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Thread: Maxwell's Demon

  1. #1 Maxwell's Demon 
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    I'm new here, and i'm looking for an explanation for maxwell's demon.


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    KJW
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    The way I see it, the greater the thermodynamically unfavourable deviation from thermodynamic equilibrium, the less likely the conditions will be met for further deviation from thermodynamic equilibrium, thus slowing down the rate that the deviation increases to the point that in the limit of infinite time, the deviation will be limited to some maximum value. Actually, it might be worthwhile for me to derive this mathematically, or even to simulate Maxwell's demon on a computer, so thanks for raising this question.


    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by LorenaF View Post
    I'm new here, and i'm looking for an explanation for maxwell's demon.
    First, welcome to the Forum!

    Second, your query can be interpreted a couple of ways. It's not entirely clear if you are looking for a description of the demon, or for a resolution to the apparent paradox resulting from the demon's actions, so I apologize if the following is more than you were interested in.

    The setup is a container full of some gas, and a wall that divides that container into two sections. Temperature describes an average quantity reflective of the kinetic energy of the gas, and that temperature is initially the same for the gas in both sections. Some molecules will be hotter than average, and some cooler. A tiny door in that wall is operated by the demon, who allows only hotter-than-average gas molecules zip to the right-hand section, say, and cooler-than-average ones to the left. The demon's actions eventually produce a cool compartment on the left, and a hot compartment on the right; the demon has reduced entropy, in violation of the second law of thermo.

    The conventional resolution begins with an acknowledgment that the demon must process information in order to carry out the sorting, and so any proper accounting of entropy and energy must comprehend how the demon goes about his business. Measuring the velocity of a molecule by itself need not involve energy dissipation (there are, at least in principle, adiabatic methods), but energy must be dissipated if information is ever deleted (a result proved by an IBM researcher named Rolf Landauer). Any practical realization of a demon would run out of data storage space in some finite time and thus begin deleting old data. At that point, the associated dissipation would rescue the second law.

    That's the short version, in any event. If you search the web for Maxwell's demon, you will undoubtedly come across many explanations, and many criticisms of those explanations. This particular topic was responsible for several heated discussions when I was a student. The class got the poor professor to reverse himself several times. We all felt as if net negative learning had taken place by the time we'd moved on to another topic.
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    KJW
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    It is worth noting that is finite, thus satisfying the conditions for Zeno's paradox.

    I'm not particularly happy with the explanations of Maxwell's demon that have been given to me, which involve the energy required to detect the approaching molecules. This seems like a red herring to me, as well as preventing Maxwell's demon from being amenable to mathematical analysis. Given that the second law of thermodynamics is statistical, the obstruction to Maxwell's demon should also be statistical.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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