Meissner Effect in a strong field?

[Wiz700]

While studying the M. Effect, a lot of sources state: "If a superconductor is applied in a weak field",
I assume the effect would work even in a strong field?

Weak or strong I think the Meissner Effect will always be applied as long as the superconductor is below critical temp.

Or am I wrong?

 

[Wiz700]

Another thing, when the Meissner Effect occurs, is the levitation due to a force? Or just an effect when a magnetic field is expelled from an object?

My understanding is that, a magnetic field is generated by the superconductor that is equally the same as the initial field, some may say it's the "perfect eddy currents", those currents would cancel the applied magnetic field, since it generates the same fields in an opposite direction. Not sure though...

 

[f95toli]

While studying the M. Effect, a lot of sources state: "If a superconductor is applied in a weak field",
I assume the effect would work even in a strong field?

Weak or strong I think the Meissner Effect will always be applied as long as the superconductor is below critical temp.

Or am I wrong?

If the field is too strong you "quench" the superconductivity and it goes normal, this happens at f field known as the critical field. This field is not very strong for most conventional superconductors (a few mT) although some materials (NbTi etc) have relatively high Bc which is why they are used for superconducting magnets.

 

[Wiz700]

If the field is too strong you "quench" the superconductivity and it goes normal, this happens at f field known as the critical field. This field is not very strong for most conventional superconductors (a few mT) although some materials (NbTi etc) have relatively high Bc which is why they are used for superconducting magnets.

Could you please dumb that down a bit, I really didn't understand what you said (Rookie here).
In the future can we create superconductor that can handel fields over 1T? Current now can superconductors handel strong fields? 1T+?

What does Bc mean?


I generally assumed that by exposing a superconductor to any field(weak/strong), the Meissner effect will always occur. I'm 100% sure that if I keep the condutor cool, as much as I can, this Quantum phenomena will not break.

(Please do explain the ideas simply, I'm still new to this idea. I barley finished most of the materials in E&M. thank you!)

 

[Wiz700]

"The magnetic field of these surface currents cancels the applied magnetic field within the bulk of the superconductor"

Via Wikipedia

That means there will be no force between the supercondutor and the source of the magnetic field?

 

[f95toli]

Could you please dumb that down a bit, I really didn't understand what you said (Rookie here).
In the future can we create superconductor that can handel fields over 1T? Current now can superconductors handel strong fields? 1T+?

Some superconductors can already handle well over 1T, I believe the limit for superconducting magnets is somewhere around 15-17 T.

What does Bc mean?

Critical magnetic field

I generally assumed that by exposing a superconductor to any field(weak/strong), the Meissner effect will always occur. I'm 100% sure that if I keep the condutor cool, as much as I can, this Quantum phenomena will not break.

The Meissner effect will always occur as long as the field is smaller than the critical field, if the field is higher than that the superconductor will go normal and the Meissner effect will go away.

(Please do explain the ideas simply, I'm still new to this idea. I barley finished most of the materials in E&M. thank you!)

This is one of those areas where the "basic" case is quite simple. However, it gets quite complicated if you want to understand the details. For example, the Meissner effect is quite different for type I and type II superconductors , the latter case is the most common in the real world since type II superconductor have higher critical fields meaning it is what you use in applications (and all high-temperature superconductors are type II). However, type II is also the more complicated case. I believe there is a good Wikipedia article that describes the difference between type I and II.

 

[Wiz700]

I have no idea what so ever about Bc,
Just the fact that if the applied field is > than the Bc it breaks the super conductivity, What is the critical magnetic field anyway?!

I understand a superconductors Tc, but not Bc at all!
But I do know the features of Type I and Type II conductors, I honestly favored typed II.
+ I know type I will not allow a field to penetrate it, but in some cases type II will example: Flux pinning?However, to sum things up.
The Meissner effect expels the magnetic field(i.e cancels the applied field) Is there a force?! I assume B = 0 Thus, F = 0 since the force is a function of the field.Btw, thank you f95toli for all the help!

 

[Wiz700]

Thanks for the move!
Who ever is responsible :)

 

[Wiz700]

But what confuses me the most, is how can the applied magnetic field can break the superconductor state?
The idea of "Bc" still confuses me the most!

If we can constantly keep the superconductor cooled can a stronger field break this Quantum phenomenon?

 

[mfb]

Superconductivity depends on 3 factors: temperature, (external) magnetic field and current. The magnet will quench if the combination of those 3 is too large.
The critical values can be expressed in 3D-graphs like that (from cern.ch).

All superconductors have a maximal temperature (but then they need zero current and external magnetic field), a maximal external magnetic field (but then they need zero temperature and current) and a maximal current density (with zero temperature and external field). All 3 cases are not really interesting for real applications, so you have to consider all 3 parameters.

 

[Wiz700]

Sorry but I don't understand you're point...

