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Homework Help: Adiabatic condition and equation of state

  1. Jan 13, 2015 #1
    1. The problem statement, all variables and given/known data
    8.02 × 10−1 moles of nitrogen gas ( γ= 1.40) is contained in a volume of 2.00 × 10−2 m3 at a pressure of 1.00 × 105 Pa and temperature of 300 K. The sample is adiabatically compressed to half its original volume. IT behaves as an ideal gas.

    (i) What is the change in entropy of the gas?

    (ii) Show from the adiabatic condition and the equation of state that TV γ-1
    remains constant, and hence determine the final temperature of the gas

    2. Relevant equations
    PVγ = A (constant)

    3. The attempt at a solution
    For I, I think that because the compression is a reversible adiabatic process there will be no entropy change.

    It is ii, that I am stuck on. I think that the equation of state will need to be rearranged to give P=nRT / V and that this will need to be substituted in to the adiabatic condition to give (nRT Vγ) / V = constant which feels close but I cannot think of the last step.

    Or is it better to start with PVγ = constant
    PVVγ-1 = constant
    nRT *Vγ-1 = constant and ignore n and R seeing as they themselves are constants? giving TVγ-1 = constant
    (I am not sure whether n and R can just be ignored or whether this method is even correct or valid).

    or some other way entirely, any insight would be appreciated.
  2. jcsd
  3. Jan 13, 2015 #2
    Both methods are correct, and give the correct result. In the first method, Vγ/V=Vγ-1

  4. Jan 14, 2015 #3
    I think the answer might be along the lines:
    for adiabatic process PVgamma= A
    so fitting this into the equation:

    then you get TgammaP1-gamma=A/(nR)gamma

    Use the same principle to substitute P=nRT/V into the adiabatic condition
    hope that helps

    Sorry about the subscripts, my first post and new to forums. But since I always check here for help, is only fitting that I help when I can
  5. Jan 14, 2015 #4
    Hi Astoreth. Welcome to Physics Forums.

    It looks like LivvyS had already solved the problem correctly in post #1. Do you feel that there was a problem with what he/she did?

  6. Jan 14, 2015 #5

    sorry about that. No, the answer was not incorrect, but given this:

    I thought I'd help.

    I am sorry if I overstepped. I was only trying to help

    Also, i didn't noticed that it was marked as resolved.

    Again, am sorry
  7. Jan 14, 2015 #6
    No need to apologize. As you said, you were just trying to help.

    The OP used one method to solve it in post #1, and got it correct. And he tried another method to solve it, but couldn't complete the final step. In post #2, I pointed out how to complete the final step, which, when applied, leads to the same correct solution.

    Again, Welcome to Physics Forums. It's great to have you in our community.

  8. Jan 14, 2015 #7
    Thanks, Chet :)
    Is good to be here
  9. Jan 14, 2015 #8
    All help much appreciated, thanks guys!
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