Science Out of the Box has more than shower curtains on its mind. Here's one of the queries sent in by NPR listeners. We hope you get a charge out of the answer. Send us your questions about puzzling scientific phenomena. We can't reply to all of the the submissions but will answer some of the questions in the weeks to come.
I'd love to know why metal in the microwave sparks. - - Rebecca Jackson
It’s all about electrons -- and the shape of the metal object.
Let's say we try to defrost a fork. We shut the microwave oven, sealing the fork inside. Press "Defrost" and microwaves, a form of electromagnetic radiation, pelt the fork. The fork absorbs the microwaves like an antenna, and electrons inside the fork thrash back and forth.
Those thrashing electrons accumulate at the tines, which is why the shape of the utensil is important. Spoons, smooth foil, and other small, smooth metal objects without pointed ends can sometimes escape the microwave unscathed. But nuke a fork, twist-ties, crinkled foil or other metal objects with sharp points, and you're asking for trouble.
The electrons build up in these sharp points, creating a strong electric field in the air around them. When that electric field becomes strong enough, it causes free electrons in the air to accelerate and jar loose more electrons, which then go on to jar loose even more electrons. This chain reaction causes an electrical charge in the air, which we see as sparks -- or, in microwave-speak, "arcing" (pronounced "AR-king").
Arcing can spark fire in paper and other materials. Notable culprits are Chinese takeout boxes with metal handles and "homemade" microwave popcorn bags sealed with metal staples. These kinds of fires may not destroy the microwave, but they produce a really mean smell.
Metal isn't always needed to cause arcing: some foods can create sparks, too. Hot dogs can promote arcing because of the uneven mixing of salt and additives. Carrots can promote arcing because of the minerals in the soil in which they grew.
Other foods pose another danger: super hot explosions. Certain liquids can become superheated, which means they reach temperatures above their boiling point, without boiling. Then, when you go to retrieve your tea or coffee, or drop in a sugar cube, the liquid boils explosively. Liquids in closed containers cause similarly dramatic explosions, as steam builds up on the inside, putting pressure on the container. Fresh eggs have been known to explode as well.
But the hands-down winner might be microwaved grapes, which cause the type of do-it-at-home-explosion that only a You Tube user can adequately illustrate.
For more information on microwave pyrotechnics, visit the USDA's website on "Microwave Ovens and Food Safety."
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