How does a rice cooker know when to turn off?

How does a rice cooker know when to turn off? — JS, Tokyo, Japan

The rice cooker turns off when there is no longer enough liquid water on its heating element to keep that element’s temperature at the boiling temperature of water (212° F or 100° C). As long as the element is covered with liquid water, it is hard for that element’s temperature to rise above water’s boiling temperature. That’s because as the water boils, all of the thermal energy produced in the heating element is converted very efficiently into chemical potential energy in the resulting steam. In short, boiling water remains at 212° F even as you add lots of thermal energy into it.

But as soon as the liquid water is gone (and, fortuitously, the rice is fully cooked), there is nothing left to keep the heating element’s temperature from rising. As more electric energy enters the element and becomes thermal energy, the element gets hotter and hotter. A thermostat, probably a bimetallic strip like that used in most toasters, senses the sudden temperature rise. It releases a switch that turns off the electric power to the rice cooker.

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