Level of detail. (1 Viewer)

[Nooblet94]

How much detail do we need to go into for the Meissner Effect? Do we need to know about vortex pinning and whatnot, or is it sufficient to simply say that there's an induced magnetic field that opposes the field of the magnet causing it to float?

Same sort of question for superconductivity in general; do we need to go into all the detail about how/why cooper pairs form? Or do we just say stuff like there's a net positive charge blah blah so cooper pairs form that can pass through the lattice with 0 resistance?

I've heard different things from two different teachers.

 

[Shadowless]

How much detail do we need to go into for the Meissner Effect? Do we need to know about vortex pinning and whatnot, or is it sufficient to simply say that there's an induced magnetic field that opposes the field of the magnet causing it to float?

Same sort of question for superconductivity in general; do we need to go into all the detail about how/why cooper pairs form? Or do we just say stuff like there's a net positive charge blah blah so cooper pairs form that can pass through the lattice with 0 resistance?

I've heard different things from two different teachers.

I'd also like to know... also there's something about phonons and stuff, do we address that in the Cooper pair type of questions?

 

[deswa1]

I don't think you need to know about vortex pinning for Meissner (I thought it was quantam pinning but they'll be the same thing). For BCS theory, I would probs go along the lines of the lattice distortions -> positive charge -> phonons -> creates Cooper pairs which place the electrons in a lower energy state and therefore no resistance

 

[Nooblet94]

I don't think you need to know about vortex pinning for Meissner (I thought it was quantam pinning but they'll be the same thing). For BCS theory, I would probs go along the lines of the lattice distortions -> positive charge -> phonons -> creates Cooper pairs which place the electrons in a lower energy state and therefore no resistance

So for Meissner you would just go B field creates perpetual current in superconductor which has its own B field that repels the field of the magnet? Would you go into field expulsion too? The syllabus is so vague

 

[deswa1]

So for Meissner you would just go B field creates perpetual current in superconductor which has its own B field that repels the field of the magnet? Would you go into field expulsion too? The syllabus is so vague

Yeah I know- it pisses me off. I think if we had a question on Meissner, I would answer it like this:

The Meissner Effect refers to the expulsion of a magnetic field from a superconductor. In its superconducting state, surface currents flow to a penetration depth which induce a magnetic field that expels the external magnetic field entirely from the superconductor. These currents can flow permanently due to zero internal electrical resistance. This expulsion of the magnetic field is what allows the superconductor to 'float' above a magnet- note that this is distinct from the formation of Eddy currents as a change of flux is not neccessary.

Obviously it depends on the mark value of the question- I might not write all that + in a test I would probs be more to the point but this is just my general idea. Also, if I missed anything in the above or made mistakes, can you point them out

 

[someth1ng]

The Meissner effect is a properly of superconductors where a superconductor will produce persistent currents within itself to oppose and cancel out any magnetic flux that attempts to penetrate the superconductor and hence, all magnetic flux is expelled from the superconductor.

Pretty similar to deswa1's definition of the Meissner effect.

As for the Cooper pairs one, you need to know about phonons (transfer), region of net positive charge, attraction of a second election and hence, zero resistance.

 

[RishBonjour]

Also, for BCS theory, i think its wise to stick to whats in most text books (adjacent electrons create cooper pairs) rather than saying cooper pairs can form from different lattice "levels" (which, according to some syd uni guy is "true")
Its a theory after all, many believe its absolutely bs.

 

[Nooblet94]

Just while we're talking about it, I love how BCS theory is actually wrong but we're taught it anyway.

 

[deswa1]

Just while we're talking about it, I love how BCS theory is actually wrong but we're taught it anyway.

Is it wrong? I thought it was correct for Type 1 superconductors but didn't explain Type 2? I might be wrong though. As far as I know, they've still got no idea how Type 2 superconductors work.

My personal favourite is in industrial chem for the extraction of sulfur and they teach us the Frasch Process as the only/dominant way to get sulfur. From wikipedia: "As of 2011, the only operating Frasch mines worldwide are in Poland and since 2010 in Mexico. The last mine operating in the United States closed in 2000."

So yeah, I'm pretty sure that this is NOT the main way they get sulfur nowadays... (it is though what they used to do and its pretty simple to understand so that's why they teach this instead of the Claus Process)

 

[someth1ng]

The BCS theory isn't necessarily wrong. The "BCS theory" that they teach us in HSC is not actually the BCS theory.

