Meissner Effect in a strong field?

[mfb]

Yes, for reasons given in the previous posts.

Hence, the magnetic flux lines during the Meissner effect is expelled

Apart from a thin surface area, where this expelling happens, and where you get a force between the field (source) and the SC.

 

[Bill_K]

A superconductor is a perfect diamagnet. This means that the presence of an external magnetic field induces persistent currents within a small layer ("penetration depth") of its surface. The B field of these currents exactly cancels the external B field within the superconductor, but not outside it. As you see from the diagram in an earlier post, the combination of the two fields outside results in a bowing out of the magnetic field lines.

Magnetic field lines don't like to be compressed, or stretched either. There's an electromagnetic stress tensor that describes this mathematically. But intuitively, field lines behave like rubber bands. Bowing out the field lines near a superconductor takes energy, and indicates the presence of a repulsive force (pressure) between the object and the external field source.

For a symmetrical situation, the pressures on either side of the object are equal and opposite. If the external B field is nonuniform, there will be a net force, and this is what can be used to levitate the object.

 

[Wiz700]

So, if I move a superconductor near a strong magnetic field, I will feel a strong force of repulsion(Assuming its superconductive state stays constant)?
It's as if bringing two magnets in opposite poles repelling each other? But the behavior of multiple experiments shows otherwise, its as if... There is a weak force that causes levitation. A superconductor floats perfectly on-top of a magnetic field. The force only causes levitation but, it does not push them strongly apart like two magnets would.

What's also mind boggling to me... Is the idea of motion with the Meissner effect, Ideally if we bring a superconductor on a track filled with powerful magnets, and give a bit of force, it moves freely. Of course there is levitation, Hence the force being applied to expel the magnetic flux lines. But I think it's wrong to compare this repulsive force equally as if having two dipoles repelling each other.

I *think* there is a force just expelling the field as you stated Bill, but that force is not strong enough to push both objects apart from each other?

 

[ZapperZ]

So, if I move a superconductor near a strong magnetic field, I will feel a strong force of repulsion(Assuming its superconductive state stays constant)?
It's as if bringing two magnets in opposite poles repelling each other? But the behavior of multiple experiments shows otherwise, its as if... There is a weak force that causes levitation. A superconductor floats perfectly on-top of a magnetic field. The force only causes levitation but, it does not push them strongly apart like two magnets would.

You need to be very careful here and fully understand the situation.

1. The repulsive force is similar to any other magnetic force.

2. It then means that it has a dependence on distance. At some point, the repulsive magnetic force equals the weight of the superconductor.

3. Most, if not all, of these demos are done using a Type II superconductor (typically, YBCO, since it has a Tc well above liquid N2 temperature). This means that there are magnetic flux lines that can penetrate the superconductor, while it is still in the superconducting state. This is important because these flux lines resists being twisted around, etc., and thus provide the stability to the "levitation". This answers why it "floats perfectly".

What's also mind boggling to me... Is the idea of motion with the Meissner effect, Ideally if we bring a superconductor on a track filled with powerful magnets, and give a bit of force, it moves freely. Of course there is levitation, Hence the force being applied to expel the magnetic flux lines. But I think it's wrong to compare this repulsive force equally as if having two dipoles repelling each other.

There's nothing here in what you've said that made it wrong. It is backed by both experiment, and theoretical description. I get the SAME effect from two ordinary magnets.

Zz.

 

[Wiz700]

You need to be very careful here and fully understand the situation.
Most, if not all, of these demos are done using a Type II superconductor (typically, YBCO, since it has a Tc well above liquid N2 temperature). This means that there are magnetic flux lines that can penetrate the superconductor, while it is still in the superconducting state. This is important because these flux lines resists being twisted around, etc., and thus provide the stability to the "levitation". This answers why it "floats perfectly".

Strange, having the flux lines penetrating the superconductor is the result of flux pining I assume?
Initially, when the superconductive state occurs with the type II SC, the flux lines are penetrated correct?

Based on the graph posted earlier, the superconductor of Type II will have nothing penetrating it, however, it will in the vortex state?

 

[f95toli]

Strange, having the flux lines penetrating the superconductor is the result of flux pining I assume?
Initially, when the superconductive state occurs with the type II SC, the flux lines are penetrated correct?

Based on the graph posted earlier, the superconductor of Type II will have nothing penetrating it, however, it will in the vortex state?

In order to get the "floating" effect you (usually) need to put the magnet on top of the SC BEFORE it goes below Tc (i.e. before you pour liquid nitrogen on it). If you just cool a SC and THEN try to "balance" a magnet on top of it you will find that it is very difficult, and it is a bit like trying to balance magnet on top of another. This is due to the fact that unless you cool the SC in a magnetic field there won't be any pinned flux lines and there is no "minima" for the magnet to rest in. If you move the magnet around above the SC you will feel a resistance not only when you try to push it down, but also when you move it sideways.
This is why it is stable.It IS possible to arrange things so that you can levitate a magnet above a type I superconductor (this is "permitted" since the SC behaves like a perfect diamagnet), it is just very difficult.

