Superconducting ring moving in magnetic field.

Hello.

I got this question, it pursues me in my sleep lately.

What will happen with a superconducting ring, passed through magnetic field?
I mean like will the electrons even flow in it, taking that magnetic field is banned out of the material? And if they flow will the ring need extra force to move like ordinary conductor ring would do?

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I think a current flowing in the ring is necessary to make the total magnetic field inside the ring = 0. And a current placed in an external magnetic field does experiences force and torque.

Many years ago, I was inside a large electromagnet (an old cyclotron magnet, 4 meter diameter poletip, 75 cm gap), running at 1.5 Tesla. I was trying to pull a 60 cm long section of a 30 cm high aluminum I-beam into the center of the magnet. The eddy currents generated in the aluminum as I pulled it through the fringe field were counteracting the magnetic field penetrating the aluminum. After about 30 seconds (due to L/R time constants in the aluminum), the eddy currents decayed away and I was able to get the aluminum I-beam inside the magnet.

I believe the superconducting ring will behave the same way, but with an infinite decay time constant, because the currents induced in the ring will be persistent currents (like in MRI magnets). The induced currents and the force to pull a superconducting ring into the magnet will not decay away.

Bob S

Agree with Bob S.

The fact that it's a superconductor make no difference at all except that once a current begins to flow it will not stop.

In my previous post, i assumed that the superconducting ring's axis of rotation (AR) was aligned parallel with the magnetic field. The AR could also be perpendicular to the magnetic field, in which case it could be pulled into the magnet with little effort. But then when the superconducting ring is in a uniform magnetic field, a torque would be required to rotate the AR to be parallel to the magnetic field. Because the induced currents in the ring are persistent, the ring will always want to rotate back so the AR is perpendicular to the magnetic field.

I had a similar situation with an aluminum plate in a 1 kilogauss magnetic field in a ferrite magnet oscillating at 500 Hz. The aluminum plate wanted to rotate such that the normal to the plate was perpendicular to the oscillating magnetic field. If I held it such that the normal to the plate was parallel to the field, it got hot due to eddy currents.

Bob S

Yes sorry i forgot to say that ring's axis of rotation is parallel with magnetic field lines. I did some wiki on the subject and the picture looks more clear to me now but i just can't get my hand on how the current is induced in the ring. How I see it, magnetic field is cutting through the conductor and current is induced however superconductor excludes magnetic field so there is nothing left for current to be induced or am I wrong ??

There is never any magnetic flux inside the superconducting ring. The current induced in the ring exactly cancels the flux from the magnet (Lenz's Law). Think of the current as an eddy current induced by Faraday's Law, but with an infinitely long decay time constant.

Bob S

Magnetic flux is repelled from the interior of a type II semiconductor, but the field inside a superconducting loop will remain unchanged from the time it becomes superconductive. The amount of magnetic flux is quantized in units Phi0=h/2qe. This includes zero flux.