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Homework Help: Thermodynamics problem

  1. May 11, 2010 #1
    1. The problem statement, all variables and given/known data

    A heat engine operates on the cycle shown below. How much work is done by the engine (J) per cycle, if Pmax = 0.03 atm?

    http://i995.photobucket.com/albums/af79/huybinhs/workprb.gif

    2. Relevant equations

    W = P * Delta V

    3. The attempt at a solution

    Delta V which is area on the graph is 8 - 2 = 6 * 2 = 12 m^2

    W = 12 * 3039.75 N/m^2 = 36477 J = INCORRECT.

    Any ideas? Thanks!
     
  2. jcsd
  3. May 11, 2010 #2
    Work will be the area of the box the system creates.

    You can approximate the area just by looking at the graph, and using formula for area of a parallelogram, which this appears to be.
     
  4. May 11, 2010 #3
    Yes, I looked on the graph:

    Base = 6 , h= 2 so Area = 6*2 = 12, correct?
     
  5. May 11, 2010 #4
    It looks to me like h is wrong. It looks like it is 1/3 of Pmax
     
  6. May 11, 2010 #5
    Ok, so 6 * 0.01 = 0.06 then * 1013.25 = 60.795 = answer correct?

    just submited it and INCORRECT :(
     
  7. May 11, 2010 #6
    My suggestion is to convert .01 atm to Pa FIRST.

    as in .01 atm = 1013.25 Pa <-- that is now your height

    there is no need to convert the Volume so

    W = 1013.25 * 6

    Answer will be in Joules

    OH THE CONVERSIONS!!!
     
  8. May 11, 2010 #7
    6079.5 J is still INCORRECT!!! How come????
     
  9. May 11, 2010 #8
    Oh I'm very sorry, did you try negative?

    How many chance do you get?
     
  10. May 11, 2010 #9
    I got 2 left. How is it negative then?
     
  11. May 11, 2010 #10
    Because of the flow of the engine.

    Let's take it in sections:

    starting at the top left corner, the system moves down and to the right, and everything below this part of the graph would be positive. Delta V in this case would be positive.

    Once it goes back up and back to the left Delta V in this case, would be negative, and the area below it is larger.

    Direction matters here, I'm just not finding a good example. I wish I had one.
     
  12. May 11, 2010 #11
    Perfect. Could u help me with this one?

    https://www.physicsforums.com/showthread.php?t=402433
     
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