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Homework Help: Ideal Gases. Query

  1. Jan 16, 2006 #1
    I have found the following question on a previous exam paper whilst I've been doing my revision, and have a few questions about how to set up my answer.

    Calculate the entropy change of 3 moles of an ideal gas when they are isothermally compressed (T = 30° C) from a volume 5V to a volume V.

    My initial thought was to use PV = nRT, but because both n and T stay constant I then started thinking that I could just make P1V1 = P2V2.

    However from my notes I have that PV^gamma = constant which confused me as there is no mention of a value for gamma in the question.

    From here I would use the forumla W=-integralP(V)dV and then use the S=W/T formula.

    Is this the correct way to go with this question?

    Thanks for any feedback
  2. jcsd
  3. Jan 20, 2006 #2

    Tom Mattson

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    It's simpler than that. Try to answer the following questions in order.

    1.) For an isothermal process, what is [itex]dU[/itex]?
    2.) Can you find an expression for [itex]dU[/itex] in terms of [itex]dS[/itex] and [itex]dV[/itex]? (since after all you were given 2 volumes and you were asked for the change in entropy).
    3.) Can you integrate the expression found in 2.)?

    That should get you there.
  4. Jan 20, 2006 #3


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    PV^1 = constant for an isothermal process (treating it as an ideal gas ...)
    PV^gamma = constant for an adiabatic process.
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