ELI5- how do rice cookers know how long to cook the rice for no matter the different quantities
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They don’t “know”, they just boil the rice until its done.
When is it done? When there’s no liquid water left. Once all the water present has either been absorbed or vaporized, then the cooker assumes that the rice is done (and assuming you put in the right proportions of rice and water, it will be).
So, how does it know when there’s no liquid water left. That’s the part that might not be intuitive.
Boiling water at sea level remains at 100 degrees centrigrade. At higher altitudes (meaning lower air pressure), the boiling temperature is lower, but under normal conditions, it’s not going to get higher. If you put more heat into boiling water, it doesn’t heat up, it just boils faster. Therefore, if you’re continually heating a pot and it stays at 100 degrees centigrade, you can pretty much assume it has water boiling in there. But once all the liquid water is gone, there can be no more boiling, so, if you continue to add heat, the temperature starts rising again.
What rice cookers do is continually heat the pot at a certain wattage, which means that the water steadily heats up, then steadily boils. Once the temperature in the pot goes above 100 degrees centrigrade, it automatically shuts off the main boiling cycle (and typically shifts to a lower-power warming mode.
You can design an automatic switch that uses a thermocouple and programming and such, but traditional rice cookers use a simpler design. They use a switch with a permanent magnet, and a disc made of an iron alloy. The disc is held in place by the magnet, keeping a circuit closed and running the cooker, but if you heat up the disc hot enough, it will stop being attracted to the magnet (this is known as the “Curie point”). The alloy is specifically designed to have a Curie point a little higher than 100 degrees centigrade. Hence, when the water all boils away, the temperature rises, the magnetic switch opens, and the cooking circuit is broken. Simple, reliable and inexpensive. All the things engineers love.
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