1
$\begingroup$

Once an electric current is established, and then the external electric field/voltage is withdrawn, a superconductor maintains the current forever.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?

$\endgroup$
  • $\begingroup$ 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. $\endgroup$ – KalleMP Sep 19 '18 at 9:26
1
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.