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Where does the energy for Meissner effect come from? (1 Viewer)

ohrehman

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Hey all,

So there are numerous posts here about the Meissner effect and the conflicting "opinions" about it. What I haven't read though, is how a superconductor can oppose the gravitational pull of the Earth?

I don't believe you need to know this for the HSC, but I'm curious.
 

ThreeSciences

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The force placed on the superconducting material causes a change in magnetic flux. Due to Lenz's law, this change in magnet flux generates a current that gives rise to a magnetic field equal in magnitude but opposite in direction to the magnetic field that generated it. In superconductors, this perfectly cancels out, thereby resulting in the apparent levitation of a superconductor.
 

ohrehman

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Okay - but that explains Meissner effect. What I am asking is this:

Suppose you had a superconductor sitting on a magnet. As you pour liquid nitrogen over this superconductor, it will reach Tc and start levitating (due to the explanation you provided). Where does this GPE come from?
 

ThreeSciences

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Okay - but that explains Meissner effect. What I am asking is this:

Suppose you had a superconductor sitting on a magnet. As you pour liquid nitrogen over this superconductor, it will reach Tc and start levitating (due to the explanation you provided). Where does this GPE come from?
It doesn't, unless you have a strong enough magnetic field. If your magnet is stronger than the intrinsic critical magnetic field for your superconducting material, then the Meissner effect provides a force of repulsion, thereby levitating the material (or superconductor, depending on the way it's set up).

If the magnet isn't strong enough, then it or the superconducting material stay put. If this position just so happens to be in contact with the magnet, then no such levitation is observed.
 
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ohrehman

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Yeah - supposing it cools down below `Tc`, and then starts levitating. A friend of mine had an interesting explanation that the energy that levitates it is the energy that is input to initially cool down the superconductor. Which makes sense, because as the energy is radiated/ convected away, the superconductor will eventually drop down.

It doesn't explain any of the energy transformations and it seems a bit bodged, however. Again, none of this is in the syllabus, but it's bugging me that I still don't understand this.
 

shehan123

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Can i ask something that may be really stupid or correct, Isn't it true that the meissner effect cannot be displayed by placing a magnet above an already cooled superconductor? Because at this stage the eddy currents induced are causing the repulsion of the magnets through lenz law blah blah, moreover they dont dissipate because the superconductor is experiencing 0 resistance. But the meissner effect is defined as the expulsion of all field lines from within a superconductor. Therefore the only way to demonstrate this is to place a magnet over a metal, metal alloy, or compound, then cool it to critical temperatures. Superconductors are perfectly diamagnetic materials, that is, they expel all field lines to prevent their state from being destroyed. Hence in response to the already existing field lines of the external magnet, cooper pairs which travel in loops, will induce surface currents that have a magnetic field as to oppose the preexisting of the bar magnet, hence expelling all field lines (meissner effect). Following this is the levitation which is a consequence of the polarity of the surface current.
 

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