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In regards to Maxwell's Demon

  1. Jan 20, 2012 #1
    In regards to "Maxwell's Demon"

    In regards to the Maxwell's Demon thought experiment and violation of the Second Law of Thermodynamics, I have a scenario that results in (a) question(s).

    First I won't pretend to have extensive knowledge of Thermodynamics so bear with me...

    Say we have two insulated chambers of temperatures T1 and T2, and we create a small opening connecting them. Criticism of Maxwell's Demon violating the 2nd Law of Thermodynamics lies in energy expenditure by the so called "Demon" monitoring the motion of all molecules in the chambers.

    Let's say, in a new scenario, there is no demon controlling this opening. Let it be open indefinitely. Is there not a non-zero probability that all molecules possessing kinetic energy above T1 can make their way to the first chamber and all molecules possessing kinetic energy less than T2 make their way to the second chamber? If the system remains in this configuration indefinitely, will it not violate the Second Law?
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  3. Jan 20, 2012 #2


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    Re: In regards to "Maxwell's Demon"

    There is nothing preventing a temporary net imbalance.

    How would it remain that way for any length of time, let alone indefinitely? It's one thing for it to have a non-zero chance of all molecules fighting against entropy to get into one chamber, but it's another for them to keep fighting it indefinitely.
  4. Jan 20, 2012 #3
    Re: In regards to "Maxwell's Demon"

    Indeed, the difference between your case and that of that of Maxwell's demon is that in the former case you have to rely on luck, whereas in the latter Maxwell's demon will make it so.

    More drastically: Maxwell's demon can obviously power your engine. Now, following your logic, luck can also power your engine, but you'd have to wait a long time before hitting the highway huh ;)
  5. Jan 20, 2012 #4
    Re: In regards to "Maxwell's Demon"

    Some configuration in which none of the molecules "contact" the gap in the insulating wall after the transfer of high/low kinetic energy molecules.

    I understand the low probability of such a configuration considering intermolecular collisions or forces. But does a non-zero probability have ramifications on the Law nonetheless?
  6. Jan 20, 2012 #5
    Re: In regards to "Maxwell's Demon"

    Ah if that is your question: yes. The second law is a probabilistic law, it's not always true, it's just very very very likely to be true. That's what Maxwell wanted to show with his demon, actually.
  7. Jan 20, 2012 #6
    Re: In regards to "Maxwell's Demon"

    Alright great, I guess a more complete understanding of the 2nd law would've answered that for me. Thanks for your help!
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