The Meissner Effect & Lenz's Law

[xfallingstar]

Hi,

I am quite confused about how the Meissner Effect works. I know that below the critical temperature of a superconductor, it will exclude all magnetic fields going through it. But why is this so?

And I also know that as a magnet is brought near the surface of a cooled superconductor, currents will be generated within that create a magnetic field to repel the field of the magnet, thus making it levitate. My teacher said that this is not Lenz's Law working, but another phenomena. So if it's not Lenz's Law, then what gives the electrons the 'push' to make it circulate and form eddy currents? (since no magnetic fields are going through the superconductor)

 

[Blibbler]

Just as Lenz' Law is a statement of Newton's Third Law for electromagnetic induction, the Meissner effect is a statement of Newton's Third Law (conservation) for a superconductor where the mobile charge carriers at the surface can be described as acting like a form of superfluid that is "stirred" by incident magnetic flux and by the Lorentz force move, circulating around quantized vortex lines in a direction which will induce a magnetic field which will always oppose the incident flux.

But it is just Newton 3.

 

[ckreol1]

Like Blibbler said.

The eddy currents induced in the superconductor oppose the magnetic field of the floating magnet.

For a more complete explanation, with experiment, from Walter Lewin of MIT see this video, minutes 31 through 39:

http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/video-lectures/lecture-19-how-do-magicians-levitate-women/

According to this video, the eddy currents are created to oppose the floating magnet, so there is no NET magnetic field. There can be no NET magnetic field in a superconductor.

Just my 2 cents.

 

[xfallingstar]

Wow that was quite confusing! I'm only in Year 12 (final year of high school) so I have no idea what superfluid, Lorentz fofrce move or quantized vortex lines are...

But isn't Newton's Third law the law of conservation? (e.g. every action has an equal and opposite reaction) I thought it was Faraday who proposed electromagnetic induction.

ckreol - i think the 'net' thing about the magnetic field helps. So magnetic lines do initially pass through first right? So that it can induce the currents to oppose the magnetic field?

 

[ckreol1]

xfallingstar,

I bet you would enjoy watching the MIT lecture I linked to, and maybe some more of them. Tons of info and demonstrations of Lorentz force and other EM fundamentals. It's a whole series of 36 lectures and they are not at all boring and not too math intensive.

Your initial question was a good one.

IMHO, Blibbler is giving you a much more thorough and correct answer. I just based my answer on an introductory Electromagnetism lecture with no knowledge of Quantum Mechanics or Quantum Electrodynamics.

 

[Blibbler]

xfallingstar, sorry if I pitched it unintelligibly.

ckreol1 is spot on with the recommendation that you watch Walter Lewin's courses: they are brilliant; he never condescends, he always explains from first principles and he has a knack for bringing everything he talks about to life - he is a wonderful communicator and a model teacher.