Introduction - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 9

1.3.1 Electromagnetic fields

As mentioned, microwaves comprise a portion of the EM spectrum.
Therefore, understanding the behaviour of microwaves requires some basic
knowledge of electromagnetism and EM field properties. An EM field is
generally a physical field generated by electrically charged objects and is
considered one of the four fundamental forces of nature, along with gravi-
tation, weak interaction, and strong interaction. The EM field may extend
indefinitely as an EM wave through space and affect the behaviour of the
charged objects in the vicinity of the field.
Two fundamental components are necessary for a propagating EM wave
to exist: an electric field and a magnetic field. Electric fields are produced
by stationary charges; the magnetic fields are generated by moving charges
(currents). The electric field in particular, being the prime source of energy
transfer to the material being exposed to the microwaves, is a recurring
parameter in microwave heating and familiarity with it is essential. In sub-
sequent sections, the nature of electric and magnetic fields and their rela-
tionships are briefly discussed.

1.3.1.1 Electric fields

The concept of an electric field was first introduced by Michael Faraday. An
electric field is a physical quantity associated with any point surrounding
an electrically charged particle or time-varying magnetic field. An electric
field exerts a force on other electrically charged objects.
The electric field is a vector field. In SI units, an electric field is charac-
terised by volts per meter (V m −1 ) or, equivalently, Newton per coulomb
(N C −1 ). The strength (magnitude) of an electric field at a given point in
space and time is defined as the magnitude of the force that would be
exerted on a positive 1-C charge placed at that point:

F
q

e

E = (1.1)

t

where E is the electric field strength, F e is the force (N) exerted on the test
particle, and q t is the charge of the particle.
The simplest case for illustrating an electric field in the macroscale may be
two conductive plates connected to a voltage V (volts) and placed apart by
a distance d (meters), small compared to the size of the plates (Figure 1.5).
In this case, an almost-uniform electric field with the strength of V / d volts
per meter will be developed between the plates. In this example, while the
electric field everywhere between the plates is substantially constant, the
geometric discontinuity at the edges causes a local field distortion that
increases the field intensity. Ignoring the small field distortion at the edges,