Meissner Effect Question

[niktes_pr]

Is a supercooled superconductor disc which repels magnetic flux actually opaque to magnetic fields?

Is anything above such a disc, which is floating over a magnetic field, sheilded from the rest of the magnetic flux?

Does the disc cast a magnetic \"shadow\"?

[Gokul]

Yes, the disc is virtually opaque to magnetic fields below some critical field, Hc. I say \'virtually\', because there will some small penetration depth due to an exponentially decaying field inside the SC.

On the top of the disc, the field lines will come together again so as to be symmetric with the flux density below the disc. Right above (as in, just on top of the center of) the SC, the flux density will be vanishingly small, but as you go higher up, the flux density will approach its limiting value. So, there isn\'t a \"shadow\" in the way an opaque body throws a shadow, but there will be something resembling the shadow thrown by a sound reflector. The physics is completely different (there\'s no diffraction effects here), but I\'m just trying to throw in an analogy for visualization.

[lysdexia]

What about these: \"spin waves\" diffraction?

[editor]

I remember reading, somewhere, that there might be a 2% reduction in gravitation above the disc.

[Gokul]

Originally posted by editor
I remember reading, somewhere, that there might be a 2% reduction in gravitation above the disc.

You are talking about the Podkletnov Effect...which NOBODY has yet been able to reproduce, least of all Dr. Podkletnov himself.

[Gokul]

Originally posted by lysdexia
What about these: \"spin waves\" diffraction?

What about them ? They are not relevant to the OP\'s question regarding the field outside the SC disc.

[nmondal]

You will find all here:--

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html
http://www.users.qwest.net/~csconductor/Experiment_Guide/Meissner%20Effect.htm

But why?
There we have bad mathematics, which at this moment I can not recall,
actually the magnetic field penetrate to some angstrom within the surface of the disc....and then it just can not enter....

It falls to 0.

for answer to question [2] NO.

What do you mean by magnetic shadow?
You mean shielding? Than [3] is NO.



[Edited on 4-27-2005 by nmondal]

[AntonVrba]

Is the reverse also true? By reverse I mean if the superconducting disc is say crystalised from a non superconducting to superconducting disc in a magnetic field, now the electric field is removed does the superconducting disc try and maintain the magnetic field - it sounds logical.

[nmondal]

Can not get it Anton.
The moment it becomes *Super*, [Which is I guess the only MACROSCOPIC efffect of a purely quantum phenomenon ] it *repells* the magnetic field....and in effects tries to jump out of the field.

[AntonVrba]

Originally posted by nmondal
Can not get it Anton.

Do I express myself that badly?:well: lets say the disk has a switch labelled SUPERCONDUCT - On/Off.

Switch Super is off - magnetic field is applied - now lets switch Super ON and mahnetic field is removed

Originally posted by nmondal
The moment it becomes *Super*, [Which is I guess the only MACROSCOPIC efffect of a purely quantum phenomenon ] it *repells* the magnetic field....and in effects tries to jump out of the field.

I do not agree

We need to ask why the magnetic field is repelled in the first place - surely the anser lies in the fact that magnetic field is apllied - eddy currents start to flow to opose the magnetic field and we now that the induced current is a function of dB/dT i.e rate of change. Hence, if superconducting disk is manufactured in magnetic field it will retain this field i.e. it is a magnet

Am I seeing this wrong or are there other explanations to why magnetic fields are opposed?

[editor]

A superconductor is a perfect diamagnetic material. The orbital motion of electrons creates tiny atomic current loops, which produce magnetic fields. When an external magnetic field is applied to a material, these current loops will tend to align in such a way as to oppose the applied field.

[nmondal]

I am sorry that I did not express myself well.


Good Catch Anton.

Meisner Effect : *SuperConductivity* is the only quantum phenomenon known till date that has a macroscopically observable effect.

The high-school experiment of repulsion of magnetic field is known/performed by us at the high school level....
That does nothing to do with superconductivity.


To have a detailed explanation of the repelling act of the *super* thing:

http://www.physics.uiuc.edu/research/ElectronicStructure/389/pdf/389-lect25.pdf

Actually any introductory text-book on QM deals with it!

B can not exist within a super, as electrons cease to become a fermion, they form {John}Bardeen{Leon}Cooper{ Robert}Schrieffer pair, a boson with
spin {0, -1, +1 } He he...
See
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/bcs.html
The *virtual* particle that *binds* 2e is called a phonon...hard but that is another name of the sound energy of the lattice vibration.

I got a seminar in my College with a friend in Metallurgy regarding this.

That guy Berdin is the only Engineer who recieved Nobel in physics twice....

Transistor, Super.


And some more,
I do not want to put the equations here...

http://www.physicsforums.com/archive/topic/t-54701_meissner_effect.html

[laputian]

Originally posted by nmondal
Meisner Effect : *SuperConductivity* is the only quantum phenomenon known till date that has a macroscopically observable effect.

Sorry to say no: in 1995 experiments conducted at JILA (Boulder, Colorado) and MIT (Boston, Massachussetts) uncovered a so-called \"Bose-Einstein condensate\" (BEC for short) which is also a macroscopic quantum phenomenon (predicted by Einstein in 1925). For more information, in a comprehensible way, see http://www.colorado.edu/physics/2000/bec/index.html

In these last 10 years, similar results (and other even more striking) have been reported by labs around Europe, Japan, USA and Australia.

[BRH]

What about a more simple example that does not require extreme setups? I am thinking of magnetism, which is a purely quantum phenomenon with a macroscopically observable effect and you can observe it for the price of a kitchen magnet... though Nd magnets are so much cooler..