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Homework Help: Gas flows from cylinder A to cylinder B

  1. Sep 10, 2007 #1
    Hi all,

    Let's say we have 2 closed cylinders A and B, same volume, containing air inside. Two cylinders are connected to each other with a tube, with valve. Initially the valve is closed.

    cylinder A: P=100 Pa, T= 350 k.
    cylinder B: P=50 Pa, T= 300 k

    now if the valve is opened, and closed when their pressure are the same.

    assume no heat loss to or gain from surrounding.

    here my problem:
    will their temperature be the same when their reach the same pressure?

    my guess are....

    1. yes. By ideal gas law, PV/T = mR, if P1=P2, V1=V2, then T1=T2, so the temp will be the same.

    2. no. When the valve is opened, the air flows from A to B, which means the air does some flow work. By conservation of energy, the temp in B will be higher than A.

    which one of me should I believe??

  2. jcsd
  3. Sep 10, 2007 #2


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    Staff Emeritus
    Science Advisor

    When A and B become (A+B), and it's adiabatic, what is the equilibrium state.

    Certainly the pressure will equilibrate, but what can be said of the temperature?

    How? B is starting at a lower temperature than A. How could B end up at a higher temperature?

    Those are lower pressures btw. Are those gage or absolute pressures?

    Remember that 1 atm = 14.7 psi = 101.325 kPa.
    Last edited: Sep 10, 2007
  4. Sep 10, 2007 #3
    what if they started with the same temperature, other condition remains the same?

    assume the values are gage pressure values(but does make any difference?)
  5. Sep 10, 2007 #4


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    Staff Emeritus
    Science Advisor

    Then they remain at the same temperature. At some point in time, the pressures have to equilibrate.

    I'm thinking along the lines of PV=nRT, in which the pressure is in absolute terms.

    Perhaps the pressures are in absolute, in which case, they are partial vacuums, or at least very low pressures.
  6. Sep 11, 2007 #5
    what about the flow work? the air molecules which flow from A to B have some KE right? The KE will change into thermal E after the valved is closed?
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