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Maxwell's demon - a new concept

  1. Nov 20, 2014 #1
    Could somebody disprove the following concept of a Maxwell's demon:
    Let's say we have a very small container, the size and a form of a nanotube, similar to ellipse, and only two gas molecules randomly flying inside. According to probability we will quite often find molecules in the different ends of a such container. If these molecules will have different speed and energy (is it likely to happen?) then we will get the hotter and the colder molecules in the different ends of the container, what is one of the conditions for a Maxwell's demon to start his work. If hotter molecule will stay at one end of the container for a while it will bounce to its walls and heat it. And contra, the cold molecule at the same time will bounce at the other end of nanotube (container) and cool it. In this way a temperature difference between the different ends of nanotube will be created. Then we could imagine a two thermoelectric wires of an atomic thickness which are connected to both ends of the nanotube and to a small electric generator. If we need to scale this system up, we could take a two largest molecules which could only exist in nature and conduct the experiment in a deep space that gravitation and weight of molecules wouldn't interrupt us. Does nature put some fundamental restriction on size of molecules (which could have a thermal motion)?

    And also question: is probability of a thermal fluctuations is always the same on all closed systems? Or there could be some systems with increased probability?
     
    Last edited: Nov 20, 2014
  2. jcsd
  3. Nov 20, 2014 #2
    Energy and temperature aren't the same thing. The terms hotter and colder refer to macroscopic quantities of particles with macroscopic number of degrees of freedom. It doesn't really make much sense to talk about a hotter molecule and a colder molecule, unless these are really big molecule.

    The probability of a fluctuation depends on the size of the fluctuation, so it is given as a power spectrum. Some theoretical values can be obtained from the fluctuation dissipation theorem. http://en.wikipedia.org/wiki/Fluctuation-dissipation_theorem

    What do you hope to accomplish with this thermoelectric current? There will be thermal noise in the voltage coming out of the generator. If the temperature of the molecules is greater than the ambient temperature outside the generator, some energy will leak from the molecules into whatever the generator is attached to. But if whatever the generator is attached to is higher temperature than the molecules, energy will leak the other way.
     
  4. Nov 20, 2014 #3
    I do not think that the size of molecules has some principal meaning if we want to talk about them in a terms of their temperature (their speed and energy). But I even appreciate to regard a case with really large molecules. Usually, it is assumed that somehow divison of molecules by their speed is sufficient for a Maxwell's demon to do a usefull work. Up to now most of objections against Maxwell's demons were related to a question on how much energy the demon will use for his own work and that he will spend more energy for his work that will be obtained from temperature differences he will create. Many scientists claim they already demonstrated possibility to convert information to energy with help of a demon however demon spend more energy than is gained as result of his work. I'm not sure that a thermal motion of the container is a principal obstacle, otherwise the concept of demon wouldn't have a sense at all. But what is exactly the difference if instead of sorting out many small molecules, demon will sort a two huge only, with the same mass as if it would be a many small molecules? There is no even need to sort a two molecules since they will constantly fly at the different ends of a container according to a pure probability. If a two molecules will have sufficient mass, speed and energy (and difference in energy between each other) the temperature difference they will create suppose to be sufficient to overcome and exceed the thermal motion in container wall or elsewhere in the system.
    [/PLAIN] [Broken]

     
    Last edited by a moderator: May 7, 2017
  5. Nov 20, 2014 #4

    Dale

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    A large molecule has many degrees of freedom. So there are many places for the thermal energy to go.
     
  6. Nov 20, 2014 #5

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    You've assumed the "hot" molecule always moves to the same end of your nanotube, and that's right back to Maxwell's Demon.
     
  7. Nov 20, 2014 #6
    And what? Where could it go?
     
  8. Nov 20, 2014 #7
    What did you want to tell by that?
     
  9. Nov 20, 2014 #8

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    You do not know which end of the tube is hot.
     
