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Homework Help: Thermodynamics question - which process is this?

  1. Nov 3, 2014 #1
    1. The problem statement, all variables and given/known data
    Hello everyone!

    I recently did an experiment, one of the parts was rapidly releasing some gas from a large vessel. We had previously increased the pressure, let it settle and then opened a value to let some out. I am wondering which thermodynamic process this approximates to.

    2. Relevant equations
    From the data collected the temperature definitely changed by a couple of degrees and of course the pressure changed as we let some air out!

    3. The attempt at a solution
    At the moment I am stuck between two answers, either isochoric or adiabatic. As far as I know the real difference between these two processes is Isochoric happens at a constant volume while adiabatic doesn't. This is what I am trying to figure out. If you think of the vessel as a control volume then the volume is constant but if you think of all the gas originally in the cylinder as the volume then it increases massively when some of it is left in the open.

    Thanks for any help you guys can offer!
  2. jcsd
  3. Nov 3, 2014 #2
    The gas that remained in the vessel expanded nearly adiabatically and reversibly when it pushed the gas out ahead of it. The gas that got pushed out through the valve experienced a flow process involving no enthalpy change (per unit mass), a so-called Joule-Thompson effect. So there were two basic things going on simultaneously.

  4. Nov 4, 2014 #3
    Ok thanks, Chet! That will mean that some work will be done BY the gas remaining the cylinder doesn't it?
  5. Nov 4, 2014 #4
  6. Nov 4, 2014 #5
    Great, thanks very much!
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