Can a Perpetual Motion Machine Ever Be Realized?

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Discussion Overview

The discussion revolves around the concept of a perpetual motion machine, specifically focusing on a hypothetical device that utilizes a clever valve mechanism to generate energy from gas diffusion while seemingly decreasing entropy. Participants explore the implications of this idea in the context of thermodynamics, particularly the second law.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant describes a device with two compartments and a valve that allows gas particles to move in one direction, suggesting it could generate energy by collecting gas in one compartment.
  • Another participant argues that the proposed 'clever valve' is not feasible, implying that the mechanism cannot exist as described.
  • A different participant references Maxwell's demon, stating that the demon's acquisition of information about particle motion increases entropy, thus maintaining the second law of thermodynamics.
  • Another participant expresses curiosity about the interpretation of thermodynamic violations at small scales, suggesting that layman misconceptions may exist regarding energy extraction from such devices.
  • One participant mentions Maxwell's original thoughts on the subject, noting that he did not speculate on the entropy gained by the demon but emphasized that the second law applies to large numbers of particles.
  • Another participant introduces Feynman's simpler system involving a ratchet mechanism, explaining that thermal fluctuations would prevent the ratchet from functioning as intended, thereby upholding the second law.
  • A later reply references a specific publication that supports the discussion about the limitations of such systems.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of the proposed device and the implications of thermodynamic principles. There is no consensus on whether the device could work or on the interpretations of Maxwell's demon and related concepts.

Contextual Notes

Participants highlight limitations in the assumptions about the device's operation and the applicability of thermodynamic laws at small scales. The discussion includes references to specific theoretical frameworks and historical perspectives that may not be universally accepted.

FZ+
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I'm not sure if I got the name right, but anyways...

Suppose we have a nanoscopically small, but very clever device. (and with modern technology, this sort of thing is becoming almost practical...) It is basically just two compartments, with a hole in between. Contained in the compartments, we have an energetic gas. The hole is controlled by a very clever valve mechanism, that let's particles through in one way only. We let the device run, and hey presto, by simple random motion, all of the gas is gathered in one of the compartments. At the push of a button, the hole can be used to drive a turbine from the diffusion of the gas, thus generating energy - and decreasing entropy.

Obviously, such a device would break the laws of thermodynamics. So, why exactly would it not work?
 
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Because this 'clever valve' can't exist.

- Warren
 
The demon has to find out which corpuscles are moving fast and which are not. In gaining this information he increases his entropy, and voila! calculations show that the entropy the demon gains is equal to the entropy the gas system loses by his ministrations! No contradiction. This is a well-known result in modern thermodynamics.
 
Originally posted by FZ+
I'm not sure if I got the name right, but anyways...

Suppose we have a nanoscopically small, but very clever device. (and with modern technology, this sort of thing is becoming almost practical...) It is basically just two compartments, with a hole in between. Contained in the compartments, we have an energetic gas. The hole is controlled by a very clever valve mechanism, that let's particles through in one way only. We let the device run, and hey presto, by simple random motion, all of the gas is gathered in one of the compartments. At the push of a button, the hole can be used to drive a turbine from the diffusion of the gas, thus generating energy - and decreasing entropy.

Obviously, such a device would break the laws of thermodynamics. So, why exactly would it not work?
I have not been doing anything on Thermo for much time, but I will try anyway!...

If the system is isolated, the temperature of the gas will increase, and the pressure too... If the temperature increases, the entropy will do too... Let's imagine that the gas is an ideal one (it couldn't be *good* since the system is nanoscopical, but I think we can do it to see what happen). We will have
S2-S1=cLn(T2/T1)>0 because T2>T1, so no decreasing entropy...

I bet for it...
 
Last edited:
Originally posted by selfAdjoint
The demon has to find out which corpuscles are moving fast and which are not. In gaining this information he increases his entropy, and voila! calculations show that the entropy the demon gains is equal to the entropy the gas system loses by his ministrations! No contradiction. This is a well-known result in modern thermodynamics.
Fascinating !
So all this talk about thermodynamics being violated
on tiny scales in devices like the one FZ+ talks about
here is wrong/misinterpreted by the layman in terms of
being able to use that energy ?

Live long and prosper.
 
Here are Maxwell's own original
words in setting out this train
of thought:

James Clerk Maxwell
Address:http://webserver.lemoyne.edu/faculty/giunta/demon.html
 
Last edited by a moderator:
Interestingly, Maxwell made no speculations re the entropy gained by the demon, but only posited the argument that the 2nd law applies to large numbers of particles.

A much simpler system was proposed by Feynman, where the demon is essentially just a ratchet. A 'windmill' attached to the ratchet would allow for thermal fluctuations to apply forces of either direction to the windmill, but the ratchet would only allow motion in one direction, thus seeming to violate the 2nd law. The resolution is the fact that for the ratchet to be this sensitive, the pawl of the ratchet must be microscopic as well, and it would itself be subject to thermal fluctuations; essentially, it would bounce around, and the ratchet wouldn't work.

Recently, the above was put on a firm basis. See Phys.Rev.E 59, 6448 (1999). Link is

http://xxx.lanl.gov/abs/cond-mat/9902056
 

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