  10. Nov 20, 2014 #9
    Not necessary. First of all if a molecules really big we could monitor their position and speed and it will not take too much energy in comparison to the energy a generator will produce. Secondly, (though I'm not sure) we could use some type of AC generator for which there is no difference in which direction current will flow. A capacitor for example is non polar and could be charged by flow in any direction. We may use a device to charge a capacitors. Not a usefull work would you say? Also we may have a thermal detector which inform us which side is colder and which is hotter. We could connect wires from the container sides with help of some diodes. So when one end of container is hotter current flows. When it is colder current doesn't flow at all and doesn't bother us.
     
    Last edited: Nov 20, 2014
  11. Nov 20, 2014 #10

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    You might find it useful to compare your ideas to available information on "Johnson Noise" and "Boltzmann Distribution."
     
  12. Nov 21, 2014 #11
    I have a theoretical assumption:
    What if we have some hypothetical kind of matter (which is capable to create a structures)
    which doesn't have any degrees of freedom and therefore no thermal motion. Subsequently, it doesn't have temperature or its temperature is always equal to zero. In one word properties of such matter is similar to a field which neither have degrees of freedom and subsequently an infinite density. Many scientists believe that a field (for example a magnetic field) is also kind of matter. If we would have such type of hypothetical matter, would it be useful for a Maxwell's demon creation? If yes, does physics postulate absolute impossibility for a such matter to exist?
     
  13. Nov 21, 2014 #12

    Dale

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    So if I understand correctly you have a thermoelectric wire attached to a generator and you hope this will allow you to generate electrical work at equilibrium. But such a generator will consume work as often as it produces it since the voltage and current are uncorrelated.
     
  14. Nov 21, 2014 #13
    Why at equilibrium?? There is two huge molecules flying inside of container. They create huge thermal difference, as one of them is fast and the other is slow.
     
  15. Nov 21, 2014 #14

    Dale

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    You aren't doing anything to keep it in one side or the other, so even a big thermal fluctuation is still just a thermal fluctuation.
     
  16. Nov 21, 2014 #15
    Molecules constantly come in contact with container walls and exchange energy with them. As fast molecule will get to one end of the container it will very likely strike this end and heat it. Of course, you may not be able to predict which one end of the container the molecule will strike. However, how do you think AC current generator does work? For example take a look at a free piston engine:
    free-pistong-engine-image-01.jpg

    So, as you see energy is released each time at a different end of cylinder. Magnet is moved back and forth. Current flows each time in opposite direction. It does produce AC current. I do not claim it, but I guess a similar scheme could be used with help of thermoelectrics. Once a one end of the container becomes hotter current flows in one direction, once the opposite end gets hotter current reverses flow. As a result you may be able to obtain AC current with irregular voltage and frequency. It will look like a natural audio wave.
    Something similar to this:
    Audio_wave_by_DinkydauSet.png
     
  17. Nov 21, 2014 #16

    Nugatory

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    You've been trying to execute an end-run around the statistics in statistical mechanics by working with two very large molecules. The way you're thinking about it, you don't need the concept of temperature at all (and your understanding of temperature as the kinetic energy of the molecules doesn't apply in this case, so it's a good thing you don't need it).

    The system you've described so far (nanotube, two big molecules) is just a scaled-down version of a perfectly ordinary length of pipe with two perfectly ordinary balls bouncing around in it.
     
  18. Nov 21, 2014 #17
    I thought about it. I think that more likely there is some fundamental restriction on the size of molecules (or particles) which are capable to have a macroscopic thermal motion. If not, then we could have even two neutron stars (which are basically two giant atoms) flying inside some planetary size tube. However, you didn't do your consistent critics of the idea.
     
  19. Nov 21, 2014 #18

    Dale

    Staff: Mentor

    This is the key point. To produce a power the current and the voltage must be in the same direction, otherwise you the generator absorbs power rather than producing it. There is nothing in your scheme which makes the current and the voltage both go in the "power" direction at the same time. So your generator will be randomly absorbing mechanical energy as often as it is randomly producing it.
     
  20. Nov 21, 2014 #19
    So why do you believe that current and voltage will be not in the same direction? When you heat a one end of thermoelectric, voltage and current contradict each other? Why would they?
     
  21. Nov 21, 2014 #20

    Dale

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    Because they are both thermally fluctuating. Your setup doesn't change that in any way.
     
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