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Homework Help: Work using a P-V Graph

  1. Aug 5, 2007 #1
    1. The problem statement, all variables and given/known data

    One mole of gas is initially at a pressure of 3.9 atm, volume of .29 L and has an internal energy equal to 108.1 J. In its final state the gas is at pressure 1 atm and volume .7 L, and its internal energy equals 169 J. (There are 6 different questions using this data).

    For the path IAF, calculate the work done on the gas. Answer in units of J

    okay well there is a graph to better visualize the problem but i can't exactly draw it out but basically the path IAF makes a right angle (If you start from point I you go down and then right). I is at 3.9 atm and .29 L, A is at 1 atm and .29 L, and F is at 1 atm and .7 L.

    2. Relevant equations

    Work = Pressure x Change in Volume

    3. The attempt at a solution
    Well to start it off, there is no work done from I to A so therefore you only need to find the work from point A to F
    W from path A to F= (1 atm)(.41 litres)
    W=(101300 N/m^2)(.00041 m^3)
    so you should get
    41.533 N*m
    41.533 J

    and somehow when i submit that in, it comes at as wrong... help? :uhh:
  2. jcsd
  3. Aug 6, 2007 #2

    Andrew Mason

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    It is asking for the work done ON the gas. Is the gas doing work or is work being done on it here?

  4. Aug 6, 2007 #3
    okay so if it's work being done on the gas.. then the equation should be
    Change in Internal energy = Q-W
    but you don't have specific heat or final or initial temperature..
  5. Aug 6, 2007 #4

    Andrew Mason

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    When gas expands, it DOES work. So in this case, it DOES work, so the work DONE ON the gas is negative. Just put a - sign in front of your answer.

  6. Aug 6, 2007 #5
    oh geeeez
    pardon my ignorance...
    thanks so much :)
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