The REAL BCS theory is about electrons of opposite spin forming pairs (Cooper pairs) which give a net spin of 0 - like bosons which do not need to follow the Pauli exclusion principle. This means that all the pairs can occupy the lowest energy state and hence, cannot lose energy.

The problem with the BCS theory is that it can be applied to Type I superconductors but not type II superconductors because HTS have temperatures that would theoretically break any Cooper pair that could form.

 

[deswa1]

The BCS theory is necessarily wrong. The "BCS theory" that they teach us in HSC is not actually the BCS theory.

The REAL BCS theory is about electrons of opposite spin forming pairs (Cooper pairs) which give a net spin of 0 - like bosons which do not need to follow the Pauli exclusion principle. This means that all the pairs can occupy the lowest energy state and hence, cannot lose energy.

The problem with the BCS theory is that it can be applied to Type I superconductors but not type II superconductors because HTS have temperatures that would theoretically break any Cooper pair that could form.

Oh right- thanks for the explanation Haha I've noticed that with a lot of HSC phys/chem- they teach some very advanced concepts but oversimplify them so we can understand them at the expense of being totally accurate

 

[someth1ng]

Yeah, they definitely do that quite a fair bit - they lose the whole idea but I guess it's possible to build on from the knowledge they give us.

 

[Nooblet94]

They really shouldn't teach us advanced stuff like that without proper explanation IMO. It just leads to everyone going "but how/why does that even work?" and the teacher floundering because they themselves have no idea either.

 

[RishBonjour]

They really shouldn't teach us advanced stuff like that without proper explanation IMO. It just leads to everyone going "but how/why does that even work?" and the teacher floundering because they themselves have no idea either.

this. e.g. for planck/einstein and black body, none of the text books / teachers explain it properly. even past hsc answers. ".. planck hypothesed e=hf and that solved every problem we ever had.."

 

[GoldyOrNugget]

Hey guys, wanna see something funny?

Many popular histories of physics, as well as a number of physics textbooks, present an incorrect version of the history of the ultraviolet catastrophe. In this version, the "catastrophe" was first noticed by Planck, who developed his formula in response. In fact Planck never concerned himself with this aspect of the problem, because he did not believe that the equipartition theorem was fundamental – his motivation for introducing "quanta" was entirely different. That Planck's proposal happened to provide a solution for it was realized much later, as stated above.[3] Though this has been known by historians for many decades, the historically incorrect version persists, in part because Planck's actual motivations for the proposal of the quantum are complicated and difficult to summarize to a lay audience.[4]

(Wikipedia, 'Ultraviolet Catastrophe')

 

[deswa1]

this. e.g. for planck/einstein and black body, none of the text books / teachers explain it properly. even past hsc answers. ".. planck hypothesed e=hf and that solved every problem we ever had.."

Do you know why they don't teach us why E=hf solved the ultraviolet catastrophe? Its because this is the equation to model the black body radiation curve:

I mean that's obviously too simple for HSC- they don't want everyone getting full marks

Shit haha though- it shows what some real physics is like

 

[RishBonjour]

Do you know why they don't teach us why E=hf solved the ultraviolet catastrophe? Its because this is the equation to model the black body radiation curve:

I mean that's obviously too simple for HSC- they don't want everyone getting full marks

Shit haha though- it shows what some real physics is like

haha, i searched up that a while back
Some crazy stuff. Yeah from what I hear hsc physics does NOTHING to help those studying physics in uni.
I think they realize not everyone does maths (e.g. this kid does general maths and physics - struggles hard)

 

[someth1ng]

To be honest, physics should be physics. Not a humanities subject. If people pick physics, give them physics.

 

[chriss95]

To be honest, physics should be physics. Not a humanities subject. If people pick physics, give them physics.

like differing views of planck and einstein lol

 

[kiinto]

How much detail do we need to go into for the Meissner Effect? Do we need to know about vortex pinning and whatnot, or is it sufficient to simply say that there's an induced magnetic field that opposes the field of the magnet causing it to float?

Same sort of question for superconductivity in general; do we need to go into all the detail about how/why cooper pairs form? Or do we just say stuff like there's a net positive charge blah blah so cooper pairs form that can pass through the lattice with 0 resistance?

I've heard different things from two different teachers.

Meisner effect is separate to induced magnetic fields. You should've done a practical at some point to prove this: Put a magnet on the material, then lower it below it's critical temperature. The magnetic will be observed to float up from the superconductor proving it is separate from induced magnetic fields, as there was no change in magnetic flux. In fact, an induced magnetic field should have acted against the upward motion of the magnet.