Electric field produced by a magnet

arul_k

Hello, I was just wondering how is it that a steady magnetic flux (stationary magnetic field) is capable of inducing a current in super conductor, as in the Meissner effect, where as a changing magnetic flux (moving field) is required in the case of a normal conductor.

sophiecentaur

The induction occurs when the conductor is initially brought towards the magnet (or if the field is 'switched on'). With a superconductor, the induced current just does not die away.

arul_k

Thanks sophiecentaur for your reply. I am aware that a varying magnetic field is required to induce a current in a conductor, however in the case of super conductors this dosen't seem to be so.
For example in some of the videos that demonstrate the Miessner effect a permanent magnet is placed on top of the super conducting surface, there is no relative motion or changing flux between the magnet and the superconductor, yet when the superconductor is cooled down, a current is induced in it, which as you have stated does not die away.

My question is how can a non varying magnetic field induce a current in the super conductor?

sophiecentaur

There was a varying field to start with - when the magnet was brought into position or the current was turned on with the electromagnet. Once the current was established (due to this field being introduced) it carried on. (In fact this is what I have already said - and that is your reason.) Energy was put in and it stays there.

arul_k

I need to clarify the point about "a varying field to start with"
Suppose I place the permanent magnet on the superconducting material, at say 8am, at this stage the superconducting material hasn't been cooled down so it behaves as a normal conductor, and the currents induced in it by placing the magnet on it will dissapate once the magnet is stationary wrt the material.

After this point there is no varying magnetic field as both magnet and superconductor are stationary. 4 hours later at noon I cool the super conducting material down and now, as it behaves as a superconductor, currents are induced in it which then levitate the magnet.

so how is it that the current is induced in the super conductor after a gap 4 hours between the varying magnetic field and the cooling of the superconductor?

K^2

I'm not sure if this actually takes care of it, because the sign might be wrong. But when your superconductor transitions, there is a change in magnetic field. There is a field inside superconductor before transition, and none after. That will induce a supercurrent.

That ought to be the current that produces external magnetic field responsible for magnet's levitation, but I'm having hard time checking the sign in my head, and I'm too lazy to do this properly on the paper.

sophiecentaur

I think you have to approach it believing in Energy conservation etc. and find a reason for it to be right rather than wrong.
It's quite reasonable to expect any existing field around a conductor to affect the energy needed to cool it past the critical temperature. The "sign" has just got to be right.

arul_k

So if I understand you correctly it is the change in the superconductors own magnetic field that produces the current and not the external magnetic field? That would mean that a supercurrent is produced irrespective of the exernal magnetic field.