 

[Wiz700]

There's nothing here in what you've said that made it wrong. It is backed by both experiment, and theoretical description. I get the SAME effect from two ordinary magnets.
Zz.

I need to study from better resources... I was slacking off and watched this video.
His descriptions are quite brief.

I used to believe that there is no force related between a superconductor and the external magnetic field. Mainly because the fields cancel out... But I need to study more and more.

Thank you all for pointing out my misconceptions.

 

[Wiz700]

2. It then means that it has a dependence on distance. At some point, the repulsive magnetic force equals the weight of the superconductor.

Zz.

A force between a magnet and a superconductor for example, is equal to the weight of the superconductor separated by a distance r?
While reviewing the concept. I realized... That indeed there is a force, as explained earlier, to expel the magnetic field lines there has to be energy, This is where the repulsive force comes into play.

I knew its a magnetic repulsive force... But is it the same force you'd get from two dipoles?
Meaning, the repulsive force generated by a magnet and a superconductor is equal to the force generated by to magnets?

A perfect dimagnet will always repel an external field?

I believe the repulsive force between a superconductor and a strong magnetic field from a magnet is very very weak.

 

[ZapperZ]

I think you still don't have an understanding of the Meissner effect.

The physics mechanism on how a magnetic field is excluded from a superconductor is due to the presence of supercurrent loops that generate its own field in response to the external magnetic field. The result of these two fields generally cancels out the external magnetic field that would have penetrated the superconductor. That is why you were told repeatedly that in such a state, the superconductor behaves as a perfect diamagnet!

However, as a result of this, the superconductor will also produce its own external magnetic field, and it results in a repulsive force between it, and the magnet.

You need to look up the definition of a perfect diamagnet. It is defined NOT as "always repel an external field", but rather has does has NO magnetic field inside its relevant volume.

Zz.

 

[Wiz700]

I think you still don't have an understanding of the Meissner effect.

The physics mechanism on how a magnetic field is excluded from a superconductor is due to the presence of supercurrent loops that generate its own field in response to the external magnetic field. The result of these two fields generally cancels out the external magnetic field that would have penetrated the superconductor. That is why you were told repeatedly that in such a state, the superconductor behaves as a perfect diamagnet!

However, as a result of this, the superconductor will also produce its own external magnetic field, and it results in a repulsive force between it, and the magnet.

You need to look up the definition of a perfect diamagnet. It is defined NOT as "always repel an external field", but rather has does has NO magnetic field inside its relevant volume.

Zz.

Im sorry, but I've struggled a lot while studying only EM...
So dealing with this idea is quite... Strang hence why I'm struggling here as well!

 

[Wiz700]

Magnetic field lines don't like to be compressed, or stretched either. There's an electromagnetic stress tensor that describes this mathematically. But intuitively, field lines behave like rubber bands. Bowing out the field lines near a superconductor takes energy, and indicates the presence of a repulsive force (pressure) between the object and the external field source.

Amazing amazing explanation! Because now I can use this explanation why when a high permeability piece of metal like soft iron, concentrates the flux lines through a certain point. Which explains the attraction between them!

I know this has no relevance to the Meissner effect, but it's nice to relate the ideas :)

 

[Wiz700]

I wonder...
If there are two type II superconductors facing a magnet, then cooled and reach the superconducting state. Will the magnetic flux lines penetrate BOTH conductors? Assuming the distance between each is r.

I'm guessing that indeed the magnetic flux will penetrate BOTH SC's.
Also, the time it takes to reach its superconductive state when exposed to a cooling source, would it be fast? How fast?

 

[ZapperZ]

I wonder...
If there are two type II superconductors facing a magnet, then cooled and reach the superconducting state. Will the magnetic flux lines penetrate BOTH conductors? Assuming the distance between each is r.

I'm guessing that indeed the magnetic flux will penetrate BOTH SC's.
Also, the time it takes to reach its superconductive state when exposed to a cooling source, would it be fast? How fast?

This has now become a meandering thread where you are just making things up as you go along.

If you wish to actually learn, start from the basic. Try Lenz's law, and brush up on your E&M. Otherwise, I can only see you asking these things just for the sake of asking.

A set of disjointed pieces of information does not a knowledge make!

Zz.

 

[Wiz700]

If you wish to actually learn, start from the basic. Try Lenz's law, and brush up on your E&M.

Done(Thank you for the advice!)
Now, I'll ask again(I couldn't figure this out!):

Type II superconductor will allow some of the flux lines from the external field to penetrate it, what is the best way to calculate the penetrating flux lines?
I'm wondering how many lines have really penetrated the superconductor.

 

[Wiz700]

2. It then means that it has a dependence on distance. At some point, the repulsive magnetic force equals the weight of the superconductor.

This is really confusing, generally. I know that magnetic force depends on the Strength of B and the distance. Why is the weight of such importance?

 

[Wiz700]

Just when I thought I've understood the magnetic force...

 

[Vanadium 50]

This has now become a meandering thread where you are just making things up as you go along.

Which makes this a good time to close it.