Why does metal catch on fire in the microwave? –Maura McCormick, Rockville, Maryland
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Can’t say I’ve come across any cases of metal actually catching on fire in a microwave, though my experiments in this respect haven’t been as extensive as I’d like, owing to Mrs. Adams’s refusal to sacrifice (potentially) the household microwave to the cause of science. What you do see is sparks, which may pit the metal. You get sparks because the intense microwave radiation generates a fluctuating current in the metal, a process known as induction. Due to internal resistance, at any given instant there are sharp differences in electrical potential between different points on the metal surface. Because the microwave is pumping in a lot of energy, these differences in potential increase to the point that a spark jumps the gap. Basically tiny lightning bolts, the sparks are very hot and I suppose could set fire to, say, a nearby paper plate. The metal may also become very hot and glow, due to incandescence, or even vaporize. But worse things could happen. The metal surface may reflect the microwaves back where they came from, namely the oven’s magnetron tube. That could damage or destroy the tube, which is why manufacturers tell you not to put metal in your microwave.
SWITCHING SIDES: THE INSIDE STORY
“‘The border crossing from China (where they drive on the right) to Pakistan (where they drive on the left) merely has a sign at the side of the road that says “Entering Pakistan, Drive Left” and for those going the other way “Entering China, Drive Right.”‘
