|Why does metal, like a fork or aluminum foil
spark when placed in a microwave?
Metals have an effective free electron gas which
responds to externally applied electric or
magnetic fields as a good reflector. (This is why
metals are shiny). In a microwave oven, there is a
strong periodic field at about 2.2GHz which
normally excites vibrations in OH bonds -- thus
heating water and oils. However, Metals of the
right shape while reflecting the energy can have
large potentials on the surface. If two different
potentials are close, then like an antenna, the
metal sponsors an arc. This can only happen if
there are regions with discontinuities in close
proximity-- Many microwave ovens have metal frames
and/or grates -- these are specially designed to
not be effective antennas.
Inside a microwave you have an intense oscillating
electromagnetic field. This field will create
electric current inside everything that is placed
within the "cavity" of the oven. The intensity of
the current generated and therefore of the heat
liberated in a body will be proportional to the
electrical resistance of the body. Insulators
(high resistance) will experience fairly low
currents and an amount of heat that can be
controlled and used for cooking purposes. Metals
are much more conductive, and
field creates very high currents on them. These
currents liberate enormous amount of heat and there
you have, the metal melts, sparks, explodes...
Electronmagnetic waves, like those emitted in a
microwave oven, cannot penetrate metals, instead
inducing surface electrical currents in the metal.
Where there is a bend in the metal, the
electromagnetic fields are concentrated, and the
sharper the bend, the more the electromagnetic
fields are contracted. With very sharp corners,
such as the tines of a fork or the edges of
aluminum foil, the fields can get so concentrated
that they are strong enough to strip electrons off
of the molecules of the air. This ionization of
the air is the spark that you see. How do you
think this effect affects the design of lightning
Basically, if you have a piece of metal in the
microwave, charges in the metal move around. If
there is a part of the metal that is very thin,
such as with aluminum foil or a fork, a high
voltage could build up that exceeds the breakdown
voltage of air and cause a spark.
related page I found regarding some experiments
that people have done with their microwave oven.
I don't really recommend trying these at home.
Wait until you have your own oven to damage and
you pay for your own health
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