturation magnetisation is read off the height of the loop, not its area. Two materials can reach the same saturation value and still enclose very different areas.
• (b)extent of demagnetization — Coercivity — the reverse field needed to demagnetise — is one width of the loop, and it does contribute to the area. But the area is the combined effect of retentivity and coercivity over a whole cycle, so 'lagging' describes it better than demagnetisation alone.
• (d)minimum energy — The direction is backwards. The area enclosed by the loop measures the energy dissipated as heat per unit volume per cycle, so a larger area means more energy lost, not less.

Take a ferromagnetic sample through a full cycle of magnetising field and the plot of B against H closes into a loop rather than a line. The loop records that B depends on the material's history as well as on the present H. Its height at the top gives saturation, its intercept on the B axis gives retentivity, its intercept on the H axis gives coercivity, and its enclosed area equals the energy converted to heat per unit volume per cycle. Soft magnetic materials such as soft iron and silicon steel have narrow loops and are used in transformer and motor cores; hard materials such as alnico and steel have wide loops and make permanent magnets.

Standard physics states the meaning of the loop area as energy dissipated per cycle, and none of the four options says that in so many words — option (d) even inverts it. Among what is offered, 'extent of lagging' is the description that follows, because the area grows precisely when B lags H more strongly, through greater retentivity and greater coercivity. It is worth carrying both readings into the hall: the area is the hysteresis loss, and it is also the visible measure of how much the material remembers.

• Hysteresis is the lagging of flux density B behind the magnetising field H.
• Retentivity is the magnetisation left when H is reduced to zero; coercivity is the reverse field needed to remove it.
• The area enclosed by the B-H loop equals the energy dissipated as heat per unit volume per cycle.
• Soft magnetic materials have narrow loops and are used for transformer cores; hard materials have wide loops and make permanent magnets.
• Hysteresis loss is one of the two main core losses in a transformer, the other being eddy-current loss.

• Reading the loop's area as a measure of saturation, which is its height instead.
• Assuming a large area is desirable; for a transformer core it is exactly what has to be avoided.
• Confusing retentivity with coercivity — one is what remains, the other is what it takes to remove it.

A single-concept item on the B-H curve, phrased so that a candidate who only remembers the picture and not what its area means will guess.

No directly related past PYQ was found.

• practice — not a real PYQ

  The energy lost as heat per unit volume per cycle in a ferromagnetic material is given by

  • (a)the height of the hysteresis loop
  • (b)the area enclosed by the hysteresis loop
  • (c)the slope of the hysteresis loop
  • (d)the coercivity alone

  Answer(b) the area enclosed by the hysteresis loop — which is why transformer cores use materials with the thinnest possible loop.

• practice — not a real PYQ

  Soft iron is preferred to steel for the core of a transformer mainly because it has

  • (a)higher coercivity
  • (b)a narrow hysteresis loop
  • (c)a wider hysteresis loop
  • (d)higher electrical resistance

  Answer(b) a narrow hysteresis loop — less energy is dissipated in each magnetising cycle.