The Meissner Effect (1 Viewer)

[vds700]

When a magnet is placed above a superconductor, currents flow in the superconductor to create a magnetic field to repel the external magnetic field right? Is this due to Lenz's law?

 

[Forbidden.]

The superconductor is a perfect diamagnet* which can repel a permanent magnet.
So surface currents are induced onto the superconductors surface to produce a magnetic field that exactly cancles the externally applied field.
As long as the superconductor is under the critical temperature.

By definition of the Meissner effect in terms of superconductors:

An applied magnetic field is expelled by the supercondcutor so that the field is zero in its interior.

* diagmagnetism - When an external magnetic field is applied to a diamagnetic substance, a weak magnetic moment is induced in the direction opposite to the applied field, causing diamagnetic substances to be weakly repelled by a magnet.

 

[vds700]

The superconductor is a perfect diamagnet* which can repel a permanent magnet.
So surface currents are induced onto the superconductors surface to produce a magnetic field that exactly cancles the externally applied field.
As long as the superconductor is under the critical temperature.

By definition of the Meissner effect in terms of superconductors:


* diagmagnetism - When an external magnetic field is applied to a diamagnetic substance, a weak magnetic moment is induced in the direction opposite to the applied field, causing diamagnetic substances to be weakly repelled by a magnet.

ok so its not Lenz's Law then, its the perfect diamagnetism of superconductors.

Also, is this a property of both type I and type II superconductors?

 

[independantz]

Doesn't it occur because when a superconductors is below it's Tc it expels a magnetic field, the relative motion of the magnet in this magnetic field causes currents to be induced in the superconductor as indicated by faraday's law. These eddie currents flow in such a way to create a magnetic field that opposes the orginaly change in flux that created them as indicated by Lenz' Law.As the eddy currents are able to flow without resitance they are able to flow indefinitely. The pole that is facing the superconductor from the magnet (eg. north) is the same pole that is placed on top of the superconductor to repel the magnet. As there is no resistance, the force is equivalent to the weight force of the magnet causing it to levitate.

 

[Triple777ER]

Below the critical temperature, when a magnet is bought near the superconductor, due to the movement of the magent, an eddy current is induced in the superconductor which gives rise to a magnetic field to try and oppose the original magnetic field, ie, to oppose the movement of the magnet towards the superconductor. Therefore the magentic field is directed vertically upwards. So, when the gravitational force = opposing magnetic field of superconductor, the magnet stops moving and hovers above the superconductor.

Please feel free to correct me

 

[DrHectic]

I don't think Lenz's law is a good way to explain magnetic levitation. If a magnet is placed on top of a superconductor which is then lowered to its critical temperature it will levitate. There has been no relative change in flux to induce eddy currents in this case as the magnet was sitting still on top of the superconductor.

Yes, it makes sense that eddy currents would provide the opposing magnetic fields, but only if there is relative movement.

The Meissner effect does explain magnetic levitation...something about how superconductors exclude magnetic fields when they reach their critical temp. I don't fully understand this.

However, I don't think just stating the Meissner effect and/or Lenz's law would give you the full 2/3/4 marks for a question on "what causes magnetic levitation" or similar. Does anybody know what the BOS has recommended as the answer for this one? The diamagnetism one sounds like the best to me.

 

[adnan91]

Superconductors below the critical temperature have a property that does not allow any sort of magnetic field to penetrate its interior. The external field induces a current to flow that produces a magnetic field inside the superconductor which just balances out the magetic field that would have penetrated it originally. Draw a rough diagram and u'll be sweet

 

[Lucid Scintilla]

Hi dudes, I don't understand a frigging thing you're saying, unless you're Forbidden., DrHectic, or adnan91.
(Sorry, it's to do with the structure of their posts.)

Heck, I'm replying to (almost) every post now.

When a magnet is placed above a superconductor, currents flow in the superconductor to create a magnetic field to repel the external magnetic field right? Is this due to Lenz's law?

Hey buddy, while that's one explanation, it's wrong.

Superconductors are diamagnetic, meaning that below T_c they will expel (external) magnetic flux (and above it, its behaviour is paramagnetic). Aaand... with no magnetic flux, no change in flux; no change in flux, no induction (Faraday's law); no induction, no opposing, retarding force (Lenz's law).

ok so its not Lenz's Law then, its the perfect diamagnetism of superconductors.

