Meissner Effect: Levitating a Magnet & Lenz's Law

[cragar]

When we see people levitate a magnet over a superconductor, Does the magnet cause electrons to flow in the superconductor and this creates a B field to hold up the magnet. Is this similar to lenz's law. And why does the stationary magnet cause electrons to flow, I thought we needed a change in magnetic flux to produce a voltage according to Faraday's law.
Any input will be much appreciated.

 

[cragar]

Someone has to have something on the Meissner effect.

 

[cragar]

come on know one has anything on this .

 

[Curl]

Nah, I'm pretty sure the magnet must drop a little bit before it can stop in a levitated state.

If it drops, it will create a current. The current will keep existing forever (as long as the material is superconducting) so the B field is about constant.

 

[DrDu]

The Meissner effect is conceptually different from the current induced in an infinitely good conductor by Faradays law of induction. In a superconductor, the current is directly proportional to the magnetic vector potential A. That's called Londons equation.

 

[DrDu]

Btw, not only superconductors show levitation in magnetic fields but in principle any diamagnetic substance. E.g. I have some high purity graphite which also floats over some permanent magnets.

 

[cragar]

interesting thanks for the answers, And I've also seen where they float a magnet over the superconductor and they turn it over and it still stays there. So does it like attract it and then repel it .

 

[K^2]

The reason why nobody is giving you good answers is because Meisner Effect is quite complicated. You need to understand a good deal of quantum mechanics and solid state theory to understand where it comes from and how it works.

The basic statement that arises from this is that magnetic field within a superconductor cannot change without destroying superconducting state. The later is only possible if you excite the superconductor to a higher energy state, so you have to input a certain amount of energy. You can do so by increasing temperature or increasing external field, for example.

From here, all of the properties follow. If you attempt to change external field, the current inside superconductor changes, canceling the external field. So if you move a magnet towards a superconductor, superconductor itself becomes a magnet that repels the one you moved closer. Hence, levitation. You can also pass critical temperature with some external field already present. Then this field becomes "frozen" in the superconductor, and you can later use it as a powerful magnet. Particle accelerators and magnetic resonance spectrometers often use this.

By the way, induction of current in superconductor has little to do with classical Faraday's effect, exactly because the field never changes. Since dB/dt inside superconductor is zero, so is the electric field created by Faraday's Law. The actual process is entirely Quantum, and has to do with effects of vector potential on electron wave function.

P.S. All of the above is valid for Type I superconductor, and not necessarily for Type II, which is a bit different.

 

[cragar]

Thanks K^2 for you answer , you seem to know a lot .