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Free expansion of an ideal gas

  1. Dec 5, 2004 #1
    For the free expansion of an ideal gas into an evacuated chamber, I know that Work=0 and Q=0. Correct? This implies that the change in internal energy dU also equals 0. Since U depends solely on T, this should mean that the final temperature of the chamber is equal to the temperature of the ideal gas entering it. But what about the final pressure?
    It's an ideal gas, so p1V1=RT1 and p2V2=RT2, but T1=T2, so p1V1=p2V2.
    If you could point me in the right direction, that would be very helpful!
  2. jcsd
  3. Dec 5, 2004 #2
    You've done all the work. What's the problem?
    All you need now in order to find the final pressure, is data on the system. (for example, the temperature, the volume before and after, or any three pieces of data).
  4. Dec 5, 2004 #3
    The problem is that I only have the temperature and pressure of the entering gas, and no other data.
    Any ideas?

    Here's the actual problem: An ideal gas is allowed to expand freely into an evacuated chamber. The pressure of the entering gas is 5 bar and it has a temperature of 45 degrees Celsius.
    Find the final pressure and temperature of the gas.

    Are the assumptions I made (that W=0, Q=0, dU=0, and dT=0) correct?
    Please help me!!!
  5. Dec 5, 2004 #4
    Yes, they are. And you need to know one more thing, they haven't given you enough data. (For different chamber volumes, you'll get different results...)
  6. Dec 5, 2004 #5
    Is there a different approach to this problem other than the one I've taken?
    I know:
    Q=0, W=0, dU=0, p1, and T1=T2.
    How do I get p2?
  7. Dec 6, 2004 #6


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    With P1 and T1, you work out V1. And with V1 and V2 (you must know how much the gas is expanded in volume) you work P2 out.
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