Superconductors @#*$&!!!! (1 Viewer)

[Michaelmoo]

Ok. So I'm really geting fukn sik of this thing. The more I try to let it go, the more it comes back in a question to haunt me.

With regards to superconductors there are two views:

(i) Currents present in a superconductor such that T<Tc results in the expulsion of all magnetic fields. The net result is that the superconductor will exert an equal and opposite force on the magnet above it, resulting in its levitation. The precise details of this phenomenom are well beyond the scope of the syllabus. However, this is the TRUE reasoning behind the meissner effect.

(ii) A magnet above a superconductor such that T<Tc will continue to fall down (to the superconductor) due to gravity. This gives rise to a change in magnetic field, which in turn induces eddy currents in the superconductor. The net effect is that these eddy currents "oppose the original change that created them", i.e. expell the magnet and its magnetic field.

In reality, the second reasoning is completely wrong. Lenzes law does not apply to superconductivity.

Although, some fukn HSC references have decided to go with the second option (even though it's wrong). Sometimes, WHOLE questions can only be addressed using the second "theory".

Which one is widely accepted in the HSC. What do I use?

 

[Bdogz]

I've had the same issue, im goign with the 1st option since some credible notes that i have obtained said eddy currents are not induced. As for which is widely accepted in HSC i have no idea considering im sitting the exam in about 2 weeks lol

 

[helper]

You need to explain in 1 why there are currents. Say they analgous to eddy currents.

 

[Michaelmoo]

You need to explain in 1 why there are currents. Say they analgous to eddy currents.

I guess you could say that. Although I still think it would be dangerous making any reference to eddy currents. But like I said, some HSC books are making more than just a reference to it. They're saying they are eddy currents (which is wrong).

This leaves me worried.

 

[helper]

In the past, the idea of analgous to eddy currents has been accepted.

Or say that as the magnet approaches, currents are set up to produce a magnetic field to exclude the magnetic field from penetrating the superconductor.

 

[untouchablecuz]

becuase electrical resistance is zero, the eddy currents will continue to circulate indefinately and thus the magnetic field will be produced indefinately (assuming the semiconductor is continually cooled)

 

[Aerath]

What my teacher tells me to write is that below the critical temperature, the superconductor becomes diamagnetic, and expels the magnetic field, hence there is no magnetic flux through the superconductor. We don't need to know how.

He says that eddy currents is just wrong. =\

 

[boxhunter91]

It becomes a superconducter when the conductor goes from being paramagnetic to diamagnetic as Aerath said.

 

[helper]

Its a bit like the tuck and trailer diagram in most text books. Its incorrect physics but at high school level it is used to explain it.

 

[untouchablecuz]

BCS theory is also wrong but we still are required to learn it

conclusion: doing HSC physics does not make us physicists

 

[darkchild69]

Yep, as mentioned above, use the term "diamagnetic" this is when an object can create a magnetic field which opposes a magnetic field (or more correctly, a change in magnetic flux) which is incident on it and curve the lines of magnetic flux away from the material, or in the case of superconductors completely exclude them from the material. However, i would not say it changes from being paramagnetic to becoming diamagnetic as essentially all materials are diamagnetic at all times, it just that either ferromagnetism or paramagnetism usually overcomes this diamagentism as it is very weak, except for superconductors.

This is why in some Superconductor videos you see, when people have nfi they just try to slowly release the magnet and it doesn't hover... i.e., they havent set up the current to oppose the magnet!

In good SC videos, you will see the experimenter bring the magnet towards the SC quickly (setting up the current) then slowly withdraw the magnet and then slowly bring the magnet back towards the SC and make it balance

 

[untouchablecuz]

YouTube - How Superconducting Levitation Works

this video doesnt help

...the flux trapping effect? attractive and repulsive forces in equilibrium

dammit, this whole hsc course needs review

 

[clintmyster]

Despite knowing all this paramagnetic and diamagnetic, I know my school never mentioned these two words but they did say that these concepts (BCS mainly though the meisner effect to some extent) don't work just like textbooks like jacaranda describe. That being said, HSC markers aren't going to mark you down for the fact you said what the textbook says. I mean that would just be unfair to teachers teaching their students what they believe is 'within the syllabus'. Its like before we started 4u that people thought the squareroots of negative numbers wasn't possible. For the time being, it made sense because we were not really required to use this knowledge. Same applies for phys. We just need to say what happens as we see it. Whilst applying lenz's law and saying that eddy currents are induced and the magnetic field expels the magnet causing it to hover may be wrong, it should be enough.

 

[helper]

Heres another video that explains levitation using the meissner effect.

It also explains Maglex trains and how that isn't the meissner effect or flux pinning, which is implied in the first video.
:: AMPS - Superconductor Applications ::

 

[youngminii]

I think I'm gonna go remember diamagnetic and use that, even though my school/tutor never taught me that (my genius friend taught me what para/ferro/diamagnetic meant)
I've never even heard of explanation #2, I've personally always used #1

 

[Michaelmoo]

Yep, as mentioned above, use the term "diamagnetic" this is when an object can create a magnetic field which opposes a magnetic field (or more correctly, a change in magnetic flux) which is incident on it and curve the lines of magnetic flux away from the material, or in the case of superconductors completely exclude them from the material.

In reality, even this declaration is wrong. Some magnetic field does penetrate a small region of the superconductor. This region is known as the "London penetration depth". The magnetic field's ability to continue penetrating will deteriorate exponentially as it progresses through. This interaction plays an important role with regards to the surface currents (located in the Lpd), which in turn gives rise to the superconductor's diamagnetic properties.

That's another thing that confused me. But I guess that's beyond the HSC course, we should stick with as you said "completely excludes".

HSC physics is definately superficial. Why the fuck are they simplifying these concepts? They're just making it harder and more confusing for us.

 

[study-freak]

In reality, even this declaration is wrong. Some magnetic field does penetrate a small region of the superconductor. This region is known as the "London penetration depth". The magnetic field's ability to continue penetrating will deteriorate exponentially as it progresses through. This interaction plays an important role with regards to the surface currents (located in the Lpd), which in turn gives rise to the superconductor's diamagnetic properties.

That's another thing that confused me. But I guess that's beyond the HSC course, we should stick with as you said "completely excludes".

HSC physics is definately superficial. Why the fuck are they simplifying these concepts? They're just making it harder and more confusing for us.

+1

But it's also that we can't possibly learn the full theory because they require the knowledge of quantum mechanics, etc, that we simply don't have time to learn.

But BCS and suprconductivity shown in HSC textbooks are definitely confusing/wrong.

 

[MrMMMan]

Wikipedia is sexy