Physics of Magnetism

CHAPTER 8. SOME BASIC CONCEPTS AND UNITS 79

On the basis of Maxwell’s equations applied to a situation in which there are no electric
currents, one has

and for the flux density one has

For a bar magnet of finite dimensions, one may therefore write

where the integration is performed over the whole space. This integral may be written as
the sum of the integral over the volume of the bar magnet and the integral over the rest of
the (free) space

and

This result shows that the integral over the volume of the magnet (first term in Eq. 8.25) has
to be negative, which is possible only if inside the magnet have opposite direction.
In other words, inside the magnet exists a magnetic field with a direction opposite to that
of the magnetization and hence the name demagnetizing field.
The demagnetizing field depends on the shape of the magnet. For a homogeneously
magnetized ellipsoid it can be expressed as

where the demagnetization factor is dimensionless with values ranging between zero

and one. This factor is a sensitive function of the geometry of the magnet. Examples of

demagnetizing factors pertaining to shapes of simple geometry are listed in Table 8.1.

Using Eqs. (8.21) and (8.26), one has for the induction inside the magnet

The units used for describing the magnetic properties of the various magnetic materials

considered in the literature are far from being uniform. Throughout this book, the Standard

International system of units (SI) has been used, that was adopted in 1960 by the Conférence