The Combined Gas Law and You

Good morning class. Let’s review one our high school physics lessons. The Combined Gas Law says pV/T = k, where p is pressure, V is volume, T is the temperature (as measured from absolute zero), and k is a konstant. When the value (P * V) goes up (as in when volume increases faster than the pressure decreases), the temperature will drop.

Let’s pretend pressure is 1000 units (of whatever), volume is 10 units, and our temperature is 400 units. Plug those values in, and K = 250. If we drop pressure to 10 units and increase volume to 1000 units, K is still 250 (because it’s a constant), so we can calculate T = 40 units in our imaginary system of units. 40 is obviously much less than 400.

For a real life application, take, for example, this tiny volume of carbon dioxide at high pressure:

Geniuine Innovations CO2 Cartridge

… discharged into this large volume 26″ x 2.6″ balloon tire in the rain at the corner of Alma and Lytton in Palo Alto:

Urbana Bikes

The result is an inflater chuck frozen solid to the valve stem:

As an exercise to the reader, discuss this in terms of real world units of pressure, volume and temperature and how the inflater chuck can be removed from the valve stem so I can continue on my journey.

7 Comments

  • Eric
    March 2, 2010 - 12:50 pm | Permalink

    Wow. Bummer. Nice presentation of the lesson in physics (or chemistry, where I think I first learned this), but sort of bad timing, huh? I'm not a CO2 canister person, so I tend to experience the other side of things, where the pump and the tire get warm from the compression of the air… I hope you were close to your destination or at least a bike-racked bus line.

  • Sam Berdoux
    March 2, 2010 - 2:15 pm | Permalink

    Let some more rain fall on it until the ice melts. Shouldn't take long.

  • March 2, 2010 - 2:48 pm | Permalink

    The minipump would have probably worked better today :-)

    I just walked across to the gas station, bought a cheap cup of coffee and dumped the hot liquid on the valve!

  • March 2, 2010 - 2:49 pm | Permalink

    Yep :-)

  • March 3, 2010 - 6:41 am | Permalink

    Well, while winter camping when something freezes and needs thawing, we don't have handy gas stations. But there is a handy source of warm water available…

  • March 3, 2010 - 11:53 am | Permalink

    Right you are, Brother Yam. Not sure that's advisable on a busy street corner, though.

  • October 23, 2010 - 11:21 am | Permalink

    “high school physics lesson” – interesting post

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