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Homework Help: Energy change of apparatus when gaws suddenly expands

  1. Nov 23, 2012 #1
    Energy change of apparatus when gas suddenly expands

    1. The problem statement, all variables and given/known data
    A gas is confined in a cylinder by a piston. The initial pressure of the gas is 7 bar(700kPa), and the volume is 0.10m^3. The piston is held in place by latches in the cylinder wall.The whole apparatus is placed in a total vacuum. What is the energy change of the apparatus if the restraining latches are removed so that the gas suddenly expands to double its initial volume, the piston striking other latches at the end of the process?

    Data known:
    P(initial) = 700kPa
    V(initial) = 0.10m^3
    V(final) = 0.2m^3

    2. Relevant equations
    Energy change = W = PΔV

    3. The attempt at a solution
    Since the gas suddenly expands (not a quasi-equilibrium process), the PV ln (V2/V1) equation cannot used. But still, my lecturer gave me zero. I want to know the right way to solve this problem. God bless you all.
    Last edited: Nov 23, 2012
  2. jcsd
  3. Nov 25, 2012 #2


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    If it's surrounded by a vacuum and the expansion happens rapidly then yes the process is probably adiabatic...


    You didn't actually tell us what answer you gave or showed how you got it.
  4. Nov 26, 2012 #3
    Thanks for the help. I'll confirm with my lecturer. I suggest since it's expansion against vacuum, there should be no energy change.
  5. Nov 27, 2012 #4


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    I'd assume that the gas is doing work against the (assume finite mass) piston during the expansion, so work done by the gas is converted into KE of the moving piston. This energy is lost with a thud when the piston hits the stop "latch", so no useful work is done, but you can still calculate the work done and the energy lost by the gas (for an adiabatic expansion).
    Last edited: Nov 27, 2012
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