Meissner Effect (1 Viewer)

[speersy]

O.k i thought i knew what the meissner effect was.
Here is what i thought it was:
The relative motion of a magnet above the superconductor induces eddy currents in the superconducor which by Lenz's law set up a magnetic field to oppose the magnets motion. Since the superconductor has no resistance the induced currents and magnetic field is as strong as the magnetic field of the magnet. So the interaction of these two magnetic fields causes the magnet to hover above the superconductor.

There is obviously huge flaws in this explanation like why if the magnet is stationary are eddy currents and hence a magnetic field induced in the superconductor?

Can someone please explain

 

[withoutaface]

The eddy currents are induced when the magnet moves into the field of the superconductor, and those currents continue to flow due to negligable resistance, hence opposing the force that created them, and keeping the magnet afloat.

 

[Xayma]

If you have a magnetic field before it goes superconductive and there is no motion it wont hover, if however it is moved then it will begin floating.

 

[Jase]

well.. to get the magnet near the superconductor in the first place, a bit of moving has to be done right? It can't just materialise statically above the superconductor. So basically, it will always induce no matter how slight, because physically, it has to move into the range of the field in order to be considered interactive.

Oh wait woops.. i get what you mean Xayma, have a superconductor above critical temp and rest it on the magnet, then decrease temperature until superconductivity kicks in?
hmm, i guess nothing much would happen then? But i have no accurate idea of what really happens..
. I guess it wouldn't do anything then, but i honestly have no accurate idea.

 

[Xayma]

Ok the superconductor is a conductor (duh), if it has relative velocity between it and a magnetic field an eddy current will be created inside.

This eddy current is set up in such that a way that its magnetic field opposes the magnetic field that created this.

As there is no resistance the eddy current will continually increase in strength until it is at a point where the created magnetic field is strong enough to expel the external magnetic field and oppose it making it hover.

 

[speersy]

so how come if there is no relative motion between the magnet and the superconductor, the magnet still levitates for example

Q22 of success one HSC Physics
"when a small magnet is placed on a material that will become a superconductor, and then the superconductor material is cooled, it is observed that when the critical temperature for the superconductor is reached, the magnet begins to levitate and hovers above the superconductor. Explain why the magnet begins to levitate once the superconductor has been cooled to the critical temperature.

3 marks

And another problem say when a magnet hovers above a superconductor there would be a point when the motion of the magnet is zero. Then the force would stop and the magnet would fall. You might then say that the eddy currents then kick in and the magnet moves upwards. But how it is possible for the force of the magnetic field created by the eddy currents is such that it is larger than the magnetic field that created it

 

[Xayma]

In any realistic scenario there will be some relative motion.

It won't fall, the eddy currents once created do not stop due to a lack of internal resistance in the superconductor.

 

[speersy]

so what happens at the point when both the magnetic fields are balanced and the motion stops?

 

[Jase]

wait a sec... the very first bit of eddy currents induced can keep it levitating for a long long time, regardless of whether the magnet is in motion or not.

when the fields are balanced, it levitates, when the motion stops (if it ever does), it still levitates.

Im confused..

 

[Lazarus]

You're all right...

In any normal material, the eddy currents would dissipate after the motion stops, because they encounter resistance in the materal. This removes the repulsive magnetic field.

In a superconductor, there is (essentially) no resistance, and so the eddy currents continue circling and continue to generate an opposing magnetic field. This keeps the other magnet suspended above the superconductor. The fields are balanced.

 

[speersy]

Omg

Oh yeah...........
I finally got it. Thanks everbody

 

[Li0n]

If you want the full details
http://www.users.qwest.net/~csconductor/Experiment_Guide/Meissner Effect.htm

otherwise at HSC level its cool

 

[andysoul]

yeah, we had the same problem when we got told to do a superconductors assignment. from my research it seemed to be that the magnet will lift up, even if there is no relative motion between it and the superconductor when it goes below it's Tc. but we worked out that any small motion between the two would make it move, and that motion would make it move even more, until it got too far away for that effect to have any effect, a kind of positive feedback. when we tried it at school the magnet lifted off the superconductor when it got cold enough, but i agree with your theory that you can't isolate all of the relative motion, and once it has moved the tiniest bt it'll lift up to it's full height.

i wonder if the super conductor experiment has ever been tried in space, and they tried to isolate it from any movement? it'd be interesting to see what happened.

from my reading people explain the effect by the 'total exclusion of all magnetic fields from the superconductors interior' and that makes the magnet, hover, but i couldn't understand WHY

 

[junior04]

If you have a magnetic field before it goes superconductive and there is no motion it wont hover, if however it is moved then it will begin floating.

Doesnt the eddy currents in a superconductor expel all field lines, which would make the magnet hover even if there is no motion?

 

[Rakija]

Doesnt the eddy currents in a superconductor expel all field lines, which would make the magnet hover even if there is no motion?

You seem to be reading from textbooks that give oversimplistic explanations and then using that simplistic explanation to counter the detailed explanations given already. Many textbooks say that superconductors expel magnetic field lines, but this is because if we placed a magnet above a super conductor and let it go, gravity would force it downwards inducing eddy currents in he superconductor thus repelling it upwards by lenz's law. If we drew a diagram of the field lines in this scenario it would be clear that the superconductor expelled the magnetic field.

 

[junior04]

You seem to be reading from textbooks that give oversimplistic explanations and then using that simplistic explanation to counter the detailed explanations given already. Many textbooks say that superconductors expel magnetic field lines, but this is because if we placed a magnet above a super conductor and let it go, gravity would force it downwards inducing eddy currents in he superconductor thus repelling it upwards by lenz's law. If we drew a diagram of the field lines in this scenario it would be clear that the superconductor expelled the magnetic field.

Someone back me up on this but i think what your talking about (induced eddy currents from the magnet causing repultion by lenz's law, and so cause the magnet to float) is called perfect diamagnetism, whereas a superconductor actively excludes magnetic field lines even without relitive motion (the Meissner effect).

 

[helper]

You are correct but the arguement goes past the level of the HSC so I was staying out of it.

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

 

[junior04]

yeah i was thinking that, but it was getting personal so i had to win.

 

[mr_guy99493]

alternate problem

ook, how about if the magnet is held under the superconductor?
it will fall again, then lenz's law will kick in and suspend it below the superconductor?
our teacher is a dick so i when i asked him he pretty much told me that i am an idiot.
should also hold it to one side of the superconductor too shouldnt it... as long as its pretty close

 

[Xayma]

Doesnt the eddy currents in a superconductor expel all field lines, which would make the magnet hover even if there is no motion?

No it expels field lines that try to enter after it reaches the superconductive state.