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Random thoughts on Maxwell's demon

  1. Feb 24, 2012 #1
    This just came into my head. I don't think I really understand the significance of Maxwell's demon.

    Please don't try to explain it to me. It's just a random thought.
     
  2. jcsd
  3. Feb 25, 2012 #2

    Ivan Seeking

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    Re: PF Random Thoughts

    Sorry, you can't make that statement and request at PF and expect cooperation. :biggrin:

    Maxwell's demon is a famous thought experiment that seemed to suggest that it is possible to get free energy - to make heat flow from cold to hot.

    I was watching the original Andromeda Strain movie tonight. I've seen this a number of times but never realized I had only seen a highly edited version of the original movie. Suddenly there were scenes coming up I'd never seen before! That's always a nice susprise on a movie you thought you knew forwards and backwards. I only had it on because I was working and not really paying attention. Only by chance did I look up at just the right time.
     
  4. Feb 25, 2012 #3
    Re: PF Random Thoughts

    The "entity" which is what Maxwell actually called it, just stands there and opens a door when more energetic molecules happen to be aimed at the door. He doesn't open it for less energetic particles. All he is doing is separating hotter from colder.

    This is not impossible, and happens all the time in Nature: hot air rises by itself, no "demon' necessary. Separate the top half of any container of air from the bottom half, and you have what the demon did.
     
  5. Feb 25, 2012 #4
    Re: PF Random Thoughts

    The container needs to be isolated. Without gravity, the top half would not get hotter than the bottom half.
     
  6. Feb 25, 2012 #5
    Re: PF Random Thoughts

    What do you mean by "isolated"? And I'm sorry for not stipulating this wouldn't work in outer space. Didn't think anyone would go there.
     
  7. Feb 25, 2012 #6
    Re: PF Random Thoughts

    Isolated means that energy is not allowed to pass through the walls of the container. The energy to lift the hot gas comes from outside the box.
     
  8. Feb 25, 2012 #7
    Re: PF Random Thoughts

    Eh? The air has to have some non-zero temp to begin with. Put some air in a thermos and the hotter, more energetic particles automatically work their way to the top while the less energetic ones sink. The colder ones can be packed more closely together, the more energetic ones rise to the zone of least density.
     
  9. Feb 25, 2012 #8
    Re: PF Random Thoughts

    You can most easily see what I am saying by trying to create a horizontal temperature gradient. Or by moving the container to a place where there is no gravity.
     
  10. Feb 25, 2012 #9
    Re: PF Random Thoughts

    It doesn't work horizontally (without an "entity") and I already agree it wouldn't work vertically without gravity.
     
  11. Feb 25, 2012 #10

    Ivan Seeking

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    Re: PF Random Thoughts

    Surely you understand what the significance of free energy would be? It would mean there would be no need for an energy supply, like oil. It would mean you can get something for nothing. Maxwell's Demon appeared to violate the second law of thermodynamics - loosely stated, there are no free rides. In it's simplest form the 2nd law says that without the addition of energy to the system, heat flows from hot to cold. I have a favorite quote for this issue.

    -------- Arthur S. Eddington (British Astrophysicist, 1882-1944) in The nature of the Physical World (1928)

    I'd never seen the theater release; only the version edited for TV. That had never occurred to me before.
     
    Last edited: Feb 25, 2012
  12. Feb 25, 2012 #11
    Re: PF Random Thoughts

    Here are Maxwell's original words (it's not long):

    http://web.lemoyne.edu/~giunta/demon.html

    It's not really about violating the second law. It's about having to view a huge number of individual things as a uniform thing.
     
  13. Feb 25, 2012 #12

    Ivan Seeking

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    Re: PF Random Thoughts

    Your statement makes no sense. Maybe you should stick to art. :biggrin:
     
  14. Feb 25, 2012 #13
    Re: PF Random Thoughts

    Yes it is. I took this from your link.

     
  15. Feb 25, 2012 #14
    Also, make note of this from the link:

    For the purpose of this discussion, heat is the same thing as energy, i.e. the envelope should permit no passage of energy. The examples you have given us all depend on gravity to provide the energy for the heated gas to rise. This energy is in the form of a potential gradient which comes from outside the container.
     
  16. Feb 25, 2012 #15
    That's what I'm saying: gravity is the demon, and this happens all the time. The air near the ceiling is always at a higher temp than the air near the floor.
     
  17. Feb 25, 2012 #16
    Re: PF Random Thoughts

    The gedanken is prelude to this:
     
  18. Feb 25, 2012 #17

    256bits

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    That's the 'gravity demom' /
    Maxwell's demon would make the air hotter near the floor and cooler near the ceiling.
     
  19. Feb 25, 2012 #18
    No, the demon is not allowed to work across the walls of the container. Maxwell's demon is inside the container.
     
  20. Feb 25, 2012 #19
    Ah! I think you're right! Vertically speaking, this is what Maxwell would be describing.
     
  21. Feb 25, 2012 #20
    I know. The original remark to Ivan merely points out the automatic separation of hot from cold without any human effort. Meaning: if you really want to separate hot air from cold without any effort, let gravity do it; no impossible demon necessary. The main implication of that being; even if you miraculously violated the 2nd law, all you'd get is the same thing that happens all the time anyway by gravity in any room. This doesn't represent any amazing "free energy". People ignore it, take it for granted. When they imagine of it being done miraculously in violation of the 2nd law, suddenly it becomes psychologically important, despite the fact it's not enough of a temperature separation to do very much with.
     
    Last edited: Feb 26, 2012
  22. Feb 26, 2012 #21
    Thanks for the replies.
     
  23. Feb 26, 2012 #22
    If you start with constant temperature air, you won't get any separation of hot and cold air. This would allow you to run a heat engine by putting an object in the hot zone and letting it radiate its heat to the cold zone, thus creating a temperature difference.
     
  24. Feb 26, 2012 #23
    We just had a detailed thread about this demon here: https://www.physicsforums.com/showthread.php?p=3784856#post3784856

    The bottom line is that Maxwell's demon is still a convincing thought experiment for attacking the second law. There are false rumours all over the internet that information theory somehow helps by pointing out limits on the efficiency of the demon, but all that theory starts by assuming the 2nd law and deriving the limits from there. It would thus be circular to use the limits to rescue the 2nd law from the demon.

    I don't think gravity is much of a demon though. It cashes in gravitational potential energy to do the sorting, which is perfectly legal under the 2nd law.
     
  25. Feb 26, 2012 #24
    I know. If your goal is to break the law, gravity won't help. If your goal is to separate air by temperature and harvest "free" energy thereby, gravity has been there all along, no demon necessary. That is all I was ever saying.
     
  26. Feb 26, 2012 #25
    But you appeared to be saying that the demon doesn't do anything that gravity can't. I'd say there is something special about the demon: he breaks the second law. Gravity never threatened to because it harvested GPE to do the separation. The demon manages the separation in the complete absence of gravity. So where's the energy coming from? That's a rather demonic party trick.
     
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