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Basic thermodynamics, reversible engine process

  1. Nov 8, 2015 #1
    1. The problem statement, all variables and given/known data
    http://i.imgur.com/jmLqca9.jpg

    pic of question

    2. Relevant equations
    W = p x dV, Q = dU + W etc

    3. The attempt at a solution
    I know what the different stages are, a-b and c-d are isobaric, d-a and b-c are isochoric
    and I believe the answer to a, the net work done in one cycle, is zero as dV is zero.

    but the further questions I'm lost. can anyone help or point me in the right direction?
     
  2. jcsd
  3. Nov 8, 2015 #2
    The net work done is not zero. Let us go through each part step by step.
    1, First, since the given values are p0, V0, we must find the values of pressure volume and temperature of each of the corner states, in terms of these values. Can you do that, and write them down?
     
  4. Nov 8, 2015 #3
    The net work is not zero. How much work is done from a to b? from b to c? from c to d? from d to a?
     
  5. Nov 8, 2015 #4
    Continuing from there, you should first calculate the work done in each process in the cycle, and then add them to give you the answer to a. Let me know if you are comfortable with that.
     
  6. Nov 8, 2015 #5
    ok i used pv=nRT taking R to be 8.31

    p and V are known at all times so I just found the temperatures for each point.
    a - pV/8.31
    b - 3pV/8.31
    c - pV/8.31
    d - pV/24.93

    then for the work since 2 are isochoric they are zero... the others
    a-b W=2pV
    c-d W= -2pv/3

    adding them gives 4pV/3

    is any of that right? haha
     
  7. Nov 8, 2015 #6
    b to c and d to a both zero work since isochoric afaik

    a to b W=2pV
    c to d W= -2pV/3
     
  8. Nov 9, 2015 #7
    This is all correct so far. In your equations for the temperatures, leave the R in (for now), instead of substituting the 8.31. Now, from your results for the temperatures, what is the change in internal energy ΔU from a to b? from b to c? from c to d? from d to a?

    Chet
     
  9. Nov 9, 2015 #8
    I appreciate your help, but I was able to complete the question by myself after you guys got me rolling. I'm fairly confident I got the right answers.

    Thanks again, Connor
     
  10. Nov 9, 2015 #9
    Excellent. Congratulations!
     
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