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Calculating work done by gas?

  1. Jun 13, 2012 #1
    In a cylinder (with a piston) containing gas, why do we use the external pressure, instead of the pressure of the gas, to calculate work?
  2. jcsd
  3. Jun 13, 2012 #2
    I think it would be because all the internal forces cancel each other out.
  4. Jun 13, 2012 #3
    Then why does the piston move up due to internal pressure?
  5. Jun 14, 2012 #4
    Internal pressure changes in all processes except isobaric. External pressure does not. Also, the work done on the system by the surrounding is easier to understand in terms of external pressure.
  6. Jun 15, 2012 #5


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    Gold Member

    And if the original question is on the work done by the system.
    That is, the pressure of the gas in the cylinder expanding against the piston.
    Then dW = Force x distance = pressure x Area x distance = p dV
    Work = ∫ V1 to V2 pdV
    And with PV = n RT
    Work = nRT ln V2/V1
    For isothermal expansion
    Last edited: Jun 15, 2012
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