Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Microwave ovens

Learn more physics!

Most recent answer: 10/22/2007
Q:
I’ve found that microwaves can ruin just about anything.It can boil water and cook meat. But after using it for so many times, why desn’t the waves affect the machine itself negatively?
- Jacob (age 14)
Middleschool , Arcadia, CA, US
A:
You are right -- microwave ovens can ruin food. They can cause potatoes and eggs to explode, and can cook foods non-uniformly, making them less appealing than foods that are cooked more traditionally. For more information on how microwaves cook foods, please see and .

Microwaves are rapidly oscillating electric and magnetic fields, confined inside the metal box of the oven. The metal walls of the oven act like mirrors which reflect microwaves back to middle of the box. The reason the metal walls act like mirrors is because they are good conductors of electricity. See for a description of why electrical conductors make good mirrors. The metal walls of a microwave oven are usually either painted or lie behind a layer of plastic so that they can easily be cleaned. If the resistivity of the metal is low, then the metal will not heat up when it reflects the microwaves and will not be damaged. Since the metal walls have some resistance, most microwave ovens include cooling fans to cool down the microwave source and possibly also the metal walls so that heat deposited in the walls will not damage the oven.

It may be possible to damage a microwave oven by leaving it cooking on high power for a long time (particularly if there is no food in it). But the amount and rate of damage (if any) depends on how well the cooling fan works and whether the oven is protected in other ways (some may even have temperature sensors that shut down the oven before the components get too hot, or they may have a fuse protecting the capacitor in the microwave source). As long as they don't get too hot to damage anything, the metal parts should not wear out. The cooling fan may wear out after a long time, not because of microwaves but because of dust, grit, or loss of lubrication.

Tom

(published on 10/22/2007)

Follow-up on this answer.