Once an electric current is established, and then the external electric field/voltage is withdrawn, a superconductor maintains the current forever. Such a process neither violates the first law nor the second law of thermodynamics and therefore, not a perpetual motion machine of any kind. Is this process reversible? If yes, how do we reverse it and doing so why would there be no net change in the entropy of the universe?
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asked Sep 19, 2018 at 5:35
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$\begingroup$ There is no violation as you are not removing any stored energy while it is just sitting there so there is no perpetual motion against friction which would violate the laws. $\endgroup$– KalleMPCommented Sep 19, 2018 at 9:26
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$\begingroup$ @KalleMP Aren't the electrons perpetually moving? $\endgroup$– Bob DCommented Dec 9, 2019 at 20:52
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$\begingroup$ I don't understand what process you a referring to. The act of cooling down a superconducting ring is not reversible (there's heat transfer). If you just talking about the resulting state, well, that's not a process either, it's a state. $\endgroup$– psitaeCommented Dec 11, 2019 at 1:45
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$\begingroup$ If you added a bounty, you should specify what's insufficient about the existing answer -- it seems to completely answer the question to me. $\endgroup$– knzhouCommented Dec 13, 2019 at 3:29
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If you spin an object in outer space it will continue spinning 'forever'. Spinning up an object in outer space is a reversible process - presumably, you could attach the motor/generator and run the process in reverse, turning rotation into electricity. Of course your motor is going to have some resistance and friction so some energy will be lost.
The same is true with superconductors. You can 'spin up' the supercurrent and then spin it down and you get (theoretically) the same energy out as you put in. This is the basis for superconducting energy storage. As with the spinning object, the conversion mechanism isn't going to be perfect so some energy will be lost.
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$\begingroup$ "If you spin an object in outer space it will continue spinning 'forever'". Even outer space has some particles which would present a tiny amount friction to the object which, over a sufficiently long, will slow the object down. I think it would be better to say "it will continue spinning almost forever". $\endgroup$– Bob DCommented Dec 9, 2019 at 20:58
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$\begingroup$ @BobD a supercurrent is expected to also relax, via phase slips, over some time that grows exponentially with the system size. Neither of these complications are really relevant to the question, I believe. $\endgroup$– RococoCommented Dec 14, 2019 at 16:50
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$\begingroup$ @Rococo I'm not so sure it isn't relevant. In thermodynamics a reversible process is one in which there are no dissipative forces. Perhaps its defined differently in electrical. $\endgroup$– Bob DCommented Dec 14, 2019 at 16:53
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$\begingroup$ @BobD There is no such thing as a perfectly reversible process. Neither superconductors nor spinning objects are truly reversible. This is why I put 'forever' in quotes. $\endgroup$– A NejatiCommented Dec 15, 2019 at 7:13
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