Superconducting induction coil

[vin300]

Looked at Maxwell's eqns after quite some time,and was wondering, what happens if we apply an alternating flux to a superconducting coil? An emf should be produced, but there's actually no emf in a superconductor!

 

[f95toli]

Well, a superconducting coil behaves more or less just like an ideal lossless coil. If the coil is part of a closed circuit you will induce a current.
Although -depending on the exact layout= flux quantization might also be important which course wouldn't be the case for a normal circuit.

 

[f95toli]

double post...

 

[Bob S]

See Meissner Effect in
http://en.wikipedia.org/wiki/Meissner_effect
Read about Type I and Type II superconductors in
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html
and
http://www.superconductors.org/
During ramp-up of magnetic fields in magnets with superconducting coils, eddy currents induced by the emf within the multi-wire superconducting cable (often carrying over 10,000 amps) is a serious problem, and special coatings on the individual wires have been developed to minimize them.
Bob S

 

[GRDixon]

Looked at Maxwell's eqns after quite some time,and was wondering, what happens if we apply an alternating flux to a superconducting coil? An emf should be produced, but there's actually no emf in a superconductor!

Actually, there is. It's what engenders the current in the coil. Note, however, that it is not what maintains the current, once the current has been established. A nonzero emf is necessary to maintain an established current (against ohmic resistance) only in the case of non-superconducting coils. Owing to the low mass of conduction electrons, only a very brief time-varying flux is normally required to engender large currents in superconducting coils. And in the case of a superconducting coil, these currents will persist indefinitely after the flux has stopped varying in time. One of my favorite demonstrations is where a superconducting washer is dropped down on a bar magnet. Current is engendered in the washer as the magnetic flux through it varies in time. The direction of the current is such that its own magnetic field opposes the external field that set it up (Lenz's Law). Once the current has become great enough, the washer stops descending and "floats" indefinitely, part way down the bar magnet, seemingly defying gravity.