Fundamentals of Electromagnetics - RF/Microwave Interaction with Biological Tissues - page 13

the material. In this case, the magnetization is the magnetic dipole moment per
unit volume , in amperes per square meter .
The total magnetic field in a material is the sum of the applied magnetic
field and an induced magnetic field, resulting from the magnetization of the
material. Earlier in this section, the perfect electric conductor has been defined
as an equipotential material in which all the points are at the same electric
potential. In such a material, the total electric field must be zero, which implies
that the material must develop an induced electric field such that the sum of
the applied field and the induced field vanishes in all points of the material.
On the other hand, one may define the perfect magnetic conductor as the dual
of the perfect electric conductor, in which all the points are at the same mag-
netic potential. In such material, the total magnetic field must be zero, which
implies that the material must develop an induced magnetic field such that the
sum of the applied field and the induced field vanishes in all points of the mate-
rial. The definition of the perfect electric conductor is very useful, largely
because there are in nature materials whose properties are very close to those
of the perfect electric conductor, for example, gold, silver, and copper. There
is, however, no physical material with properties very close to those of the
perfect magnetic conductor.
The induced magnetic field is calculated by taking into account the geom-
etry of the problem and the boundary conditions, which can of course be com-
plicated. The human body is essentially nonmagnetic, hence transparent to
magnetic fields, except however for some localized magnetic properties.
A new vector field is then defined, known as the magnetic flux density ,
in webers per square meter , similarly to the magnetization, or tesla , defined as

B

BHM = m 0

(

+

)

Wb m

2 ,

T (1.23)

-

This definition is totally general, applying to all materials. It indeed holds for
materials in which [3]:

1. The magnetization vector has not the same direction as the vector mag-
netic field, in which case the material is anisotropic .
2. The magnetization can be delayed with respect to the variation of mag-
netic field. This is the case in all lossy materials , and all materials are
lossy, so this is a universal property. It is neglected, however, when the
losses are reasonably small.
3. The magnetization is not proportional to the magnetic field, in which
case the material is nonlinear .

In all other cases, that is, when the material is isotropic , lossless , and linear ,
the definition (1.23) can be written

BH = m (1.24)