Page 343 - Schaum's Outline of Theory and Problems of Applied Physics

328 MAGNETISM [CHAP. 27

Bar magnets with poles facing each other are equivalent to parallel current loops whose currents are in opposite
directions [Fig. 27-12(a)]. Such loops repel. Bar magnets with opposite poles facing each other are equivalent to
parallel current loops whose current loops are in the same direction [Fig. 27-12(b)]. Such loops attract.

S N N S

Like poles repel Parallel loops with opposite currents repel
(a)

S N S N

Unlike poles attract Parallel loops with similar currents attract
(b)
Fig. 27-12

SOLVED PROBLEM 27.16
How does a permanent magnet attract an unmagnetized iron object?
The presence of the magnet induces the atomic magnets in the object to line up with its field (Fig. 27-13), and
the attraction of opposite poles then produces a net force on the object.

S N

S N

Fig. 27-13

SOLVED PROBLEM 27.17
An unmagnetized iron rod is placed inside a solenoid. The current in the solenoid is then increased
from zero to a maximum in one direction, decreased back to zero, increased to a maximum in the other
direction, brought back to zero, and so on. Plot B versus H for the iron rod, and discuss the shape of the
resulting curve.
The required curve is shown in Fig. 27-14. At a, both H and B are 0. As H increases, B increases slowly at
first, then rapidly, and finally levels off at a maximum value at b. The rod is now saturated, and a further increase
in H will not change B. Saturation occurs when all the magnetic domains in the rod are aligned with H. The curve
from a to b is called the magnetization curve of the material.
When H is brought back to 0 from H b , B lags behind so that B = B c at point c when H = 0. This is an example
of hysteresis. The value of B c is called the retentivity of the material. To demagnetize the rod completely, H must
be reversed in direction and increased to H d , the coercive force. The greater the retentivity, the stronger the residual
magnetization of the rod; the greater the coercive force, the better able the rod will be to keep its magnetization
despite the presence of strong magnetic fields. Thus a good material for a permanent magnet should have both a high
retentivity and a high coercive force.