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Pressure due to volume change

  1. Apr 22, 2012 #1

    I have a closed system of fixed volume with an piston pump attached. The medium is air and the initial environmental condition are at standard atmospheric. I would like to know the formulas to use that would provide me with the new pressure when the piston is at top dead center (i.e. traveled a full stroke). Would I be correct in assuming that temperature is negligible when no heat (or fuel) is supplied to the system during the cycle?

  2. jcsd
  3. Apr 22, 2012 #2
    If you know the initial volume and pressure, you would use P1V1=P2V2, where P1 and V1 are the initial pressure and volume and the other side of the equation is the pressure and volume after the piston has moved.

    I would also say that temperature is negligible. It would change slightly, but it's not relevant because it's a result of the pressure change.
  4. Apr 23, 2012 #3
    Actually, air heats up as it is compressed, like in a diesel engine. Specifically for adiabatic compression, T·V γ-1 = constant, where γ = 7/5 for air. There is a similar relation for the pressure increase. See http://en.wikipedia.org/wiki/Adiabatic_process
  5. Apr 23, 2012 #4
    I know. I just meant that, from the description of the problem, temperature wasn't relevant, not that didn't occur.
  6. Apr 24, 2012 #5
    We can't say whether the temperature change is 'slight' unless we know the relative volumes between piston down and piston up, which the OP failed to mention.
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