# Levitating magnet and rotating superconductor,

by labview1958
Tags: levitating, magnet, rotating, superconductor
 P: 37 Look at this figure. What will happen to the repulsive force between magnet and superconductor?Would the repulsive force increase with rotation?
 P: 728 The picture's way too small. I cannot make out any of the words.
 P: 555 Levitating magnet and rotating superconductor, i just see a disc and some gray lines around it.
 P: 37 The tiny magnet is on a scale. The SC is rotating above. Normally there would be a repulsive force between a non-rotaing SC disk and the tiny magnet. What happens if we put the SC into rotation? Does the repulsive force increases, decreases or remain the same?
 Emeritus Sci Advisor PF Gold P: 29,238 I am guessing that this is the typical demonstration that most of us have either seen, or even demostrated ourselves, of magnet levitation on a superconductor due to the Meisner effect. The key thing to keep in mind here is that the scales of such a demonstration, be it the physical size and the magnetic field strength, are relatively small. So what you have here is that all the magnetic fields from the magnet is sufficiently expelled by the superconductor, creating a levitating effect. Under such circumstances, the rotation of the superconductor doesn't do anything special because the fields from the magnet and the superconductor do not intertwine with each other. In fact, if you have seen such a demonstration, it is often that either the superconductor has some spin to it, or the demonstrator him/herself will impart a spin to it just to show that it is truly levitating. I certainly have done that myself. Now, if you make this more complicated (which as physicists, we always want to do since we always ask "what if...?"), if the superconductor is heavier and require a stronger magnetic field, then what I have just said above may no longer be true. In most of these demonstrations, one tends to use a high-Tc superconductor, usually the one that has the acronym YBCO. This is because one can easily make this compound superconducting using just liquid nitrogen. But YBCO is a Type II superconductor. It means that, depending on the strength of the external magnetic field, it can still have a bulk superconducting effect but still lets some magnetic flux lines passing through it. So, if the magnetic field is sufficiently strong enough, you will have the situation where not all the field is expelled by the superconductor, but rather a few "stray" ones will actually penetrate the superconductor. When this occurs, then the superconductor will not be as free to spin, twist, shake, shiver, boogie, etc. as it can in the earlier case. The magnetic field lines penetrating the superconductor do not like to be twisted and bent. I believe this principle is being used in some magnetic levitation application to stablize the levitated object. Zz.
P: 555
 Quote by labview1958 The tiny magnet is on a scale. The SC is rotating above. Normally there would be a repulsive force between a non-rotaing SC disk and the tiny magnet. What happens if we put the SC into rotation? Does the repulsive force increases, decreases or remain the same?
Thanks for explaning the tiny unresolvable diagram, labview.

Normally, as you realize, due to Meissner, when the superconductor is supported, the magnet can be made 'float' above the superconductor.

However, a fact not realized by many, except those who have experimented considerably with such, is a fact that is pertainent to the resolution of your question.

First, the superconductor can be freely suspended BELOW a fixed magnet provide it is not too weighty compared to the strength of the magnet.

This is possible provided you first place the magnet in contact with the superconductor beneath it BEFORE lowering the SC temp. below T(c). THis allows for the magnetic flux to penetrate the SC, and then when it goes superconducting the flux lines will become 'trapped' within the SC and will remain frozen in position. Amazingly these trapped flux lines in the SC will interact with attractive force and will tend to stabilize the SC against any movement toward the ground due to its weight, suspending the SC below the magnet as long as it remains below T(c).

Moreover, in such a case, the interaction of the magnet's non-linear field and the SC is such that there is a flux variation which can alternate from repulsive to attractive with position.

Having said that, if we reverse the position of magnet & SC to conform to your diagram, with magnet arranged below the fixed SC--(forget the rotation & the scale for a moment), then the magnet 'should' also stay suspended below the SC .
However, the experiment is rarely set up that way (with the SC fixed on top), and the reason I personally have never done it that way, is because of the inherent difficulty in keeping the SC below T(c) due to the awkwardness of keeping it in cryogenic fluid without a contaner below in which it can reside.

So , experimentally, hopefully you can see how pinned flux lines providing an attractive force can alter any preconceived (mis)conception that a 'scale' will somehow reliably measure the force that may arise strictly as a result of rotation.

Furthemore, frozen flux lines cannot be safely excluded simpy by dropping the temperature below T(c) in advance of bringing the magnet near because the magnetic field lines can still penetrate the SC to some degree if the magnetic field exceeds a certain critical value.

In general, then, even though rotational effect is an interesting question, being fully aware of other aspects of SC's is necessary to guide experimental set-ups that will preclude misinterpretation of data.

