Twisting water molecules
Why Does a Microwave Heat Food, Not the Plate?
A microwave flips a radio field billions of times a second, and only water molecules follow it.
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Step 1 of 5
A microwave makes radio waves, not heat.
Nothing inside the oven is hot to begin with. A tube called a magnetron fills the metal box with radio waves, which the walls keep bouncing around.
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Step 2 of 5
A water molecule has two charged ends.
Water is lopsided: the oxygen side is slightly negative and the hydrogen side slightly positive, so the whole molecule behaves like a tiny magnet.
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Step 3 of 5
The field flips, so the molecules flip.
A charged molecule lines itself up with the field around it. When that field reverses, every water molecule has to swing round to match.
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Step 4 of 5
All that twisting is heat.
Molecules cannot swing round without shoving their neighbours. That jostling spreads through the food, and jostling molecules is exactly what heat is.
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Step 5 of 5
Dry things have nothing to twist.
Glass and dry ceramic have no free water molecules to swing, so the waves pass straight through and the plate only warms by touching hot food.
Microwave ovens flip their field about 2.45 billion times a second, far faster than water molecules can comfortably follow.
The short version
A microwave heats food by flipping a radio field billions of times a second, twisting water molecules until the jostling becomes heat.
Try it yourself
Microwave a dry ceramic mug for ten seconds, then one holding water. Only the water gets hot; the dry mug hardly changes.
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