How a microwave oven heats food, and why the middle stays cold
Hot and cold spots: why the plate turns inside a microwave oven
Take the turning plate out of a microwave, put in a bar of chocolate, and it melts in a neat row of spots. The waves inside the box are not spread out evenly. A simple kitchen test shows where the heat goes, and why the plate has to turn.
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Chocolate again, like in Percy Spencer's pocket. Rono lays it on the floor of the oven, where the plate used to turn. Then he runs the microwave for 20 seconds.
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The chocolate does not melt all over. It melts in a few spots, with hard chocolate in between. So inside the box, some places get a lot of energy, and some get almost none.
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It comes from the bounce. Waves going one way meet waves coming back from the wall. In some places they add up into strong waves, and in others they cancel out. This is called a standing wave, and its hot and cold places never move.
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Rono measures the gap between two melted spots: about 6 centimetres. Each wave has two strong places, so the spots sit half a wave apart. One whole wave is about 12 centimetres long.
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That is why the plate turns. It carries the food through the hot places and the cold places, again and again, so each part of the food gets its share.
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Rono offers the warm chocolate to Miva. She steps back from the oven and frowns.
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Quick check
A broken plate.
Show the answer
Some parts are hot and some are still cool, because the bowl sat in the same hot and cold places. Stir it, or turn the bowl by hand halfway through.
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Go further
More about the hot places.