superconductivity - yes there are 10's of threads but this question is different (1 Viewer)

[Squar3root]

I understand that a permanent magnet can hover above a superconductor, but why does it do that?
i know the meissner effect expels the internal magnetic field, but there is no change in magnetic flux
so shouldn't it just like rest on the surface?

 

[iBibah]

When the super conductor is below critical temperature, resistance is zero. So when the initial current is induced due to Lenz's Law, it just keeps going as there is no resistance. And the magnetic poles created stay there as a result. Hence the hovering.

 

[Squar3root]

When the super conductor is below critical temperature, resistance is zero. So when the initial current is induced due to Lenz's Law, it just keeps going as there is no resistance. And the magnetic poles created stay there as a result. Hence the hovering.

don't eddy currents "disappear" eventually?

 

[iBibah]

View attachment 28732

don't eddy currents "disappear" eventually?

I'm not trolling at all. Think about how eddy currents "disappear". They leave as heat due to meeting resistance. But when a superconductor is below critical temperature, resistance is zero, so how else can the eddy current disappear?

Why do you think superconductors are so good, because we could transmit electricity at 100% efficiency, because none is lost, because there is no resistance.

Not sure why you thought i was joking lol

 

[someth1ng]

When the super conductor is below critical temperature, resistance is zero. So when the initial current is induced due to Lenz's Law, it just keeps going as there is no resistance. And the magnetic poles created stay there as a result. Hence the hovering.

This is wrong.

Read on diamagnetism - persistent currents are not related to eddy currents.

 

[iBibah]

This is wrong.

Read on diamagnetism - persistent currents are not related to eddy currents.

I know they are not eddy currents generated, but i thought for the HSC we are allowed to call them that? A question on this is in 2003 HSC Q23a, while marking notes say "current" alone, there is no mention of eddy currents. Is it accepted in the HSC?

 

[someth1ng]

I know they are not eddy currents generated, but i thought for the HSC we are allowed to call them that? A question on this is in 2003 HSC Q23a, while marking notes say "current" alone, there is no mention of eddy currents. Is it accepted in the HSC?

No, you cannot say that they are eddy currents.

 

[iBibah]

No, you cannot say that they are eddy currents.

I've been deceived....

thanks for your help!

 

[Squar3root]

I've been deceived....

thanks for your help!

We both learned something new today, threat could be in his years HSC

 

[Randox]

So are we supposed to say its just due to the fact that:... a superconductor (when cooled below its critical temperature) will exclude magnetic fields. This will cause a magnet to levitate assuming the weight force of the magnet can be balanced (the magnet doesn't weigh too much) with the repulsion force between the superconductor and magnet.

If this is correct, when do we talk about eddy currents in regards to meissner effect/superconductors. All the HSC answers don't talk about eddy currents but for some reason all the textbooks do..

So confused.

 

[Squar3root]

So are we supposed to say its just due to the fact that:... a superconductor (when cooled below its critical temperature) will exclude magnetic fields. This will cause a magnet to levitate assuming the weight force of the magnet can be balanced (the magnet doesn't weigh too much) with the repulsion force between the superconductor and magnet.

If this is correct, when do we talk about eddy currents in regards to meissner effect/superconductors. All the HSC answers don't talk about eddy currents but for some reason all the textbooks do..

So confused.

Yeah, physics in focus explicitly says eddy currents but not once have I seen success one mention it

i magnet's weight has no effect. i was able to stand on it and still hover

 

[Fizzy_Cyst]

Eddy currents arise due to Lenz/Faraday (change in flux) and oppose motion of conductor etc.., currents in a superconductor to not arise due to this and do not oppose motion of the conductor, they arise due to the Meissner Effect.

Just term them 'current loops'

The whole 'explain why a magnet levitates above a superconductor' is ridiculous how they want you to explain it anyways. Complete and utter crap!

They want you to say what Randox said, but the magnets dont just levitate, they are also suspended underneath the superconductor if you turn it upside down! Really, you need to look into flux trapping or 'quantum locking' to explain the phenomenon more completely

 

[Web Addict]

Wait... So, what do you actually write in the HSC exams?

 

[study1234]

Wait... So, what do you actually write in the HSC exams?

I'm confused too. Can someone please confirm?

 

[tutorhunt]

Personally, I think you could get away with "As a material reaches critical temperature, it begins to actively exclude magnetic fields from its insides, thus achieving near perfect diamagnetism, this phenomenon is the Meisner Effect. As a permanent magnet is placed within range of the superconductor, the superconductor develops current loops on its surface, adopting a magnetic field that repels the permanent magnetic but also levitating it, this is a demonstration of the Meisner Effect." I read a bit on the Quantum Vortex and flux pinning, and they seem to be different to the Messiner effect. I don't think they are in the HSC syllabus either, and I know for a fact that many teachers do not understand nor even know many Quantum effects, I'd rather be safe then sorry, I'd stick with basic syllabus answers, but do not call the currents eddy currents, completely different things.

 

[hjed]

The messiner effect is cause by currents in the superconductor making it act like a solenoid, but this is not induced by the movement of the magnet. Basically the model in the physics syllabus for how Eddy currents are induced doesn't work in superconductors. Also interesting fact, but the meissner effect only works in type 1 semiconductors, it gets really weird in type 2s (but we don't need to know that)
BTW We had a question in the trial where I lost marks for saying that the messner effect was caused by lenz's law inducing eddy currents.

 

[someth1ng]

I don't think they are in the HSC syllabus either, and I know for a fact that many teachers do not understand nor even know many Quantum effects, I'd rather be safe then sorry, I'd stick with basic syllabus answers, but do not call the currents eddy currents, completely different things.

Yeah, that stuff isn't in the syllabus...and nobody understands quantum mechanics, don't worry.

 

[Fizzy_Cyst]

Yeah, that stuff isn't in the syllabus...and nobody understands quantum mechanics, don't worry.

I understood it til all the numbers changed to letters.

If you are going to stray outside of the syllabus and talk about flux trapping or quantum locking, make sure you cover what is in the syllabus first!

 

[tutorhunt]

Yep, I still find it silly that they make us explain these quantum phenomenas with literally no understanding of quantum physics ourselves. :x

 

[anomalousdecay]

I understood it til all the numbers changed to letters.

If you are going to stray outside of the syllabus and talk about flux trapping or quantum locking, make sure you cover what is in the syllabus first!

Can we just say that the perfect diamagnetism is caused by surface current loops which prevent a flux change in the superconductor, so that all magnetic fields are excluded inside the superconductor?

Yep, I still find it silly that they make us explain these quantum phenomenas with literally no understanding of quantum physics ourselves. :x

That's why you do Age of Silicon lol. Very logical topic.