PostHeaderIcon Heat: We’re Doing it Wrong!

Oops. We’ve been teaching meteors wrong. I’ll bet it’s my fault.

The Short:

Meteors do not heat up due to friction with the atmosphere, unlike what our classic hand-rubbing demonstration leads you to believe. They heat up due to compression of the atmosphere. Mostly.

The Story:

So imagine a boat speeding across Lake Washington. See that bow shock, the beginning of the wake, which forms in front of your imaginary boat? That’s kinda what’s happening. Of course, water doesn’t compress (being a liquid), so you get up-and-down waves in water instead of compressed density waves, but gas like an atmosphere is VERY good at compressing, so for our meteor we’ll get nice compression. The speeding meteor-particle (yes, the tiny sand-sized piece of rock) compresses the air in front of it, like the boat’s bow shock.

Bow Shock

Bow Shock

Density Waves:

We talked about density waves a little while ago when we were discussion the galaxy. Instead of up-down or side-to-side waves, density waves are just more stuff or less stuff in a given area. In the case of compression heating, we’re squishing more stuff (air) into a smaller area.

Compression Heating:

When you compress a gas it heats up. Well, almost always. There are some weird cases, but this isn’t one of them. This is classic PV=nRT. What?! Am I allowed to write that on the internet? Yes. This is one of the things you learn early on in a chemistry class that has even a little math in it. Basically the Pressure and Volume of a gas are directly related (=) to the number of atoms, R (just a number), and Temperature. So, if you change one of those things, the others have to change too… oh, and you can’t change n or R.

Alice, I’m not listening anymore:

Okay, come back. This is why a bicycle pump heats up when you’re pumping. It’s not the friction of the little thing inside, it’s that you’re pressurizing gas. This is why aerosol cans (shaving cream, hairspray, those compressed air cans you get for dusting out your computer) cool off when you use them.

The New Way:

You can still do hand-rubbing. Until we come up with a kinesthetic way to teach compression (suggestions?), you might as well – we’re still talking about heat, and that’s a great demonstration of heat. Just mention that it’s not friction that burns up the meteor – it’s compression heating (like a cooling aerosol can).

Alice Enevoldsen

Want More?

Thanks for tipping me off Phil.

Here’s some more from NASA.

7 Responses to “Heat: We’re Doing it Wrong!”

  • Sean says:

    Far out! And I love the equation…it actually makes sense to me, which I find kind of scary. Am I turning into a scientist??


  • Beth says:

    My educational universe collapsed for a moment…until you gave me permission to do the hand-rubbing demo again. Whew!

  • Andrew says:

    Huh, very cool, I did not know that!

  • Holly says:

    I love the way you wrote this one. It made me laugh- and you cleverly reeled be back in right after the math portion. Bravo!

  • Dan says:

    Huh. Thanks for the clarification on this. For demonstration purposes, how about holding a deep breath and then attempting to compress your torso. Do this a few times, and you will get warm. Too bad it doesn’t make you glow. By the way, what is R? Just a number and you can’t change it?

    • Alice says:

      Yes, R is a constant:
      R = universal gas constant = 8.3145 J/mol K

      I suspect getting warm by holding your breath is more about being active and bringing your blood pressure up than about pv=nRt, but I could be wrong.

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