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Homework Help: Baffling Thermodynamics Graph

  1. Jan 12, 2010 #1

    jacksonpeeble

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    Gold Member

    1. The problem statement, all variables and given/known data
    The pressure and volume of a gas are changed along the path ABCA in the graph. The vertical divisions on the graph represent 4.00 105 Pa, and the horizontal divisions represent 5.00 10-3 m3. Determine the work done (including algebraic sign) in each segment of the path.
    cj6_p15-10alt.gif
    (a) A to B = 0
    (b) B to C
    (c) C to A


    2. Relevant equations
    Area of Triangle=.5lwh
    (0th and 1st Laws of Thermodynamics)


    3. The attempt at a solution
    What exactly am I supposed to figure out, and how am I supposed to go about doing this? I'm thrown off by the graph and what to do with it? What is it saying about the scale? If someone can explain HOW to get the numbers that I need to make the calculations (and why to use them), I can probably work it out (I assume I use the area of a triangle in there somewhere, but perhaps I'm wrong).
     
  2. jcsd
  3. Jan 12, 2010 #2

    sylas

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    Don't worry about the area... try rather to start by looking at each edge. That's what the question is asking.

    For example... in going from A to B, what are the values for volume and pressure and how do they change? Can you figure out anything about the work done, just for this simple transition?
     
  4. Jan 12, 2010 #3

    ideasrule

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    You're supposed to be figuring out the work done. Work is P*delta-V, so the work done from A to B, for example, is the area under the graph from A to B.

    There's no scale on the PV diagrams. The question's telling you what each grid represents.
     
  5. Jan 12, 2010 #4

    jacksonpeeble

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    ...I hate to do it, but I'm going to have to ask for additional clarification. This just isn't clicking. :]
     
  6. Jan 12, 2010 #5

    ideasrule

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    OK, one step at a time. Do you know why dW=P*dV? This is analogous to dx=v*dt, so it follows that because the area under a velocity-time graph is displacement, the area under a pressure-volume graph is work.
     
  7. Jan 12, 2010 #6

    jacksonpeeble

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