Perfect, huh?

It's not Lenz's law.

Also, is this a property of both type I and type II superconductors?

Both. Type is according to composition, and is indicative of T_c.

It's a property of all superconductors, and is conditional upon the temperature.

Doesn't it occur because when a superconductors is below it's Tc it expels a magnetic field, the relative motion of the magnet in this magnetic field causes currents to be induced in the superconductor as indicated by faraday's law. These eddie currents flow in such a way to create a magnetic field that opposes the orginaly change in flux that created them as indicated by Lenz' Law. As the eddy currents are able to flow without resitance they are able to flow indefinitely. The pole that is facing the superconductor from the magnet (eg. north) is the same pole that is placed on top of the superconductor to repel the magnet. As there is no resistance, the force is equivalent to the weight force of the magnet causing it to levitate.

No.

Below the critical temperature, when a magnet is bought near the superconductor, due to the movement of the magent, an eddy current is induced in the superconductor which gives rise to a magnetic field to try and oppose the original magnetic field, ie, to oppose the movement of the magnet towards the superconductor. Therefore the magentic field is directed vertically upwards. So, when the gravitational force = opposing magnetic field of superconductor, the magnet stops moving and hovers above the superconductor.

Please feel free to correct me

Sorry, mate, nope.

I don't think Lenz's law is a good way to explain magnetic levitation. If a magnet is placed on top of a superconductor which is then lowered to its critical temperature it will levitate. There has been no relative change in flux to induce eddy currents in this case as the magnet was sitting still on top of the superconductor.

Ting!

Yes, it makes sense that eddy currents would provide the opposing magnetic fields, but only if there is relative movement.

Ting again!

But, you haven't mentioned the fact that expulsion of magnetic field lines signifies the end of induction-explanation days.

The Meissner effect does explain magnetic levitation...something about how superconductors exclude magnetic fields when they reach their critical temp. I don't fully understand this.

That and quantum pinning.

The Meissner effect; "the phenomenon of the expulsion of magnetic fields from the interior of a superconductor, thus resulting in the repulsion of magnets when placed above it."

Okay, guys, this has to do with the dot pot before (on Cooper pairs), and the fact that superconductors are diamagnetic.
When a magnet is placed over a superconductor's surface, magnetic fields will try to penetrate the superconductor. However, if this does occur, the superconductor's state of having zero B in the interior of the conductor will be destroyed. Thus, to keep the material superconductive, a surface current, produced by the formation of Cooper pairs travelling in loops, is induced to induces a magnetic field which opposes the external magnetic field.

However, I don't think just stating the Meissner effect and/or Lenz's law would give you the full 2/3/4 marks for a question on "what causes magnetic levitation" or similar. Does anybody know what the BOS has recommended as the answer for this one? The diamagnetism one sounds like the best to me.

Diamagnetism; and yes, yes it should.

Superconductors below the critical temperature have a property that does not allow any sort of magnetic field to penetrate its interior. The external field induces a current to flow that produces a magnetic field inside the superconductor which just balances out the magetic field that would have penetrated it originally.

Ah, matey, this is where you fall short. The external field actually doesn't penetrate to induce.

Also, try to use scientifically correct terminology; "diamagnetism" instead of "a property that does not allow any sort of magnetic field to penetrate its interior". Also, it (also) prevent penetration of the surface.

Draw a rough diagram and u'll be sweet

Wise.

While the eddy current explanation would be great and all, in theory, in truth a change in magnetic flux is required to induce eddy currents.

Most textbooks should cover this, but their explanations will probably be an eddy current explanation, a deductive reasoning one ('hey, we know a couple theories; but this and that don't work, so thus diamagnetism, case closed!'), or non-explanations.
What are others' views on this?

 

[dolbinau]

Q. Explain the Meissner Effect

If a material is in a superconducting state and is exposed to an external magnet field, then by its diamagnetic properties and to prevent the loss of its superconducting state, surface currents are (induced?) to exclude the magnet field. This explains why magnets are seen to hover above superconductors as the (produced?) field is equal and opposite to the magnet.

Is this sufficient enough for 3 marks?

 

[henry08]

I'd give you 2/3 marks.

Lenz's law is in no way involved in the production of the Meissner Effect.

 

[dolbinau]

I didn't mean to imply Lenz's law at all...elaborate?

