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Air expansion

  1. Jul 28, 2010 #1
    So they say that compressing air heats it up and expanding air cools it off. But why is it that when you pucker you lips, like to whistle, and blow, the air is cooled.But if you blow air onto your palm with your mouth open, its warm, how is that compressing the air to make it warmer?

  2. jcsd
  3. Jul 28, 2010 #2


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    Staff: Mentor

    In neither case will you do significant cooling of the air via expansion. Our lungs simply don't generate enough pressure for significant expansion.

    The cooling effect of blowing a thin jet of air is almost certainly due to entraining surrounding ambient air and blowing it onto your hand.
  4. Jul 28, 2010 #3

    thats what i thought but my book for school seems to disagree, or at least the author does. Paul g hewit physics 1010 11th edition.

    i agree with you, its just the ambient air being pushed around.
  5. Jul 28, 2010 #4


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    Your lungs heat up the air, and blowing slowly onto your palm doesn't induce much flow from the surrounding air, so it is not cooled much before reaching your palm. Blowing a thin stream at a faster speed induces more flow from the surrounding air, so the flow feels cooler. This is assuming that the ambient temperature isn't well above the air temperature in your lungs, where the effect would be reversed.
  6. Jul 29, 2010 #5
    When you hold your hand further away you feel cold because:

    1) The stream of air has cooled to approximately room temperature
    2) Moisture off your hand is evaporating, causing the "wind chill effect".

    When you hold it close you feel warm because your body temperature is ~36 degrees, and its warmer inside your lungs than on the skin of your palm.

    I figured this out on my own when I was 11.
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