Fundamentals of Electromagnetics - RF/Microwave Interaction with Biological Tissues - page 11

e¢ r

Dipolar

Ionic

Electronic

e¢¢ r

(Hz)

10 3 10 6 10 9 10 12 10 15

FIGURE 1.3 Permittivity as a function of frequency (from [3], courtesy of De Boeck,

Brussels).

imaginary parts of a relative permittivity satisfying the second-order equation
of movement. When the damping term decreases, the variation of the real part
of the permittivity becomes steeper while the maximum of the imaginary part
increases and tends to infinity at the oscillation frequency.
When submitted to electric fields, an actual material exhibits a variety of
relaxation and resonance phenomena. Figure 1.3 represents a typical variation
of the real and imaginary parts of the relative permittivity as a function of
frequency.
Some materials exhibit unusual properties when submitted to electric fields.
For instance, barium titanate and some other materials are ferroelectric :The
electric polarization does not vary linearly with the amplitude of the applied
field, exhibits hysteresis, and varies significantly with temperature. A critical
temperature, above which ferroelectricity disappears and termed the Curie
temperature , is associated with the phenomenon. Some other materials are
called antiferroelectric : Their electric dipoles are very much organized as well;
they are antiparallel, however.

1.3.2 Magnetic Field and Flux Density

The magnetic field is derived from Ampere's law, which expresses the inter-
action between two loops of wires carrying currents. The total force experi-
enced by a loop is the vector sum of forces experienced by the infinitesimal
current elements comprising the second loop. Experimentally, it has been
shown that [3, 4-7]

H

1. Two currents of equal directions attract each other, while they repel
when they have opposite directions.
2. The magnitude of the force is proportional to the product of the two cur-
rents and to the product of the lengths of the two current elements.
3. The magnitude of the force is inversely proportional to the square of the
distance between the current elements.