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Homework Help: Thermodynamics-boundary work

  1. Feb 16, 2015 #1
    1. The problem statement, all variables and given/known data
    Hello, I came across a problem where the polytropic index is not given but I've been given P1, P2, V1 and V2.

    P1V1 = P2V2 (130kPa)(0.007m3)n = (100kPa)(0.08637m3)n ----> n=1.249
    2. Relevant equations
    I was thinking I would have to use log to figure out n maybe?

    3. The attempt at a solution
    haven't been able to work it out backwards. Need your help please!
  2. jcsd
  3. Feb 16, 2015 #2

    Andrew Mason

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    If xn = k then ln(xn) = ln(k)

  4. Feb 16, 2015 #3
    the value of n is not given. I just put it there because but what if it was
    xn = ?
    solving for n
  5. Feb 16, 2015 #4
    somebody helpppp
    I have a test tomorrow
  6. Feb 16, 2015 #5
    I came to this but now I am not sure how to isolate for n

    ln(130) n*ln(0.07) = ln(100) n*ln90.08637)
  7. Feb 17, 2015 #6

    Andrew Mason

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    [tex]P_1V_1^n = P_2V_2^n[/tex]
    [tex]\frac{P_1}{P_2} = \left(\frac{V_2}{V_1}\right)^n[/tex]
    [tex]\ln(P_1)-\ln(P_2) = n(\ln(V_2) - \ln(V_1))[/tex]

    Can you work it out from there?

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