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Author Topic: Why are superconductors both attracted and repelled by a magnet?  (Read 1322 times)

Offline chris

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I saw a terrific demonstration the other day by a superconductor physicist whom we were recording for the programme next week; she had a circle of powerful magnets arranged around the outer edge of a large disc resembling a coffee table top.

Above this ring of magnets she placed a small puck (2 inch by 2 inch) of a copper oxide based superconductor cooled to -200C. The puck hovered about 2cm above the table top and, when given a push, followed the path of the magnets, in a circular path.

This is relatively simple to explain, because the superconductor generates a magnetic field that opposes the field applied by the magnets and this keeps it suspended.

But, the next thing she did was to turn the table upside down so now the puck was hanging under the table and following the same circular path.

If the magnetic effect was purely a repulsive one, the puck would have fallen to the floor. Instead it hangs there; so it seems to be both attracted and repelled from the surface at the same time.

How?




 

Offline Thebox

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I saw a terrific demonstration the other day by a superconductor physicist whom we were recording for the programme next week; she had a circle of powerful magnets arranged around the outer edge of a large disc resembling a coffee table top.

Above this ring of magnets she placed a small puck (2 inch by 2 inch) of a copper oxide based superconductor cooled to -200C. The puck hovered about 2cm above the table top and, when given a push, followed the path of the magnets, in a circular path.

This is relatively simple to explain, because the superconductor generates a magnetic field that opposes the field applied by the magnets and this keeps it suspended.

But, the next thing she did was to turn the table upside down so now the puck was hanging under the table and following the same circular path.

If the magnetic effect was purely a repulsive one, the puck would have fallen to the floor. Instead it hangs there; so it seems to be both attracted and repelled from the surface at the same time.

How?

Because negative is attracted to negative in my opinion, negative is mass in my opinion and all mass  is attracted to mass. I have viewed the same or similar video.

added - consider what stops the puck flying off when the table was not upside down.....

I drew it in basic form







« Last Edit: 12/03/2016 11:32:32 by Thebox »
 

Offline lightarrow

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I saw a terrific demonstration the other day by a superconductor physicist whom we were recording for the programme next week; she had a circle of powerful magnets arranged around the outer edge of a large disc resembling a coffee table top.

Above this ring of magnets she placed a small puck (2 inch by 2 inch) of a copper oxide based superconductor cooled to -200C. The puck hovered about 2cm above the table top and, when given a push, followed the path of the magnets, in a circular path.

This is relatively simple to explain, because the superconductor generates a magnetic field that opposes the field applied by the magnets and this keeps it suspended.

But, the next thing she did was to turn the table upside down so now the puck was hanging under the table and following the same circular path.

If the magnetic effect was purely a repulsive one, the puck would have fallen to the floor. Instead it hangs there; so it seems to be both attracted and repelled from the surface at the same time.

How?

I'm not sure because I should think a little bit more about it so this is my answer version 1.0  :)
When you make the superconductor (SC) puck approach the disk of magnets from the above, an induced current is formed on the first, because of Lenz's law this current is such to create a magnetic field on the SC puck which is (perfectly) opposite to the other field generated from the magnets, as you wrote.

But what would happen if, in this situation, you tried to pull up the puck?
Exactly for the same reason, inside the puck would be generated a current which generates a magnetic field in the opposite sense with respect as before, which would make the puck be attracted, this time, from the magnets, showing a resistance to be pulled up.

When the puck+magnets are upside down, the puck's weight pulls it downward and this, for what I wrote, generates a resistance to fall, that is a magnetic force which prevents it to fall down.

--
lightarrow
« Last Edit: 12/03/2016 12:50:44 by lightarrow »
 

Offline jeffreyH

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The answer may lie in the relationship between magnetic field strength and superconductivity. The hyperphysics website has information on this which someone more knowledgeable in this area may explain.

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/scbc.html
 

Offline chiralSPO

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My understanding is that the superconductor completely nullifies the magnetic field within itself (Meissner effect: https://en.wikipedia.org/wiki/Meissner_effect), and that there is some (fairly small) barrier to changing its response to a change in the magnetic field around it. So the superconducting object will be free to move about within a homogeneous field, but will meet some resistance in going closer to or further from the source of the field.

 
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Offline timey

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Here's a bit of back to front thinking...for what it's worth.

Could it quite simply be that the polarities of the magnets and the puck are merely reversed by their activity being contrarily positioned within the gravitational field?

That it is gravity itself that has afforded these aligned magnets to be repulsive in an upwardly oriented position, and to be in equal part attractive when oriented in a downwardly position.  The repulsive reaction is repelling the gravitational force, and the attractive reaction is repelling the gravitational force... in equal parts, due to the tables position in the gravitational field.

Leading to my questions:  Did the table have legs oriented for an upwards and downwards representation that were of 'equal' length?  If so, would the 'distance' observed between the puck and the magnets be altered if the legs of both the upward and downward orientations of the table were to afford the magnets a change in position via the magnets and the pucks activity being represented at different 'heights' within the gravitational field?
 

Offline Colin2B

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My understanding is that the superconductor completely nullifies the magnetic field within itself (Meissner effect: https://en.wikipedia.org/wiki/Meissner_effect), and that there is some (fairly small) barrier to changing its response to a change in the magnetic field around it. So the superconducting object will be free to move about within a homogeneous field, but will meet some resistance in going closer to or further from the source of the field.
As usual you are right, but in certain circumstances the flux can penetrate in quantum tubes.
Here is some additional wiki which explains why the disk gets trapped
https://en.m.wikipedia.org/wiki/Flux_pinning
 
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