[Phys-L] Re: Meisner Effect?

[Chuck Britton <cvbritton@EARTHLINK.NET>]

High School type words:

Your question is about the Meisner Effect but I would like to first
back up to the zero resistance property of superconductors. My
understanding is that the ME is not a necessary result of perfect
conductivity.

Zero resistance is easier to talk about and easier to demonstrate
(well the APPROACH to zero resistance.)

If you have the 'High Temp' superconductors - then you have access to
some NdFeBo magnets. Sliding a NdFeBo magnet across a thick piece of
aluminum (or copper) gives a real 'hands on' illustration of eddy
currents, Lenz's Law and Faraday's Induction law.
Rolling a NdFeBo cylinder down an aluminum channel is quite dramatic.

Drop a cylindrical NdFeBo magnet down a length of copper or aluminum
pipe. The magnet diameter should be small enough so that it will
slide down the pipe without twisting and sticking. Nesting pieces of
pipe will produce a measurable increase in time to fall through the
pipe. Cool the Cu or Al pipe with your LN2 and the eddy currents slow
the fall even more.
Better conductivity of the pipe is what is creating the greater eddy currents.

At this point I ask what might be expected if the pipe were a PERFECT
conductor, with absolutely ZERO resistance. Well, it wouldn't fall
through the pipe at all ! ! !

Doug Oscheroff (Noble Laureate in Low Temp Physics) cools a slab of
OFHC (oxygen free, high conductivity) copper and drops a fairly large
NdFeBo magnet from a height of a foot or so. As the magnet approaches
the block of copper it's speed decreases dramatically and it slowly
settles onto the copper. (an inch thick, six inch square of OFHC Cu
will cost 2 or 3 hundred bucks!)

Ok, now the kids should have some appreciation of the Lenz's law,
eddy current phenomena but this DOESN'T address the Meisner-Ochenfeld
Effect.

At this point I draw on an analogy with the electric field and how it
'behaves' when a conduction object is introduced. An insulator won't
necessarily affect an E field but it doesn't take a very good
conductor to 'exclude' the E field completely from within the object.
This is accomplished by having free charges spontaneously move in
response to the external field.  ok?

Well, for some strange quantum mechanical reason, a superconductor
will exclude a B field by spontaneously creating flowing charges,
current loops, that totally cancel the external B field.

E fields are shielded (excluded) by conductors. B fields are shielded
(excluded) by superconductors. (Ignoring Type II superconductors for
the nonce)

Leon Lederman was walking down the hall and stopped at the lab door
while I was going thru this with a couple of our HS students. He
didn't wince, much.

Feymann said  something like this:

        - If you think you understand quantum physics - then you DON'T.


At 11:37 AM -0400 7/5/05, David Abineri wrote:
> I am finding it very difficult to explain the Meisner effect to my high
> school students and I wonder if someone can help to put it into their terms.
>
> Explanations that I have read talk about "expelling the field"  or
> "preventing penetration of the field" by the superconducting material
> but I have a difficult time seeing this as the source of an upward force
> on the levitating magnet.  And why is this upward force able to keep the
> levitating magnet in balance so well since, if I tried to levitate one
> magnet with another, I would have a very difficult time keeping it in
> equilibrium.
>
> Can someone fill in some details that high school students, and I, might
> be able to grasp.
>
> Thanks, for any help on this,     David Abineri
>
>
> --
> dabineri@fuse.net
_______________________________________________
Phys-L mailing list
Phys-L@electron.physics.buffalo.edu
https://www.physics.buffalo.edu/mailman/listinfo/phys-l