Arggh, Why Does the Shower Curtain Attack Me?

Woman in a shower

hide captionThe water comes on, the curtain bows inward. Is it water pressure, hot air or ... a vortex in the stall?

Lambert/Archive Photos

We've got another "Science Out of the Box" entry on the mysteries of microwave sparks. And you're invited to submit your questions as well. Scroll down for details.

It strikes when we're cold, tired, naked. We may try to dodge it — but we're cornered.

The shower curtain. Why does it bow inward when we turn on the faucet? NPR's Joe Palca investigates from the scene of the crime: his bathroom. He talks with engineer Liz Marshall from Fluent, Inc., in New Hampshire, a company investigating the curtain effect.

Marshall says there are three explanations. But the simplest, she says, is full of hot air.

Traditional thinking went like this: the hot spray heats the air around it. As the hot air rises, it pulls cooler air into the shower from outside. With that cooler air comes the shower curtain. But this theory is lacking, Marshall says, because it overlooks one important fact. The curtain does the same thing in a cold shower.

Scientists have since turned to the "Bernoulli Effect," which states that when fluids accelerate, the pressure around them drops. So when we turn on the shower, the spray is surrounded by lower air pressure. The pressure outside the shower curtain stays roughly the same. That difference in pressure on either side of the curtain makes it bow in.

That theory held until about five years ago, Marshall says. Then David Schmidt, an engineer at the University of Massachusetts, simulated the shower scene on his computer. His model predicts that when the shower sprays, the air inside the shower becomes a kind of spinning vortex. The pressure at the center of this vortex is very low, as it is at the eye of a hurricane. And that low pressure, Schmidt says, could be what sucks the shower curtain in.

Marshall hopes that further modeling and field studies will settle this steaming hot question.

Microwave Antics: Don't Stick a Fork in It

Fork and pasta in a container

hide captionKids, don't put Mr. Fork in the microwave, or sparks could fly!

Getty Images/George Doyle

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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