Creator
Emeritus
PF Gold
P: 29,238
 Quote by Creator Thanks for explaning the tiny unresolvable diagram, labview. Normally, as you realize, due to Meissner, when the superconductor is supported, the magnet can be made 'float' above the superconductor. However, a fact not realized by many, except those who have experimented considerably with such, is a fact that is pertainent to the resolution of your question. First, the superconductor can be freely suspended BELOW a fixed magnet provide it is not too weighty compared to the strength of the magnet. This is possible provided you first place the magnet in contact with the superconductor beneath it BEFORE lowering the SC temp. below T(c). THis allows for the magnetic flux to penetrate the SC, and then when it goes superconducting the flux lines will become 'trapped' within the SC and will remain frozen in position. Amazingly these trapped flux lines in the SC will interact with attractive force and will tend to stabilize the SC against any movement toward the ground due to its weight, suspending the SC below the magnet as long as it remains below T(c).
This is incorrect.

First of all, if the superconductor is a Type I, you do not have any flux lines penetrating the superconductor. If you claim it does, you will have to rewrite the whole theory of superconductivity.

Secondly, what you have described is a property of a diamagnet. A superconductor is a SPECIAL CASE where even when field-cooled, the magnetic flux will STILL be expelled. That is what makes the Meissner effect different than a perfect diamagnet. There is no "attractive force" between a magnet and a superconductor even for a Type II superconductor in the vortex state.

Zz.
P: 555
I was under the impression we are talking type II here; zapper.- I hope that is clear from the context - of the usually demonstrations showing the Miessner effect. Maybe I should have made it clear.

There are conditions in which flux is NOT fully expelled.
I thought my post was clear and I gave clearly the experimental conditions in which flux is not fully expelled, at least in a high temp. SC. I'm surprised you have a problem with this, Zapper. This is pretty elementary.

 There is no "attractive force" between a magnet and a superconductor even for a Type II superconductor in the vortex state.
.

Call it what you please but it is a simple experimental fact that a type II superconductor can be suspended from BENEATH a magnet in a gravitational field and it is typically thought to be a result of pinned flux in the SC.
Whether you want to call that 'attractive' or not is up to you. Its a flux pinning effect.

As an examle see here:
Simple demo: http://www.users.qwest.net/~csconduc...n%20Effect.htm

more:
http://www.iop.org/EJ/abstract/0953-2048/15/12/321
 P: 37 Thanks zapper & creator. It is a type 2 SC. The magnet is only placed on the scale once the SC is superconducting. The SC is placed in a container of LN2 which is held by a retort stand clamp. Thus the flux is not frozen. The SC is attached to a drill and put into rotation. Now does the repulsive force increases, decreases or remains the same? Normally if we rotate copper above a magnet, the repulsive force increases with the speed of rotation due to eddy current levitation. Does that happen here?
Emeritus
PF Gold
P: 29,238
 Quote by Creator I was under the impression we are talking type II here; zapper.- I hope that is clear from the context - of the usually demonstrations showing the Miessner effect. Maybe I should have made it clear. There are conditions in which flux is NOT fully expelled. I thought my post was clear and I gave clearly the experimental conditions in which flux is not fully expelled, at least in a high temp. SC. I'm surprised you have a problem with this, Zapper. This is pretty elementary. Call it what you please but it is a simple experimental fact that a type II superconductor can be suspended from BENEATH a magnet in a gravitational field and it is typically thought to be a result of pinned flux in the SC. Whether you want to call that 'attractive' or not is up to you. Its a flux pinning effect. As an examle see here: Simple demo: http://www.users.qwest.net/~csconduc...n%20Effect.htm more: http://www.iop.org/EJ/abstract/0953-2048/15/12/321
Notice I said ".. even in the VORTEX state", meaning that I AM including Type II superconductors.

In my first posting, I described the effect in which the flux lines do not like to be twisted. The Maglev uses this effect as a stablizing factor. This DOES provide a force, but it isn't the "attractive" force that you think. The direction of the field lines is very crucial. If I put the SC in the middle of a bar magnet and then let it drop, at some point it will start experiencing the flux from the bar magnet, but this flux is parallel to the bar magnet itself. But because the SC does NOT want to have these lines twisted or bent, there is a net force pushing itself up towards the bar magnet. If you apply only a uniform field vertically, you will not get such a suspension.

The moral of the story is here is that the field orientation between "levitating" the SC and "suspending" the SC are every different, and the mechanism for causing it is also very different. You also do not get the latter effect using a Type I SC, but you do get the former.

The field cooling issue is also a problem. In a Type II, there is magnetic flux through the SC if it is greater than Hc1 no matter if it was field cooled or not. This is an intrinsic property of any Type II superconductor.

Zz.
 P: 37 Thanks for a lively discussion. However my question remain unanswered. Q: Does the levitational/repulsive force increases wiith the rotation of the bulk type 2 superconductor?
Emeritus