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Homework Help: PV Diagrams (Curve v. Straight)

  1. Jan 14, 2012 #1
    Why are some lines on a PV digram curved and some lines are straight?
     
  2. jcsd
  3. Jan 14, 2012 #2
    Be more specific. Give an example of a PV diagram you don't understand.
     
  4. Jan 14, 2012 #3
    Assuming that the no gas enters or leaves, there's a third relevant variable: temperature. If you heat something, it will either expand, increase in pressure, or a little of both.
    edit: There are only these 3 variables if nothing enters or leaves the system if the gas is assumed to be ideal (the molecules neither take up space nor attract each other).
     
    Last edited: Jan 14, 2012
  5. Jan 14, 2012 #4
    That still doesn't explain why the state at (Po, To) is curved when it goes to state (0.5Po, 2Vo). If it's directly proportional, shouldn't it be a decreasing straight line slope?
     
  6. Jan 14, 2012 #5
    Pressure and temperature are not directly proportional. PV = constant if the temperature is constant.
     
  7. Jan 14, 2012 #6
    So how do you know if one of the lines need to be straight or need to be curved?
     
  8. Jan 14, 2012 #7
    You can just make a back-of-the-envelope graph. For example if the temperature is constant (which would be expected if everything happens slowly and there's poor insulation) you can assume PV = 1 (although any constant would do) and then make a rough graph with P being 1/4, 1/2, 1, 2, and 4 (V would be the inverse).
     
  9. Jan 14, 2012 #8

    Redbelly98

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    That's a pretty general question, so here is a general answer.

    If the line is curved, then a problem statement will either:
    (1) tell you that the process is isothermal
    (2) tell you that the process is adiabatic
    (3) present you with a diagram that shows a curved line
    or (4) provide some other information that allows you to conclude that the line is curved.

    If the line is straight, then a problem statement will either:
    (1) tell you that the process is at constant pressure (for a horizontal line)
    (2) tell you that the process is at constant volume (for a vertical line)
    (3) present you with a diagram that shows a straight line (could be horizontal, vertical, or slanted)
    or (4) provide some other information that allows you to conclude that the line is straight
     
  10. Jan 15, 2012 #9
    That is so much clearer. Thank you!!!!
     
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