Elisa- I had prepared a snotty answer about how your teacher was all wrong, since when Iíve burnt things up in the microwave no plasma formed. However, my colleague Tim referred me to two sites describing how to use grapes to generate a plasma in a microwave oven: and .
The key idea is this. In ordinary use, the electric fields inside a microwave are too low to generate plasmas, which are made when electric fields tear apart molecules into electrically charged ions. However, if you violate the manufacturerís instructions and run the oven with almost nothing in it to absorb energy, the microwave energy density and hence the electric fields build up far above the usual levels. In the process they may damage or destroy the microwave oven, so, unless youíre rich, this is not a good experiment to try.
Now it turns out that grapes are just the right size to further focus the electric fields in the oven. Two grapes placed close but not quite touching will produce a large field between them, sometimes enough to make a plasma.
Again, donít do this unless you are prepared to sacrifice a microwave oven.
Mike W. (w essential corrections from Tim!)
(published on 10/22/2007)