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Variant of Maxwell's demon

  1. Apr 4, 2009 #1


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    I've not covered yet the Second's Law of thermodynamics so I don't understand it well. A friend of mine talked to me about Maxwell's demon and now I have the following question :
    Imagine a planet such that its escape velocity is about Earth's one, but without any atmosphere. Now imagine I have a composed gas (hydrogen+argon) and I free it at ground level. After some time all the hydrogen would leave out the planet and the argon would remain on the planet. So without any intervention the 2 gases are separated. How has the entropy increased?
    Thank you.
  2. jcsd
  3. Apr 4, 2009 #2

    Andy Resnick

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    There's no conflict here- both gases expand, and the entropy of the system (planet+ gases) is proportional to the volume occupied.
  4. Apr 4, 2009 #3


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    Ah ok!
    I thought that by separating the 2 gases the entropy would have decreased. Anyway I'll be soon studying the second law of thermodynamics so I guess I will understand what is entropy and understand better the example I gave here.
    Thank you for the answer. (so there's no ambiguity, no paradox nor anything wrong with the situation described above).
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