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I Gas pressures with (hypothetical) filter

  1. Mar 21, 2016 #1
    Imagine I have 2 constant-volume, constant-temperature containers. Container A has a volume of 10 L and one Container B has a volume of 5 L.

    My hypothetical gases Ideal Gas A and Ideal Gas B have the same properties as each other (temperature, mass, energy, heat capacity, blah blah blah).

    If I insert 3 moles of Ideal Gas A and 3 moles of Ideal Gas B into Container A and connect the two containers, I'd expect the pressure to equalize between the containers, and the final distribution of the gases to be
    Container A: 2 moles Gas A, 2 moles Gas B
    Container B: 1 mole Gas A, 1 mole Gas B

    makes sense.

    Now, I restart the experiment, but this time I've placed a magic filter (possibly controlled by a demon) that only allows Gas B to pass, and insert it the connection between the two containers.

    What would the distribution of the gases be?

    Obviously all of Gas A stays in Container A, thanks to the filter.

    Would Gas B move to Container B until the total pressure was equal between the two containers? Or would the partial pressure of Gas B move it until the Gas B pressure was equal. Or does it have to do with the number of moles in each container. Or....?

    Thanks for any insight anyone who understands ideal gas laws can give!
  2. jcsd
  3. Mar 22, 2016 #2

    Vanadium 50

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    Doesn't this depend on the properties of this magic filter? (e.g. can it do work?)
  4. Mar 22, 2016 #3
    Good point.

    This magic filter doesn't apply any forces to the gas any more than the container walls do -- it's a Maxwell's demon-ish type filter that simply allows the passage of one type of gas particle but not the other.

    For example if the two gases have molecules of different size, and the filter has holes that allow one size particle through and not the other.

    Would that count as doing work? I don't think it's adding energy to the system if that happens. It's definitely messing with the entropy though.

    If no work were being done by the filter, my intuition says that the two containers should still equalize in pressure. Or is it just the case that each gas equalizes in pressure, if it can?

    Thanks for your quick response.
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