 

[Shadose]

Thnks Lucid Scintilla for such a structured explanation

Q. Explain the Meissner Effect

If a material is in a superconducting state and is exposed to an external magnet field, then by its diamagnetic properties and to prevent the loss of its superconducting state, surface currents are (induced?) to exclude the magnet field. This explains why magnets are seen to hover above superconductors as the (produced?) field is equal and opposite to the magnet.

Is this sufficient enough for 3 marks?

From what I've read, I think you got it. Surface currents are produced due to the pair of electrons to prevent the penetration to maintain its 0 B interior, ie the diamagnetic property.

 

[Lucid Scintilla]

Q. Explain the Meissner Effect

If a material is in a superconducting state and is exposed to an external magnet field, then by its diamagnetic properties and to prevent the loss of its superconducting state, surface currents are (induced?) to exclude the magnet field. This explains why magnets are seen to hover above superconductors as the (produced?) field is equal and opposite to the magnet.

Is this sufficient enough for 3 marks?

"Sufficient" connotes the implication that you're confident that you're right; however, that is unfortunately not the case, mate.
You answer, as quoted, either lacks specificity or accuracy.

Thnks Lucid Scintilla for such a structured explanation

w00t!
Thanks, man; lol.

From what I've read, I think you got it. Surface currents are produced due to the pair of electrons to prevent the penetration to maintain its 0 B interior, ie the diamagnetic property.

Yeah, for shiz.
The problem now is writing it, and checking the Board of Studies for their accepted answer(s).

 

[dolbinau]

"Sufficient" connotes the implication that you're confident that you're right; however, that is unfortunately not the case, mate.
You answer, as quoted, either lacks specificity or accuracy.

As is the response , can you please be more specific about what's wrong with it? Thanks.

 

[Lucid Scintilla]

As is the response , can you please be more specific about what's wrong with it? Thanks.

Yeah, sure. Why not?

But I'd like to inform y'all, in case you don't already know this, that last year, Q23(a)-(c) were a set of question about semiconductors (in total, worth four marks); and, (c) was "explain why mag. levitation occurs in superconductor materials".

NOTE: The answer in the Odlum & Garner past paper book is sure as heck wrong, as it makes reference to Lenz's law (without actually stating it; which is incredibly stupid), and the answer in Success One ain't quite right, either.
The question was worth two, one-fiftieth of the HSC Physics paper. (Still, a mark's a mark; and it makes a considerable difference. )

Anyway, back to the request. So,

If a material is in a superconducting state and is exposed to an external magnet field, then by its diamagnetic properties and to prevent the loss of its superconducting state, surface currents are (induced?) to exclude the magnet field.

Well, that's where you'd not gain full marks, if the marker were being totally anal.

But more or less correct.

This explains why magnets are seen to hover above superconductors as the (produced?) field is equal and opposite to the magnet.

The causal link between the production of currents, by the formation of Cooper pairs travelling in loops, and this, the repulsion isn't very clear.

 

[obimoshman1234]

well lucid post up ur answer then so we can all learn from it????????????

 

[dolbinau]

Speaking of the 2007 paper and success one.

Can anyone do 24 b)? I get 11,667 not 1167 even when I use their working. Did I miss something?

10^-30*7*10^21/(6*10^-12) = 11,667

 

[Lucid Scintilla]

Speaking of the 2007 paper and success one.

Can anyone do 24 b)?

Yes.
(Wrong answer to the question, I know.)

I get 11,667 not 1167 even when I use their working. Did I miss something?

10^-30*7*10^21/(6*10^-12) = 11,667

For starters, dolbinau, if you were a peer of mine, I'd advise -- or someone I gasa/gafa (I'm pretty vulgar, sorry) -- you to use equation lines, and perhaps a column on the side to show your SI units.

m=10^-30 kg.
q=+6e-12
a=7e21 ms^-2

F=ma
F=10^-30 x 7e21
F=7e-9

E=F/q
E=7e-9 N/6e-12 C
E=1.1666...e3
E=1.2e3 NC^-1

Sorry about using e for exponent; ya know, power of ten. I'm too lazy to type out all the BBcode.

 

[henry08]

For 24. b I get 1166.67

 

[henry08]

E = F/q

= [10^-30 x (7 x 10^21)]/ 6 x 10^-12

= 1166.67 NC^-1