Wikipedia says that microwave ovens can be around 50-64% efficient at converting electricity into microwaves. Where does the energy lost at this stage go? And how much of the energy that is successfully turned into radiation ends up turning into heat in the food? What happens to the radiation that isn't absorbed by the food? Does the appliance use less power if there's less food absorbing the microwaves?
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$\begingroup$ Just found this similar post, which answers most of my questions at least qualitatively. $\endgroup$– KarlCommented Feb 21 at 4:36
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$\begingroup$ The remainder of the energy is dissipated as heat in the circuitry, most of it probably in the magnetron, which has cooling fins to keep its temperature sufficiently low. $\endgroup$– PukCommented Feb 21 at 4:49
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efficiency losses in a microwave oven occur in the power supply that feeds the magnetron, in the magnetron itself that feeds the oven cavity, and in the cavity walls.
The useful radiation output of the oven is absorbed by the food inside the cavity. Whatever is not absorbed by the food passes through it and bounces off the cavity walls and passes through the food again at a different angle.
If there is no food in the cavity, the microwave energy inside it builds up and can reach levels which may cause the magnetron to malfunction.
In the no-food case, when you turn off the oven then the buildup of radiation is dissipated in the walls so there's no burst of radiation to escape when you open the oven door.
answered Feb 21 at 18:06