Moreno Posted June 20, 2016 Posted June 20, 2016 If we would live in a Universe where absolute zero is achievable in principle, how would perpetuum mobile that exploits this effect practically look like? Just curious. Which components would it require and how would it function?
swansont Posted June 20, 2016 Posted June 20, 2016 Here's the thing: physics is inter-related in many ways. So it's really hard to make any conclusive statements about how things would occur if you decide to break one of the laws. A perpetual motion device, for example, breaks the second law of thermodynamics, but what happens to the second law of the third law goes away? Somewhere there is probably an example of a conflict that arises.
Moreno Posted June 20, 2016 Author Posted June 20, 2016 Here's the thing: physics is inter-related in many ways. So it's really hard to make any conclusive statements about how things would occur if you decide to break one of the laws. A perpetual motion device, for example, breaks the second law of thermodynamics, but what happens to the second law of the third law goes away? Somewhere there is probably an example of a conflict that arises. This is what I would be glad to know as well. If perpetuum mobile of the "third kind" will lead to the second law violation and why.
swansont Posted June 20, 2016 Posted June 20, 2016 This is what I would be glad to know as well. If perpetuum mobile of the "third kind" will lead to the second law violation and why. Why would the third law apply to this at all? Perpetual motion of the third kind is a frictionless system; it has nothing to (directly) do with the third law of thermodynamics.
Moreno Posted June 20, 2016 Author Posted June 20, 2016 Why would the third law apply to this at all? Perpetual motion of the third kind is a frictionless system; it has nothing to (directly) do with the third law of thermodynamics. Frictionless system which is capable to produce useful work indefinitely? Could you tell more on that?
swansont Posted June 20, 2016 Posted June 20, 2016 Frictionless system which is capable to produce useful work indefinitely? Could you tell more on that? That would be a machine of the first kind as well. What's to tell? They're impossible.
Moreno Posted June 26, 2016 Author Posted June 26, 2016 (edited) Usually, under "perpetuum mobile of the second kind" they assume a device which is capable to reduce total entropy of the universe. But what if total entropy stays unchanged and just cyclically increased/reduced in different parts of a system? Would it work like a perpetuum mobile (capable produce useful work) and will it violate the second law of TD? Edited June 26, 2016 by Moreno
swansont Posted June 26, 2016 Posted June 26, 2016 Usually, under "perpetuum mobile of the second kind" they assume a device which is capable to reduce total entropy of the universe. But what if total entropy stays unchanged and just cyclically increased/reduced in different parts of a system? Would it work like a perpetuum mobile (capable produce useful work) and will it violate the second law of TD? One that's capable of producing work is a machine of the first kind. If you had one that didn't increase entropy it would be a machine of the second kind. A mechanical system like that would violate the second law. https://en.wikipedia.org/wiki/Perpetual_motion#Classification
Moreno Posted June 27, 2016 Author Posted June 27, 2016 One that's capable of producing work is a machine of the first kind. If you had one that didn't increase entropy it would be a machine of the second kind. A mechanical system like that would violate the second law. https://en.wikipedia.org/wiki/Perpetual_motion#Classification 1) Machine of the second kind is NOT capable to produce work? 2) "The second law of thermodynamics states that the total entropy of an isolated system always increases over time, or remains constant in ideal cases where the system is in a steady state or undergoing a reversible process. https://en.wikipedia.org/wiki/Second_law_of_thermodynamics So, if entropy doesn't increase it doesn't necessarily violates the second law, but could be just a perfectly reversible engine?
swansont Posted June 27, 2016 Posted June 27, 2016 1) Machine of the second kind is NOT capable to produce work? 2) "The second law of thermodynamics states that the total entropy of an isolated system always increases over time, or remains constant in ideal cases where the system is in a steady state or undergoing a reversible process. https://en.wikipedia.org/wiki/Second_law_of_thermodynamics So, if entropy doesn't increase it doesn't necessarily violates the second law, but could be just a perfectly reversible engine? There are no real reversible processes. Another take on this is converting heat to work without rejected heat. Doesn't create energy, but violates the second law.
Moreno Posted June 27, 2016 Author Posted June 27, 2016 There are no real reversible processes. Why? It wouldn't violate a fundamental laws of TD?
swansont Posted June 27, 2016 Posted June 27, 2016 Why? It wouldn't violate a fundamental laws of TD? There are no real reversible processes because you can't have a real process that doesn't increase net entropy. Once you have more than two particles, or incorporate internal states, you lose the time-reversal symmetry that a reversible process requires. Reversible processes get categorized in with frictionless surfaces and elephants whose mass may be ignored. Used in idealized cases to simplify analysis, but not something you find in the real world.
Moreno Posted June 27, 2016 Author Posted June 27, 2016 What do you think about the following article: http://www.nature.com/articles/srep06208 They claim a frictioneless engine could be created. Not a real life engine again?
swansont Posted June 28, 2016 Posted June 28, 2016 What do you think about the following article: http://www.nature.com/articles/srep06208 They claim a frictioneless engine could be created. Not a real life engine again? Have they built one? "We have demonstrated the possibility to perform a fully frictionless quantum cycle in a finite-time." implies that no, they did not. And it's a quantum system, not a classical one. The application of the rules is a little different, but there are still idealizations in what they propose. All they've done is kick the can down the road. They disconnect and reconnect with the reservoir but appear to ignore any implications of doing that (from a quick skim of the article)
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