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[Phys-L] Re: metal in microwave oven?

On 03/22/05 12:26, Anthony Lapinski wrote:
I thought that any metal placed in a microwave
oven will produce sparks and damage the interior.
Can someone clarify the physics of metals in microwaves for me?

1) I agree with what Chuck B. and Daryl T. wrote.

2a) One way you can get nasty weirdness is to have an
almost-closed loop. It might act as an antenna,
resulting in a big voltage (at GHz frequencies)
across the gap. You'll get sparks ... and then
the resulting plasma will absorb.......

2b) Another type of nasty weirdness results if you
have a loop of thin wire with anywhere near the
right resistivity to be "impedance matched". It
can absorb lots of energy and get verrry hot.

2c) Another big no-no is to have metal be the
only thing in the oven, or enclosing everything
in the oven. This means (a) the enclosed food
won't cook, and (b) the microwave power unit
will be operating under no-load conditions.
Some units (older ones especially) don't handle
that too well.

3) OTOH there are lots of nice things you can do
with metal in the microwave. Others have
mentioned some examples. Any decent "microwave
cookbook" will have other examples.

My favorite example, I keep a stash of frozen
lagagna that I use as "survival food" when
I'm pressed for time. The problem is that
the microwave oven tends to scorch the corners
before the middle is cooked, or even fully
thawed. This is a not-too-uncommon when
you're trying to thaw things, because the
product is much more absorptive when thawed
than when frozen, so it is easy to get a
runaway condition.

Solution: I made a hoop out of sheet metal,
just the right size to be a "belt" around
a brick of lasagna. The joint is riveted
shut, so there's no risk of having an
almost-closed loop. The boundary condition
ensures that there will be no microwave
intensity near the hoop, i.e. at the edge
of the food, where the problem was.

After a couple of minutes of that treatment,
the middle is thawed, whereupon I remove
the hoop and everything proceeds normally.
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