Introduction - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 10

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d

E = V/d

V

++++++++

Figure 1.5 The uniform electric field between two conductive plates.

the magnitude of the electric field for this example may be obtained using
the following simple equation:

V
d

E =− (1.2)

where V is the voltage difference between the plates, and d is the distance
separating the plates. It is also immediately obvious that the value of the
electric field tangential to the plates in Figure 1.5 is zero because the plates
are perfect conductors, which by definition cannot afford any voltage drop
across their surfaces. Contrary to this example, electric fields may also
change with time. For example, an electric field may be developed as a
result of the motion of charged particles in the field.
Because the electric field can exert forces on charged particles, it is usu-
ally thought of as having a potential electric energy. The electric (or elec-
trostatic) potential energy (in joules) is the potential energy associated with
bringing together the building charges of a defined field from a state in
which individual charges are completely separated. The potential energy of
a field is directly proportional to the field amplitude. The line integral of an
electric field intensity between two points, say A and B, is defined as the
voltage between these two points, expressing the work done by the electric
field to move an unit charge from point A to B. The following sections
describe in detail how the electric energy can be calculated and how this
energy contributes to microwave heating.

1.3.1.2 Magnetic field
A magnetic field is basically a field of forces generated by the movement of
electric charges, a variation in the electric field in time, or the intrinsic mag-
netic field associated with the spin of particles. The concept of a magnetic
field was first introduced by Hans Christian Ørsted (1820), who discovered
that a conductor carrying a current produces a magnetic force field in its
vicinity (Figure 1.6). His work was later followed by that of Andre Marie
Ampere, who quantified the relationships involved.