Meissener Effect (1 Viewer)

[iheartOJ]

Hi Everyone!

I'm a little confused on how the Meissener Effect works.

According to a few textbooks the meissener effect is the result of Eddy Currents: "When a superconductor is below its critical temperature, when an external magnetic field attempts to enter a superconductor it induces a eddy current to circulate in the superconductor"

But according to other resources the Meissener effect isn't a result of Eddy Currents but is because the superconductor acts as a diamagnet as when a "magnet is on a superconductor as it is being cooled, it will jump into the air as the superconductor becomes superconducting- this shows it is not an induction phenomenon as change in magnetic flux is required to induce eddy currents. Therefore the levitation occurs due to the exclusion of magnetic fields from the superconductor"

So which explanation of the Meissener effect is correct????

 

[someth1ng]

But according to other resources the Meissener effect isn't a result of Eddy Currents but is because the superconductor acts as a diamagnet as when a "magnet is on a superconductor as it is being cooled, it will jump into the air as the superconductor becomes superconducting- this shows it is not an induction phenomenon as change in magnetic flux is required to induce eddy currents. Therefore the levitation occurs due to the exclusion of magnetic fields from the superconductor"

This is the correct explanation.

 

[iheartOJ]

This is the correct explanation.

Thanks someth1ng, thats what I thought as well!

But I got a little confused when looking at the 2003 HSC Q23 the notes from the marking centre said: "Many candidates simply described the Meissner effect without explaining the balance between the weight force and the force of magnetic repulsion resulting from the interaction of the fields of the induced current and hovering magnet." :S

 

[asianese]

According to a few textbooks the meissener effect is the result of Eddy Currents: "When a superconductor is below its critical temperature, when an external magnetic field attempts to enter a superconductor it induces a eddy current to circulate in the superconductor"

This is called diamagnetism. It is a well known effect. A superconductor that is ALREADY COOLED has a magnet brought near it. Eddy currents are induced on the surface and then the magnet is repelled.




Meisner effect is when the (warm) superconductor and the magnet are already in the same vicinity. THEN super cooling occurs (pour liquid nitrogen etc). Spontaneously the magnet is repelled by the superconductor. This is the Meisner effect. Not that there was already magnetic fields through the superconductor before it was cooled, but the meisner effect is when it spontaneously repels after it is cooled.

 

[someth1ng]

Thanks someth1ng, thats what I thought as well!

But I got a little confused when looking at the 2003 HSC Q23 the notes from the marking centre said: "Many candidates simply described the Meissner effect without explaining the balance between the weight force and the force of magnetic repulsion resulting from the interaction of the fields of the induced current and hovering magnet." :S

This is one of the many stupid things about HSC sciences. The way they worded it was quite poor and so, that's where the confusion comes from.

The sentence should really be something like: "Many candidates simply described the Meissner effect without explaining the balance between the weight force and the force of magnetic repulsion resulting from the interaction of the magnetic fields of the persistent currents and hovering magnet."

The only time that their wording is right is if you have a superconductor THEN move the magnet close to the superconductor - that would be eddy currents (diamagnetism, induction etc) because there was a change of magnetic flux but in this case, that's not correct.

 

[asianese]

Lol HSC Phys.

 

[iheartOJ]

The only time that their wording is right is if you have a superconductor THEN move the magnet close to the superconductor - that would be eddy currents (diamagnetism, induction etc) because there was a change of magnetic flux but in this case, that's not correct.

Ahh... I see what you mean!
So if the magnet is already on the 'superconductor' and then it was cooled the magnet would levitate because the superconductor acts as a diamagnet. But if the superconductor was already cooled and the magnet moved towards the superconductor the magnet would levitate because of eddy currents?
Okay, so I think I get it now!
Thanks so much!!

 

[iheartOJ]

Lol HSC Phys.

I know right! :/

 

[someth1ng]

The superconductor doesn't act as a diamagnet. He was referring to what was quoted. Diamagnetism and eddy currents are the same phenomena in this case.

The superconductor would produce persistent currents if the magnet was on it before it was cooled (there was no change of flux).