When the electromagnetic field oscillates, the polarity will change according to
the frequency. Furthermore, the polar molecules in the wood try to oscillate in
phase with these changes. However, as described earlier, these induced motions
will be slowed down by frictional, inertial and elastic forces, causing the
production of heat in the material. The heating, which occurs by microwaves, is a
conversion of the electromagnetic energy into heat. The law of conservation of
energy states that the total amount of energy in an isolated system remains
constant. However, it may change form. For example, friction turns kinetic energy
into thermal energy. Conservation of the electromagnetic field’s energy is stated
by the Poynting theorem [22]. From this theorem, the average total absorbed
power can be expressed as: