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Sharp metal edges
Metals are conductors.
That means that their electrons can move around in the metal to
different places. It also means that the electric field inside
them is zero because all the excess charge (electrons) resides on the
surface.
If the metal is in the shape of a sphere, then the charge will
distribute itself uniformly on the surface. But things people put
in the microwave are rarely spheres. Since the potential is the
same everywhere on the surface, the electric field is larger anytime
there is a point or edge, because more charge has to fit into a smaller
space.
Another way to see this is to realize
that above a surface, the electric field is equal to the surface
charge density divided by a constant. Since the
surface charge density is more at a point or edge, the electric field
there will also be larger.
This means that metal with edges or pointed ends creates a larger
disturbance in the electric field pattern in the microwave oven.
And the excess charge finds it easy to jump off to somewhere else like
the side of the oven-and this causes arcing and sparks. That is
why popcorn bags do not have staples in them and why light bulb
filaments create quite the display when microwaved (see next section).
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