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Homework Help: Molecular Model of an Ideal Gas Question!

  1. Dec 9, 2012 #1
    1. The problem statement, all variables and given/known data
    Gas is allowed to expand freely when the force of a piston is removed. In terms of the molecular model of an ideal gas, explain why the temperature decreases when it expands.

    2. Relevant equations

    Molecular model of an ideal gas

    3. The attempt at a solution

    When the force of a piston is removed, the volume expands. Give that the molecular model of an ideal gas says that molecules collide with each other, now that there's more volume, the molecules will collide less frequently. Since temperature = average KE of molecules, now that they collide with each other less, the KE decreases so temp decreases as well.

    Is this correct? I feel like I'm mixing terms here but I don't know what.
  2. jcsd
  3. Dec 9, 2012 #2
    here's an alternate answer:

    Given that P = F/A, when the volume expands, the pressure decreases. This means that the molecules hit the walls with less, and so the force exerted back to them is lower, thus their velocity is also lower. Since temp = ke = velocity of molecules, the temperature decreases.

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