Perpetuum mobile

[Moreno]

Could somebody explain in detail why is it practically impossible (as I suspect) to create an engine which works without change of the total entropy of the Universe? Would it be a perpetuum mobile if it ever existed? Could such engine exist if ideally reversible process would be achievable in principle? Would it violate any Thermodynamics Laws? 

Edited July 19, 2019 by Moreno

[koti]

23 minutes ago, Moreno said:

Could somebody explain in detail why is it practically impossible (as I suspect) to create an engine which works without change of the total entropy of the Universe? Would it be a perpetuum mobile if it ever existed? Could such engine exist if ideally reversible process would be achievable in principle? Would it violate any Thermodynamics Laws? 

Conservation of Energy prevents any engine from creating more energy (or keep it equal) than its put into it. Energy put into any engine will end up in work done with heat and friction produced on the side. Isolating a system like this from total Universe entropy would mean you would have to take the engine out of this Universe which is kind of ridiculous.

[Moreno]

I didn't claim it needs to be completely isolated from the Universe. I'm not sure it is one of the obstacles.

[koti]

2 hours ago, Moreno said:

I didn't claim it needs to be completely isolated from the Universe. I'm not sure it is one of the obstacles.

I know you didn’t claim that. I’m just saying that anything in our Universe needs to obey energy conservation which prevents perpetual devices.

[Moreno]

Only perpetuum mobile of the first kind violates the energy conservation law. Perpetuum mobile of the second kind doesn't and therefore requires different arguments to disprove it. However perpetuum mobile of the second kind reduces the total entropy of the Universe (what is statistically improbable) and therefore violates the second law of thermodynamics. But the hypothetical engine that I've proposed for discussion doesn't reduce the total entropy of the universe. It leaves it unchanged, what makes it formally different from perpetuum mobile of the second kind. Therefore it may require completely new arguments altogether. 

Edited July 19, 2019 by Moreno

[swansont]

Reversible processes and the second law are closely intertwined. You must have reversible processes to keep entropy constant, and they don't exist (for any system where you would be applying thermodynamics, at least). Thus, with entropy increasing, perpetual motion is impossible.

[Moreno]

 

1 hour ago, swansont said:

Reversible processes and the second law are closely intertwined. You must have reversible processes to keep entropy constant, and they don't exist (for any system where you would be applying thermodynamics, at least). 

Can you explain in detail why exactly? 

Edited July 19, 2019 by Moreno

[swansont]

 

1 hour ago, Moreno said:

Can you explain in detail why exactly? 

Why what?

reversible means dS = 0

https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node48.html

https://en.wikipedia.org/wiki/Second_law_of_thermodynamics#Derivation_of_the_entropy_change_for_reversible_processes

 

 

 

[Moreno]

8 hours ago, swansont said:

Why what?

Why there is no reversible process in the nature? At any level? What about quantum level?

So, if reversible processes would exist in the nature and practically achievable in a reasonable time intervals, then we would be able to create a perpetuum mobile which is capable to produce some sizable useful work? Or...not?

Edited July 19, 2019 by Moreno

[swansont]

1 hour ago, Moreno said:

Why there is no reversible process in the nature? At any level? What about quantum level?

So, if reversible processes would exist in the nature and practically achievable in a reasonable time intervals, then we would be able to create a perpetuum mobile which is capable to produce some sizable useful work? Or...not?

Because you can’t actually be in equilibrium during the process and have it take a finite time 

[Moreno]

23 minutes ago, swansont said:

Because you can’t actually be in equilibrium during the process and have it take a finite time 

More info on that?

[swansont]

20 hours ago, Moreno said:

More info on that?

It needs to be in equilibrium. This requires infinitesimal changes in whatever property, meaning an infinite number of steps. They each take a finite amount of time.

[Moreno]

7 hours ago, swansont said:

It needs to be in equilibrium. This requires infinitesimal changes in whatever property, meaning an infinite number of steps. They each take a finite amount of time.

But a hypothetical perpetuum mobile of the second kind (let say Maxwell's demon) doesn't have this problem? Or at least it is not one of the objections against it? How come? If Maxwell theorized about "demon" which is "capable" to reduce the total entropy of the closed system, could we theorize about "Maxwell's demon of the second kind" which makes system run keeping its entropy unchanged?

Edited July 21, 2019 by Moreno

[Bufofrog]

12 hours ago, Moreno said:

But a hypothetical perpetuum mobile of the second kind (let say Maxwell's demon) doesn't have this problem? Or at least it is not one of the objections against it? How come? If Maxwell theorized about "demon" which is "capable" to reduce the total entropy of the closed system, could we theorize about "Maxwell's demon of the second kind" which makes system run keeping its entropy unchanged?

 OK.  With the help of a demon you can have perpetual motion.  But this is kind of cheating, plus you would have go to hell after the experiment.

[swansont]

12 hours ago, Moreno said:

But a hypothetical perpetuum mobile of the second kind (let say Maxwell's demon) doesn't have this problem? Or at least it is not one of the objections against it? How come? If Maxwell theorized about "demon" which is "capable" to reduce the total entropy of the closed system, could we theorize about "Maxwell's demon of the second kind" which makes system run keeping its entropy unchanged?

Maxwell’s demon doesn’t actually exist, because it’s impossible.