The reason why metals or materials with
metallic surfaces are highly discouraged from
being used in microwaves is due to how microwaves
interact with metals. Lets first talk about how
microwaves work to heat food. A microwave
generates electromagnetic waves of a particular
wavelength (approximately 10 cm). Water molecules
are efficient in absorbing electromagnetic
radiation right around this wavelength. By
absorbing this radiation, the microwaves increase
the internal energy of the water molecules causing
them to vibrate and rotate faster. This increased
motion causes the water molecules to collide with
each other and with other nearby molecules,
generating heat in the process.
behave vary differently because their atomic
structure is very different from non-metals.
Metals also absorb microwaves, but the electrons
associated with metallic atoms are not
constrained, unlike those associated with water
molecules. This causes the metallic electrons to
essentially move much faster than the electrons in
the water molecules, resulting in more collisions.
This causes the metal to heat up much more
quickly and can easily reach extremely high
temperatures hot enough to melt the metal in some
cases. An additional problem resulting from
having metal inside a microwave is that the
electrons moving around the surface generate
currents. Under certain conditions, these
currents can give rise to electric discharges
which are commonly manifested by sparks of light
and popping sounds. These electric discharges, in
addition to damaging the metal, can also damage
the microwave itself.
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