Metal in the Microwave and Other ‘WTF?’ Tech Moments

Moving into my apartment, I was perplexed when I checked out my appliances. The stainless steel fridge and dishwasher were great upgrades, but the microwave made me pause. I spotted a rack inside, just like in a conventional oven. Ever since trying to soften a foil-wrapped pat of butter in the microwave as a kid, I’ve always known you weren’t supposed to put metal in the microwave…

But apparently there’s a reason you can leave that obviously metallic rack in the microwave. We checked out a script from a radio show out of the California Institute of Technology hosted by alumna Sandra Tsing Loh, “The Loh Down on Science,” that gives us a clue:

Microwave ovens produce electromagnetic waves that jiggle electrons … that motion [is what] heats up your food. The electrons in metal are mobile—they can move freely among atoms—and that’s where microwave problems start.

A thin metal like aluminum foil doesn’t have room for all the wiggling, roving electrons. Instead, they bang into aluminum atoms, and then other aluminum atoms, and the foil heats up [and] catches fire. Sharp edges and points—like on a fork—can also be prickly. Electrons congregate in the edges and points, building up negative charge. Eventually, they start leaping off, causing sparks.

But when the metal is thick [and] smooth with rounded edges [like] that metal rack, the moving electrons can bounce around freely while rarely hitting another metal atom. [The] rack doesn’t get hot!

Who knew? There was a simple physics answer for a perplexing piece of home technology. Is there anything else about your home computer, TV or appliances that makes you say, “Huh?”

Email us your question or let us know in the comments and we’ll see if we can solve the mystery together!

Image: Taryn