Civil Engineering Reference
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conditions, the electric and magnetic parameters associated with any EM
field can be calculated.
1.4 ELECTROMAGNETIC PROPERTIES
In Section 1.3.2, we briefly discussed the relationship between the EM
properties of the materials and the field parameters. We found that, to
obtain the field parameters for a specific problem, the EM properties of
the constituent materials should be known. Here, we review the most fun-
damental EM properties of materials. Knowledge of the EM properties
of materials in the microwave frequency range is essential for the proper
design of microwave applicators.
Every material has a unique set of EM (dielectric) properties affecting
the way in which the material interacts with the electric and the magnetic
waves. Materials basically possess charge particles that give rise to three
basic phenomena—conduction, polarisation, and magnetisation—when
exposed to external fields. Depending on whether conduction, polarisa-
tion, or magnetisation is the most prominent phenomenon, the material
may be classified as a conductor, dielectric, or a magnetic material, respec-
tively. Microwave heating, especially microwave heating of concrete, deals
normally with the dielectric materials in which polarisation caused by the
external field is the prominent response. Therefore, the focus of this section
is the properties of dielectric materials, especially dielectric materials acting
as loads. Electric polarisation refers simply to the creation and alignment
of dipoles within a material as a result of the relative shift of positive and
negative electric charges in opposite directions within a dielectric under the
influence of an electric field, making one side of the atom somewhat posi-
tive and the opposite side somewhat negative (Figure 1.7).
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-
+
+
+
+
+
+
+
+
+
++++++
(a)
(b)
Figure 1.7
Dipolar molecules (a) in the absence and (b) in the presence of an electric
field.
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