# I Why does a magnet above a superconductor stop spinning?

#### Idunno

Summary
Why does a magnet above a superconductor stop spinning?
I got the opportunity a while ago to play around with a magnet floating above a superconductor cooled with liquid Nitrogen. The magnet was a little cube, and I spun it with a pair of tweezers. I was surprised at how quickly it stopped spinning, it seemed to stop much quicker than could be accounted for by air friction.

So what stops it from spinning? Does a cylindrical magnet spin for longer? Can you make a perfect frictionless bearing with this setup, or not?

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#### A.T.

So what stops it from spinning?
Do you understand what stops it from falling?

#### jedishrfu

Mentor
Here's a video showing the effect:

Perhaps the interaction of its magnetic field with the superconductor provides a drag that ultimately slows it down.

#### Idunno

Do you understand what stops it from falling?
The superconductor does not allow a magnetic field to penetrate it, so charges in the superconductor circulate to create a magnetic field that is a sort of mirror image (is "mirror" the best word?) of the magnetic field from the magnet. The result is a magnetic field that allows the magnet to float stably.

My guess is that eddy currents are induced in the conducting material of the magnet, and this induces the drag. But I don't know, so that's why I'm asking.

#### Idunno

Here's a video showing the effect:

Perhaps the interaction of its magnetic field with the superconductor provides a drag that ultimately slows it down.
Thank you for that, obviously a cylindrical magnet does much better than a cubical magnet at acting as a frictionless bearing. So what's up with the cube?

#### Dale

Mentor
Are you sure it is not just air resistance? At least for the cylindrical magnet I didn’t see anything happening.

#### anorlunda

Mentor
Gold Member
My guess is that eddy currents are induced in the conducting material of the magnet, and this induces the drag.
That sounds plausible, but it is difficult to say with certainty that it is correct. You would need to model it with Maxwell's Equations.

#### Idunno

Are you sure it is not just air resistance? At least for the cylindrical magnet I didn’t see anything happening.
Oh yes, it slows down quickly. In trying to find an answer before posting here, I was all over the web, and I saw a similar question in the website Hyperphsyics here, but it wasn't answered.

#### f95toli

Gold Member
I am pretty sure it is because of the "friction" from vortex motion in the superconductor.

These kits are all based on high-Tc superconductors (usually YBCO) which are type II meaning they support vortices.
Now, you might have noticed that the magnet is only stable (doesn't "fall off) if the superconductor is cooled with the magnet already in place (typically some sort of spacer is used which is removed once the superconductor is cold). The reason for this is that cooling the superconductor in field creates vortices through the superconductor in some specific pattern. The vortices "hold" the field lines from the magnet (so to speak) and "freezes" the magnet into a some specific configuration of the magnetic field; thereby stabilizing its position.

This is the reason why if you remove the magnet and put it back again it will (usually) end up in the same position.

Now, the vortices will stabilise the magnet; but it also means that there will be some resistance if you try the spin the magnet in a way that in in any way changes the field distribution. In fact, if you try to force the magnet into a new configuration it feels like dragging it through thick jam (best analogy I can think of).

Hence, if you want the magnet to spin for a long time you need to spin it in such a way that the "symmetry" -and therefore the field distribution- does not change in any way.
This is indeed how superconducting bearings (as used in e.g. flywheels) are designed. Doing this by hand is obviously very tricky.

#### Idunno

I am pretty sure it is because of the "friction" from vortex motion in the superconductor.

These kits are all based on high-Tc superconductors (usually YBCO) which are type II meaning they support vortices.
Now, you might have noticed that the magnet is only stable (doesn't "fall off) if the superconductor is cooled with the magnet already in place (typically some sort of spacer is used which is removed once the superconductor is cold). The reason for this is that cooling the superconductor in field creates vortices through the superconductor in some specific pattern. The vortices "hold" the field lines from the magnet (so to speak) and "freezes" the magnet into a some specific configuration of the magnetic field; thereby stabilizing its position.

This is the reason why if you remove the magnet and put it back again it will (usually) end up in the same position.

Now, the vortices will stabilise the magnet; but it also means that there will be some resistance if you try the spin the magnet in a way that in in any way changes the field distribution. In fact, if you try to force the magnet into a new configuration it feels like dragging it through thick jam (best analogy I can think of).
Ok, so it takes energy to move the vorticies? Does that mean there is resistance of a sort in the superconductor? You'd think there would be no resistance to moving the charge around, but of course, I don't know.

What about eddy currents in the conductive material of the magnet?

"Why does a magnet above a superconductor stop spinning?"

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