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About Joule expansion

  1. Oct 21, 2013 #1
    I read on 27 chapter from Blundell's Thermal Physics 2nd edition.

    For ideal gases, Joule expansion doesn't cool gases. But for real gases, Joule expansion makes cooling effects. And this book(page 314) says that when gases expand, their potential energy of molecular interactions increases(since V is proprtional to 1/r). And this energy is exactly from kinetic energy of gas.(So makes kinetic energy of gas decrease)

    What does the last sentence mean? Why does temperature decrease when kinetic energy of gas decrease?

    I understand that low temperature makes average kinetic energy decrease(because of boltzmann distribution, low temperature makes low average kinetic energy)..

    Or... Is temperature defined by boltzmann distribution? If a system has energy distribution of a temperature T, then does it said that the temperature of that system T?

    I am sorry for this absurd question.
  2. jcsd
  3. Oct 21, 2013 #2


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    Staff: Mentor

    Not always. It depends on the sign of the Joule-Thomson coefficient.

    It is not defined from the Boltzmann distribution, but at equilibrium, they are equivalent information. Fit a Boltzmann curve to the distribution of velocities and you will get the temperature.
  4. Oct 22, 2013 #3

    Philip Wood

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    Dr Claude: Are you sure of this? Joule expansion is expansion into a vacuum as opposed to Joule Thomson isenthalpic expansion.
  5. Oct 22, 2013 #4


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    Staff: Mentor

    Right. I misread that as Joule-Thomson. At least the rest of my post is still correct :redface:
  6. Oct 24, 2013 #5

    rude man

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    Temperature is average kinetic energy of the molecules.

    Joule (free) expansion produces a very small change in temperature. Masuring the Joule coefficient is very difficult. As you say, for an ideal gas the Joule coefficient = 0.
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