All 3 cases are not really interesting for real applications, so you have to consider all 3 parameters.

Why do you need all 3 cases?

So far, I understand the most important thing to do is, cool down the material under Tc and achieve the superconducting state, hence the Mesinner effect is achieved(Thats how I simply understood it) why all the complications? Of those three?

Having zero resistance is the most important interest(imo). And is the most IMPORTANT application that is currently under (R&D).
Having zero resistance means a superconductor can and will induce eddy currents that creates a magnetic field equally the same as the one applied and cancels it, its amazing(i.e no force of attraction or repulsion). Can you imagine the amazing applications coming from that too?
I mean, we can make indoor flying rooms lol that's one!

 

[marcusl]

The Meissner effect can be analyzed in terms of internal energy in the superconductor (SC). The superconducting state results from a special pairing of electrons (Cooper pairs) that occurs only in SC's. This pairing results in lower energy than the normal state, which is advantageous, but also results in expulsion of magnetic field from the interior which costs energy. So long as the external field is weak, it is energetically favorable overall to be in the SC state. As the field increases, a point is reached where it costs more energy to expel field than is gained by pair formation, so the SC reverts to a normal state.

 

[Wiz700]

The Meissner effect can be analyzed in terms of internal energy in the superconductor (SC). The superconducting state results from a special pairing of electrons (Cooper pairs) that occurs only in SC's. This pairing results in lower energy than the normal state, which is advantageous, but also results in expulsion of magnetic field from the interior which costs energy. So long as the external field is weak, it is energetically favorable overall to be in the SC state. As the field increases, a point is reached where it costs more energy to expel field than is gained by pair formation, so the SC reverts to a normal state.

Advantageous indeed!

However, the external magnetic field breaks that low state of energy this the electrons will not be paired and starte to move around and resistance is no longer zero?
I believe if we can constantly keep the supercondutor cool, I think applying stronger fields will not break the effect.

Is the expulsion due to a force? Or cancelation of the forces generated by both ?!

 

[Wiz700]

marcusl, mfb, f95toli,

My understanding so far is good?
I feel like I might have a misconception of something...

 

[mfb]

So far, I understand the most important thing to do is, cool down the material under Tc and achieve the superconducting state, hence the Mesinner effect is achieved(Thats how I simply understood it)

It is not that simple.

However, the external magnetic field breaks that low state of energy this the electrons will not be paired and starte to move around and resistance is no longer zero?

Right.

I believe if we can constantly keep the supercondutor cool, I think applying stronger fields will not break the effect.

No, that is not right.
Colder temperatures allow to cancel stronger fields, but there is always a limit on the field strength.

Is the expulsion due to a force? Or cancelation of the forces generated by both ?!

No.

But still, how can the external magnetic field effect the energy state?

Magnetic fields influence electrons with a momentum, how is that surprising?

And why does would it cost more energy? If we constantly keep the (SC) cooled? Shouldn't it be constant?

I don't understand that question.

 

[Wiz700]

It is not that simple.

By looking at multiple attempts of the experiment, it really looks simple... In fact,One of the simplest things ever, I don't understand why it "might" be complicated? I fear of missing out on something important.

No, that is not right.
Colder temperatures allow to cancel stronger fields, but there is always a limit on the field strength.

Sorry! There will always be a limit you're right forgot to state that.
There is a limit as for how much we can cool the SC, But I believe by increasing the amount of matter the SC has, it might handel a stronger magnetic field at the same cooling temperature maybe?

No.

Is there even a force? Because, when the effect occurs "the levitation" some say its due to the superconductor repelling the external magnetic field , I believe it only expels the field and blocks it from penetrating it. I don't think there are forces involved...

Magnetic fields influence electrons with a momentum, how is that surprising?

Its not, I forgot.

I don't understand that question.

Forgot it about it. I stated something foolish

 

[marcusl]

Advantageous indeed!

However, the external magnetic field breaks that low state of energy this the electrons will not be paired and starte to move around and resistance is no longer zero?

No, motion is not a very useful concept here. SC is a quantum mechanical phenomenon involving electron wavefunctions.

I believe if we can constantly keep the supercondutor cool, I think applying stronger fields will not break the effect.

It is curious that someone who doesn't understand the concepts is holding and expressing firm beliefs. Your belief is completely wrong.

Is the expulsion due to a force? Or cancelation of the forces generated by both ?!

I don't think so--maybe someone else knows this one. The common viewpoint is from considering energy.

 

[Wiz700]

No, motion is not a very useful concept here. SC is a quantum mechanical phenomenon involving electron wavefunctions.

Interesting.

It is curious that someone who doesn't understand the concepts is holding and expressing firm beliefs. Your belief is completely wrong.

I don't understand it fully, It's not harm sharing firm beliefs/theories in order to be corrected, hey its sounds really simple and basic. It's not hard to connect the dots sometimes, thus new and new ideas pop easily because of its beautiful simplicity.

