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Probabilistic particle-scale valve with no moving parts

  1. Dec 26, 2014 #1
    Hi,

    Smoluchowski's trap door was proven to fail to obtaining work in a system without heat difference. The problem turns is random movement of trap door due to random particle movements. However I could not find any discussions of such system where the "trap door" did not involve moving parts. I am wandering if it would be possible to construct a specially shaped hole (with no moving parts) in a wall separating two chambers of gas filled box, such that the whole would have a higher likelihood of allowing gas molecules to pass from one way than the other and not involve any moving parts. Intuition suggests that one could shape the hole in such a way so that molecules would be more likely to bounce away / towards the hole e. g. one side being convex and the other side concave.

    However existence of such hole should be impossible as it would lead to violation of second law of thermodynamics and extraction of work from system with single heat reservoir. I would appreciate if someone would be kind enough to pinpoint the flaw the above reasoning.

    thank you

    Filip
     
  2. jcsd
  3. Dec 26, 2014 #2

    Bystander

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    Molecular sieves, zeolites, custom fullerene tubes, all with atomic and molecular scale pores? Oh, the possibilities for molecular "wasp traps!" "Not." Unfortunately (probably good fortune for all of us in terms of being the way the universe works) there does not appear to be any such thing as a nice clean "single bank off the side cushion" billiard shot collision of vapor molecules with "solid" surfaces. It's "hit and stick" on clean surfaces (which have only been prepared by cleaving in high vacuum); once a surface is in equilibrium with the vapor phase surrounding it, surface sites and adsorbed gas molecules play "hot potato" with the kinetic energy of the molecule colliding with the surface and eject another, previously adsorbed molecule from an adjacent or nearby site with the same (sometimes more, sometimes less) energy (Boltzmann runs that roulette wheel) according to the cosine(?) law for scattering.

    A non-sticky cast neutronium wasp trap? Wasp and minnow traps depend not on the ratio of the sizes of ends of conic sections, but on the asymmetry and tropisms of the animals being trapped. Molecules in field free space are not going to be travelling with specific orientations necessary to make such a trap work.
     
  4. Dec 26, 2014 #3

    Vanadium 50

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    No. Such a hole will do work on the gas without an energy source, so you have gone from violating the 2nd law of thermodynamics to violating the 1st.
     
  5. Dec 26, 2014 #4

    cjl

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    Interestingly, there are some specific cases when gas molecules do bounce off surfaces in a method more akin to a billiard ball rather than the stick and scatter you mention here. It's not common, but in a fairly high vacuum with a correctly prepared surface, specular (rather than diffuse) reflection is possible. Of course, for this to happen, the surface must be incredibly clean, the molecular weight of the gas must be much smaller than the molecular weight of the surface material, and the gas molecules must be moving quickly (usually several km/s).
     
  6. Dec 26, 2014 #5

    Bystander

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    That had me digging in the library for days when I was getting up to speed on spinning rotor gauges.
     
  7. Dec 27, 2014 #6

    Dale

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    Filip, as I told you previously, we do not discuss perpetual motion machines here on PF, not even to debunk them.

    Regarding your specific PMM proposal, even if you could set up the ideal perfect "beveled hole" you would need to analyze not only the size of the holes, but also the angle of incidence on the wall of the hole.
     
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