What Causes Sparks In The Microwave?

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We all know we're not supposed to put metal in the microwave, right? But why not? Physicist Maarten Rutgers has some scientific answers — and some of them will surprise you.

GUY RAZ, host:

Even if your sippy cup does leak, it's still easier to clean up than a microwaved Klondike bar.

Now, the following is a true story and it was witnessed by one of our producers, Alice Winkler.

Alice, take it away.

ALICE WINKLER: I was on vacation last week with my best friend from high school and her 13-year-old son, Diego(ph), took a Klondike bar out of the freezer.

RAZ: A Klondike bar.

WINKLER: A Klondike bar with the tin foil wrap around it. And it was rock hard and he wanted to get it a little soft. He liked his ice cream goopy so he stuck it in the microwave just for a few seconds, and suddenly, sparks were flying.

RAZ: So sparks started flying and it got us wondering about this. We all know we're not supposed to put metal in the microwave, but why?

Well, physicist Maarten Rutgers is on the line to try and help us out.

Dr. Rutgers, what is it about metal that makes it, you know, perfectly fine in a regular oven but, you know, then you put it in a microwave and it's this enormous menace.

Dr. MAARTEN RUTGERS (Physicist, Asylum Research): Well, actually, it's -there's nothing wrong with putting metal in the microwave. The only thing that you shouldn't put in the microwave is metalized foil wrappers, which is a very thin layer of metal with something that can burn behind it, like plastic.

RAZ: So you're saying that, you know, contrary to a manufacturer's instructions, a knife or a spoon or a metal pot will not cause your microwave to explode.

Dr. RUTGERS: As best I know, yes. That would be absolutely fine. When I was a professor at Ohio State, we had spent sometime sort of playing around with this. We specifically bought a microwave to not care if we broke it or not. And it turns out that you can put almost anything in there and it will not fail.

But you can start a fire if you put something in there like a chips bag or a Pop Tart wrapper or a Klondike wrapper.

RAZ: Would it be okay for me to, you know, heat up some pasta or rice tonight in a metal pot in my microwave?

Dr. RUTGERS: I think that would be fine. If it's a shallow metal pot, it's really not that different from the walls of the microwave itself. But microwaves reflect off of metal surfaces. So if you were to, say, try to heat up some pasta in the bottom of a deep metal cup, then the microwaves really don't have any good way to get in there, except through the top.

So it's better to use materials like glass, which tend to not absorb the microwaves, they just travel right through. And then they - once they hit the food, because they're tuned to the specifics of water in terms of heating it up very efficiently, that's how you heat the actual food.

RAZ: Mm-hmm. So can you explain why a thin piece of metal is a problem in a microwave but a thick piece of metal isn't?

Dr. RUTGERS: Quickest explanation is that microwaves will hit the surface and the heat is generated on the surface. If there is a large bulk of metal behind that surface, the heat that's generated has somewhere to go. It conducts into that bulk, keeping the surface cool.

If you have an extremely thin layer of metal, like on the Klondike wrapper on paper or plastic, that heat immediately builds up and it really has nowhere to go. That plastic does not conduct the heat very well and the temperature will rise and rise and rise until it catches fire.

RAZ: Dr. Rutgers, you have a Web site, and on this Web site, you show some experiments that you've done using microwave ovens.


RAZ: And I want to make it clear, we're not promoting any of these. And we've got to advise against doing these actually. But I watched one of the videos you posted, where you create lightning in the microwave with a single grape, a one grape in the microwave.

Can you explain how you did that?

Dr. RUTGERS: I'll explain how I did it. I don't know if I fully understand it.

(Soundbite of laughter)

But what we did is that - and I didn't invent this either, I got this from somebody else. We take a grape. We cut it in half, almost all the way so you sort of open it like a book. And there's a tiny bit of skin still attaching the two halves together. And you lay those two halves on the bottom of the microwave. And it seemed to work better in some spots than others.

We turn on the microwave and sparks start to jump from the junction between those two grape halves. And those sparks grow into giant orbs of glowing gas that rise up to the top and makes sort of a noise like that - oh, what is the name of it - the thing you always see in these Frankenstein labs.

RAZ: Yeah. Yeah.

Dr. RUTGERS: Those two metal wires of this sort of arc moving up towards the top. So as best I can understand it, that grape has a particular shape, which is conducive to receiving and amplifying microwaves. It's almost like an antenna for microwaves. It's about the right size and about the right shape.

So we get a lot of intensity of microwaves right where the two grape halves touch and it ignites the - it ignites a small flame where the grapes are touching. And once that flame is there, the flame just grows and it gets amplified by the microwaves, and it rises towards the top and then it extinguishes once it reaches the top. And then another one starts up again at the bottom and it keeps going on and on.

RAZ: That's very cool.

Dr. RUTGERS: As far as I know, it's not terribly dangerous but it is fire and it probably isn't good for your microwave. So again, I would reserve this. I would go to your local science museum and ask that they do...

RAZ: Or just go to your Web site and look at it.

Dr. RUTGERS: Or look at the movie, yeah.

RAZ: Maarten Rutgers is a former professor of physics at Ohio State. He now works for Asylum Research. It's a company in Santa Barbara that makes atomic force microscopes.

Dr. Rutgers, thanks for enlightening us.

Dr. RUTGERS: Thank you. It was a pleasure.

RAZ: You'll find a link to Maarten Rutgers' video of the flaming grape experiment at our Web site, npr.org.

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