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My son asked me a question couple of days ago: is it possible to create a filter which would let some chemical substance only one way?

It took me several seconds to think of the answer. Would such filter be possible I would take a container, fill it with the chemical substance, divide the container with filter. Some time later all the substance (suppose it's a gas) would accumulate on one half of the container. Entropy decreased, and it looks like a prototype for perpetual machine of the second kind.

I also recalled wonderful explanation by Richard Feynman why a ratchet would not work as expected if you try create a perpetual machine and I was about to start answering the question and explaining how this is absolutely impossible because of the very principal laws of nature.

But at that moment my son continued his question: well, you know, to make something like a diode?

This question I can't answer.

Why is the situation with diode is different that situation with filters and gases? I guess it IS possible to create something like a diode for gases, and it would not violate the second law of thermodynamics, but what this "something" would look like?

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    $\begingroup$ A reverse osmosis membrane lets water through, but blocks salt. $\endgroup$ Commented Sep 21, 2018 at 18:55
  • $\begingroup$ @DavidWhite Well, but the question was if it is possible to create a membrane which would let salt one direction, but block the same salt to pass in the opposite direction. $\endgroup$
    – lesnik
    Commented Sep 21, 2018 at 18:58
  • $\begingroup$ @lesnik - Sounds like a type of Maxwell's Demon for reducing the entropy of a system. Don't remember the detailed arguments against the existence of such "Demons" or mechanisms, but I'm pretty sure that the idea has been thoroughly studied, and the possibility of such Demons existing has been debunked and that the 2nd law of thermodynamics is still standing strong. $\endgroup$
    – user93237
    Commented Sep 21, 2018 at 19:24
  • $\begingroup$ @SamuelWeir I totally agree with you. I do not doubt Maxwell's Demon is impossible. The question is: how come one-way filters are impossible, but diodes are possible and work perfectly? What's the fundamental difference between one-way filter and dione? $\endgroup$
    – lesnik
    Commented Sep 21, 2018 at 19:51
  • $\begingroup$ @lesnik - Unlike a Maxwell's Demon or your hypothetical one-way chemical filter, diodes can't reduce the entropy of an isolated system and violate the 2nd law of thermodynamics. What would violate the 2nd law is if there were some electronic valve which monitors individual electrons approaching it and lets the electrons pass through it in one direction but not the other. Similarly, a Maxwell's Demon that lets approaching gas molecules pass through its gate in only one direction is a 2nd law violation, but a 1-way gas check valve (en.wikipedia.org/wiki/Check_valve) is not. $\endgroup$
    – user93237
    Commented Sep 21, 2018 at 20:07

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