Page 342 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 27] MAGNETISM 327

FERROMAGNETISM
The magnetic field produced by a current is altered by the presence of a substance of any kind. Usually the
change, which may be an increase or a decrease in B, is very small, but in certain cases there is an increase in B
by hundreds or thousands of times. Substances that have the latter effect are called ferromagnetic; iron and iron
alloys are familiar examples. An electromagnet is a solenoid with a ferromagnetic core to increase its magnetic
field.
Ferromagnetism is a consequence of the magnetic properties of the electrons that all atoms contain. An
electron behaves in some respects as though it is a spinning charged sphere, and it is therefore magnetically
equivalent to a tiny current loop. In most substances the magnetic fields of the atomic electrons cancel, but in
ferromagnetic substances the cancellation is not complete and each atom has a certain magnetic field of its own.
The atomic magnetic fields align themselves in groups called domains with an external magnetic field to produce
a much stronger total B. When the external field is removed, the atomic magnetic fields may remain aligned to
produce a permanent magnet. The field of a bar magnet has the same form as that of a solenoid because both
fields are due to parallel current loops (Fig. 27-11).

B B
I
N S

Fig. 27-11

MAGNETIC INTENSITY
A substance which decreases the magnetic field of a current is called diamagnetic; it has a permeability µ that is
less than µ 0 . Copper and water are examples. A substance which increases the magnetic field of a current by a
small amount is called paramagnetic; it has a permeability µ that is greater than µ 0 . Aluminum is an example.
Ferromagnetic substances have permeabilities hundreds or thousands of times greater than µ 0 . Diamagnetic
substances are repelled by magnets; paramagnetic and ferromagnetic ones are attracted by magnets.
Because different substances have different magnetic properties, it is useful to define a quantity called
magnetic intensity H which is independent of the medium in which a magnetic field is located. The magnetic
intensity in a place where the magnetic field is B and the permeability is µ is given by

B
H =
µ
magnetic field
Magnetic intensity =
permeability of medium
The unit of H is the ampere per meter. Magnetic intensity is sometimes called magnetizing force or magnetizing
field.
The permeability of a ferromagnetic material at a given value of H varies both with H and with the previous
degree of magnetization of the material. The latter effect is known as hysteresis.

SOLVED PROBLEM 27.15
The ends of a bar magnet are traditionally called its poles, with the end that tends to point north called the
north pole and the end that tends to point south called the south pole. Like poles of nearby magnets repel
each other, and unlike poles attract. Explain this behavior in terms of the interaction of current loops.