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What force accelerates a heated air molecule?

  1. Sep 17, 2005 #1
    Hi All,

    I have always been made to understand that the reason why gas expands or causes pressure build up when heated in a sealed container is that gas molecules speed up when heated. If this is correct, can someone in the know please explain what is happening at the individual molecular level how absorbing heat energy can cause a molecule to change its speed.

    Assume a single gas molecule in a 1 cubic meter container at temperature T1 moving at speed S1, if the temperature is increased to T2, does its speed increase to S2 resulting in more frequent collisions with the container walls? Exactly what mechanism is at play to cause the increase of speed and in what direction does the speed increase? If the speed does indeed increase, then there must be acceleration which implies a force. What kind of force is this? Why does this force always act in the direction in which the molecule is already moving?

    Thanks for any help.

  2. jcsd
  3. Sep 17, 2005 #2


    User Avatar

    Staff: Mentor

    You have to consider where the heat comes from. In a chemical reaction like burning, the heat energy comes from chemical energy of released/reformed chemical bonds. Think of chemical bond energy as a rubber-band (actually, its magnetic) - when the energy is released, the molecule formed/liberated is essentially flung away in a random direction, like a slingshot. A container full of, say, hydrogen and oxygen that are ignited ends up with just faster moving water molecules.

    Now, that heat energy can then be transferred via blackbody radiation (the heat you feel by standing near an open flame) or conduction/convection. Conduction and convection work literally by having energetic molecules hitting less energetic ones and transferring their momentum, like balls on a billiards table.
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