I don't think so--maybe someone else knows this one. The common viewpoint is from considering energy.

Fair enough.
I rather avoid considering energy, tends to confuse me.

 

[Wiz700]

I noticed a lot of people prefer type II (SC), why so?

 

[Wiz700]

Is the expulsion due to a force? Or cancelation of the forces generated by both ?!

Its somewhat confusing... Some sources say that the superconductor expels the applied magnetic field because it generates currents to cancel the applied fields.
And some say, lenz law is applied. And a magnet or superconductor is repelled... Which one is right?
If the field is canceled or expelled there shouldn't be any force what so ever... Why use the term "repled" unless there is a force?

 

[Dreak]

Wiz, the cirital field Bc depends on the temperature...

The stronger the field, the lower the temperature of the material must be to become a SC.

 

[Wiz700]

Wiz, the cirital field Bc depends on the temperature...

The stronger the field, the lower the temperature of the material must be to become a SC.

You're right.
However, do you all think, the larger(i.e more matter) the SC at the same temperature, it can create a stronger field, and handle stronger fields?

In order to keep the effect I understand that we most constantly keep the SC in a cool state.

 

[mfb]

No, the size of the superconductor is not relevant.

 

[Wiz700]

No, the size of the superconductor is not relevant.

Mass of the SC is not relevant? shouldn't it amplify the effect?
I guess those 3 things you mentioned mfb are the things I should concentrate on.

 

[Wiz700]

Is there a force between a SC and the applied magnetic field though?
All the resources I read, states it "expels" the applied magnetic field, so in order to "expel" it, the SC just repels the field?!

Magnetic field is partly excluded from the superconductor. Hence, the same repulsion as between a magnet and a diamagnetic.

Can someone please explain the expulsion/cancellation of the applied external magnetic field, and how there should be a force of repulsion?

Makes no sense to me! If there is a cancellation of the fields of the SC and the applied magnetic field there should be no force...

 

[mfb]

Mass of the SC is not relevant? shouldn't it amplify the effect?

No.

I guess those 3 things you mentioned mfb are the things I should concentrate on.

Indeed.

Is there a force between a SC and the applied magnetic field though?

Yes. That is the idea behind all magnetic levitation experiments.

All the resources I read, states it "expels" the applied magnetic field, so in order to "expel" it, the SC just repels the field?

The magnetic field goes around the superconductor.
You can write the total magnetic field as sum of an external field (going through the superconductor) plus an internal field (generated by the superconductor) - both cancel exactly in the superconductor, so its interior is free of magnetic fields (assuming type-1 superconductors here).
You still have the interaction of those fields (or the internal currents and the external fields, if you like), leading to a force.

 

[Wiz700]

The magnetic field goes around the superconductor.

We could assume the magnetic field is avoiding the superconductor,

http://upload.wikimedia.org/wikipedia/commons/b/b5/EfektMeisnera.svg

Just like putting an object in a stream of water, the water goes around the object but not passing through it.

You can write the total magnetic field as sum of an external field (going through the superconductor) plus an internal field (generated by the superconductor) - both cancel exactly in the superconductor, so its interior is free of magnetic fields (assuming type-1 superconductors here).

Makes perfect sense when they cancel out, the superconductor has induced currents that do that. But why isn't it just like poled dipoles? They repel each other? Why "expel" a external magnetic field? Because of the equal magnetic fields B₁ - B₂ = 0?

About the Type I superconductors... I know that Type II would sometimes "allow" some of the external magnetic field to penetrate it's surface. But that is an effect of related to it's temperature I think, at some point. The Type II conductor acts like Type I by now allowing those flux lines to penetrate it(Not sure).

You still have the interaction of those fields (or the internal currents and the external fields, if you like), leading to a force.

How? We agreed they cancel out each other, so there really shouldn't be a force. I think that's why they don't "attract" or "repel" each other like two dipoles?
I doubt there is a force between a superconductor and a magnet, the forces just cancel out since the fields do.
The mind boggling thing is, the fields cancel each other out, how would their be a force?

 

[Wiz700]

Also, I assume if there was really a force of repulsion or attraction( in Type II SC's) levitational motion wouldn't be possible?

Useless the magnetic forces are canceled and only the external applied force (e.g My hand) will be applied to the superconductor.

 

[Wiz700]

I noticed a lot of people prefer type II (SC), why so?

I guess they have higher Critical temperatures than type I.
Also this diagram is interesting.

The vortex static is amazing(i.e flux pinning?)

 

[Wiz700]

So, is there a force between a SC, and the external magnetic field?
Yes?(Why?)
No?(Why?)
I assume no, because the two fields cancel each other. Hence, the magnetic flux lines during the Meissner effect is expelled, and avoids the SC, and will not penetrate through it, like any other object.

 

[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.