Exploring the Impossibility of a Perpetual Motion Machine

In summary, the demon forces particles through a hole in a barrier, which causes the gas to diffuse and generate energy. However, because the valve can't exist, the device would not work according to the laws of thermodynamics.
  • #1
FZ+
1,604
3
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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  • #2
Because this 'clever valve' can't exist.

- Warren
 
  • #3
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.
 
  • #4
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...
 
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  • #5
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.
 
  • #6
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 [Broken]
 
Last edited by a moderator:
  • #7
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
 

1. Can a perpetual motion machine really exist?

No, according to the laws of thermodynamics, it is impossible for a perpetual motion machine to exist. This is because energy cannot be created or destroyed, it can only be converted from one form to another.

2. Why do people still try to create perpetual motion machines?

Many people are intrigued by the idea of a machine that can run forever without any external energy source. Some may also see it as a way to achieve unlimited free energy. However, these attempts ignore the fundamental laws of physics.

3. What are the main challenges in creating a perpetual motion machine?

The main challenge is overcoming the law of conservation of energy. In order for a machine to continue moving without an external energy source, it would have to produce more energy than it consumes, which is impossible.

4. Are there any machines that can run forever?

No, there are no machines that can run forever without an external energy source. Even machines that use renewable energy sources, such as wind or solar power, will eventually wear down and require maintenance or replacement parts.

5. Could advancements in technology make perpetual motion machines possible in the future?

No, the laws of thermodynamics are fundamental principles of physics and are not subject to change. Therefore, it is highly unlikely that perpetual motion machines will ever be possible, no matter how advanced technology becomes.

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