Is Meissner effect same as Lenz's law principle ?? (1 Viewer)

[jamesfirst]

I'm confused. Some sources say that it's different to Lenz's law, but I just think it's logically that the principle can be applied to Meissner effect.

 

[Shadowdude]

Meissner effect is about superconductors repelling magnetic fields. Lenz's Law is about electromagnetic circuits inducing currents that repel the change in flux or something.

They aren't the same.

 

[jamesfirst]

Meissner effect is about superconductors repelling magnetic fields. Lenz's Law is about electromagnetic circuits inducing currents that repel the change in flux or something.

They aren't the same.

Then how does Meissner effect work ?? Why the repulsion??


Some sources say that there are currents induce in superconductors and Lenz's law may apply..

 

[Shadowdude]

I haven't touched HSC Physics in about six months, but from what I can recall - the Meissner effect occurs as when in a superconducting state, the superconductor creates electric currents that cancels out the magnetic field through the superconductor itself.

I can see how you could get confused, but if I recall correctly - Lenz's law is about how an electromagnet repels changes in its flux. The Meissner effect on a superconductor does not decay at all, so there is no change in flux - ever as long as the magnetic field through the superconductor remains the same.


Hopefully someone else who has a more recent memory of HSC Physics can help out! So I do hope I'm wrong, so that someone can get to you. But as I recall, what I said I think is correct.

 

[jamesfirst]

But in superconductors, the current can exist forever as long as the critical temperature stays constant. So if eddy currents are initially induced in the superconductor with an initial changing magnetic flux, wouldn't it make sense to think that the magnetic flux will permeate forever until the temperature rises?...


So the flux will stay there to levitate the magnet. That's what my teacher told me as well.

 

[Fizzy_Cyst]

But in superconductors, the current can exist forever as long as the critical temperature stays constant. So if eddy currents are initially induced in the superconductor with an initial changing magnetic flux, wouldn't it make sense to think that the magnetic flux will permeate forever until the temperature rises?...


So the flux will stay there to levitate the magnet. That's what my teacher told me as well.

What they want you to know for the HSC is similar to Lenz' Law.
Below critical temperature, superconductors are perfect diamagnets. A diamagnet is a material which completely expels any magnetic field lines incident on it. However, Superconductors are a funny type of material, you are correct, according to Lenz' and Faraday's Law, an eddy current should be formed to produce a force to oppose the change in flux which gave rise to it, but within a Superconductor below its critical temperature, change in flux is always zero. (As otherwise you would have a potential difference, and according to Ohms Law, seeing as R=0, then you would have infinite current)
Basically, what you have is the magnetic field from the small magnet itself superpositioning with the magnetic field formed via the generation of induced current to produce a region of zero magnetic field inside the superconductor. But, then again what happens in Type 1 SC is a great deal different to what happens in Type 2 SC!!

It's quite a complex process, suffice to say what they want u to know in the HSC:
1. Magnet approaches SC below Tc
2. Change in flux induces eddy currents in SC
3. These eddy currents produce a magnetic field which produces a force to oppose change in flux which caused it
4. This produces a repulsive force on the magnet
5. Causes magnet to levitate above SC below Tc

 

[jamesfirst]

THANKS!!!!!

A rep for you !!! That was a really nice and neat answer. Thank you .

Phew, lucky I don't have to fix my written syllabus notes for this

 

[Aindan]

Meisenner effect is different to Lenz's Law. Understandably, you may think that it produces eddy currents as a result of changing magnetic flux. BUT:

There is an experiment where you let a magnet sit on a super conductor. As you cool the superconductor below its critical temperature, the magnet begins to rise. Now, in this scenario there is no change in magnetic flux as the magnet was there all along, rather the superconductor seeks to expel the magnetic field and hence this allows the magnet to rise by producing its own magnetic field to expel it. This is due to the Meisenner effect rather than lenz's law.

 

[jamesfirst]

We didn't perform the experiment like that.


We first added the liquid nitrogen to the ceramic and dropped the magnet. The teacher said that the magnet won't levitate if it's initially placed on the ceramic before the liquid nitrogen went in...

 

[Aindan]

i've never performed any experiment - our school to poor, but i'm pretty sure it's a legit experiment

 

[Fizzy_Cyst]

We didn't perform the experiment like that.


We first added the liquid nitrogen to the ceramic and dropped the magnet. The teacher said that the magnet won't levitate if it's initially placed on the ceramic before the liquid nitrogen went in...

Your teacher is definitely wrong with that statement! The SC itself does not experience a change in flux and in fact more or less "mirror-images" the magnetic field from the SC
Perfect diamagnetism, or any diamagnetism for that matter is not studied in the HSC course, neither is flux pinning, both of which you really need in order to explain Meissner effect properly.
Taking what you have learnt in the HSC syllabus, particularly from M&G, it is reasonable to talk about Faraday and Lenz Law.
IMO the best answers in response to a "Explain how a magnet can levitate above a SC" type question are answers which mention perfect diamagnetism as it shows that extra personal research (or extra notes in class) have been used.

The experiment which Aindan is talking about is the real deal. If you put a small, low-strength magnet on a SC and then cool the SC to below Tc, it will begin to rise off the SC and levitate due to the magnetic field being expelled from the SC.
From what you are taught in the HSC course, if this happens (which it does), you could then reasonably conclude that due to the magnet rising, this would cause a change in flux which would give rise to an eddy current which would pull the magnet back down -- this obviously does not happen.
This really shows that the levitation is not due to the generation of an eddy current, rather is due to the expulsion of the magnetic field of the small magnet.

 

[jamesfirst]

Is it okay to answer like this:


The magnet will levitate due to Meissner effect and due do the relative motion between the rising magnet and the SC, it will induce eddy currents in the SC which will produce its own magnetic flux to oppose the levitating magnet's flux. Thus, the magnet is able to levitate.

 

[SpreadTheWord]

Jamesfirst - im sure it says that the Meissner effect proved that eddy currents didn't occur within superconductivity. Therefore both Lenz's law and the Meissner cannot be the same in this case.