Fundamentals of Electromagnetics - RF/Microwave Interaction with Biological Tissues - page 3

There is an interesting feature to note about microwaves: They cover,
indeed, the frequency range where the wavelength is of the order of the size
of objects of common use, that is, meter, decimeter, centimeter, and millime-
ter, depending of course on the material in which it is measured. One may,
hence, wonder whether such wavelengths can excite resonance in biological
tissues and systems. We shall come back later to this question.

1.2 FIELDS

Investigating the interaction of EM fields with biological tissues requires
a good physical insight and mathematical understanding of what are
fields . A field is associated with a physical phenomenon present in a given
region of space. As an example, the temperature in a room is a field
of temperature, composed of the values of temperature in a number of
points of the room. One may say the same about the temperature distribution
inside a human body, for instance. We do not see the field, but it exists,
and we can for instance visualize constant-temperature or isothermal
surfaces.
There are fields of different nature. First, fields may be either static or time
dependent . Considering, for instance, the temperature field just described, the
room may indeed be heated or cooled, which makes the temperature field time
dependent. The human body may also be submitted to a variety of external
sources or internal reasons which affect the temperature distribution inside
the body. In this case, the isothermal surfaces will change their shapes as a
function of time.
Second, the nature of the field may be such that one parameter only, such
as magnitude, is associated with it. Then, the field is defined as scalar . The tem-
perature field, for instance, inside a room or a human body, is a scalar field.
One realizes that plotting a field may require skill, and also memory space, if
the structure is described in detail or if the observer requires a detailed
description of the field in space. This is true even in the simplest cases, when
the field is scalar and static.
On the other hand, in a vector field, a vector represents both the
magnitude and the direction of the physical quantity of interest at points in
space, and this vector field may also be static or time dependent. When
plotting a static scalar field, that is, one quantity, in points of space already
requires some visualization effort. On the other hand, plotting a time-
dependent vector field, that is, three time-varying quantities, in points of space
obviously requires much more attention. A vector field is described by a set
of direction lines , also known as stream lines or flux lines . The direction line is
a curve constructed so that the field is tangential to the curve in all points of
the curve.