Page 344 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 27] MAGNETISM 329

Fig. 27-14

Increasing −H beyond d produces an increasing −B until the rod saturates again at e, where B e =−B b . When
H is returned to 0, again B lags behind and this time has the value B f at H = 0, where B f = B c . Increasing H then
returns B to B b to complete the hysteresis loop bcdefgb.
The area enclosed by a hysteresis loop is proportional to the energy dissipated as heat during each magnetization
cycle. A broad hysteresis loop with high values of retentivity and coercive force is characteristic of a suitable material
for a permanent magnet, since a great deal of work must be done to change its magnetization. However, a material
with a narrow hysteresis loop is better for such applications as transformer cores which must undergo frequent
reversals in magnetization; the smaller the area of the loop, the greater the efficiency of the transformer.

SOLVED PROBLEM 27.18
How can a magnetized piece of iron be demagnetized?
One method is to heat the iron, since all ferromagnetic materials lose their ability to retain magnetization
◦
beyond a certain temperature, which is about 760 C in the case of iron. Another method is to bring the iron through
a succession of hysteresis loops of smaller size, as in Fig. 27-15. To do this, the iron can be placed in a solenoid
connected to a source of alternating current, and then the current is gradually decreased to zero.

Fig. 27-15