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In my college level thermodynamics class, the teacher gave us the example of a closed system which consisted of a box thermally insulated. A piston fully divides the box in two parts, and each is filled with some ideal gas (lets call them G1 and G2). The gases can't exchange heat through the piston and so, according to the First Law of Thermodynamics, all work done by the gas must correspond to a variation of internal energy.

Now, suppose G1 and G2 have the same pressure, but different temperatures. The teacher told us that, even though the pressure was the same, the piston would move towards the gas with the lowest temperature until an equilibrium in which both pressure and temperature were equal.

Why does it happen??

Thanks in advance!

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  • $\begingroup$ Why can't the gasses exchange heat through the piston? $\endgroup$
    – user253751
    Mar 20, 2015 at 6:08

3 Answers 3

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if the pressure is the same, there in no net force on the piston, so it will remain at rest, so neither gas will expand, and this will stay this way because there is no heat exchange that could change the pressure on either side. Ask your teacher to return his diploma.

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Your teacher is wrong... in a way. What he probably meant was same mass, different temperture.

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    $\begingroup$ how would reinterpretations make his statement correct? $\endgroup$
    – user66432
    Mar 20, 2015 at 4:30
  • $\begingroup$ If you interpret his statement as " take two ideal gases with the same mass, then heat one. The piston will move into the colder gas, compressing them until the pressures are in equilibrium. $\endgroup$
    – Jimmy360
    Mar 20, 2015 at 4:33
  • $\begingroup$ The main problem is if they have the same pressure but different temperatures, then the one with the higher temperature must have less volume $\endgroup$
    – Jimmy360
    Mar 20, 2015 at 4:34
  • $\begingroup$ And if pressure is the same $\endgroup$
    – Jimmy360
    Mar 20, 2015 at 4:36
  • $\begingroup$ There would be no motion $\endgroup$
    – Jimmy360
    Mar 20, 2015 at 4:36
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It doesn't. They'd have to exchange heat through the piston.

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    $\begingroup$ Well, comments can be used for such short sentences. If it's gonna be an answer, then it'd be nicer to brief it. Would you be so kind to explain it, given the level of OP? :) $\endgroup$ Mar 20, 2015 at 5:03

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