Contradictory Physics (1 Viewer)

[SlaminSammy]

We HSC students are being robbed of correct physics understandings.

From what i have read! Most texts books say two things

1 -- That a superconductor excludes a magnetic feild!
2 -- That the meisner effect is created by induced eddy currents composed of cooper pairs that oppose the magnetic field which created them (lenzs law) and hence levitate a magnet

The second point (which every stupid textbook and internet site states) contradicts the first point! To induce a circular eddy current the magnetic feild of a magnet must "thread" the conducting material. BUT IF WERE TALKING ABOUT SUPERCONDUCTORS WHICH ARE DIAMAGNETIC (magnetic feild excluding) MATERIAL WHEN THEIR TEMPERATURE IS COOLED TO NEAR ABSOLUTE ZERO - then eddy currents cannot be created.

If anyone understands why a magnet levitates on its own field please explain this. DONT GIVE MISHMASS EXPLAINATIONS OF THE REPULSION OF THE MAGNET BY EDDY CURRENTS PLEASE! - it is untrue. thanks

 

[helper]

Firstly no text bookhas what you said because a current can't oppose a magnetic field. The magnetic field produced by the currents may.

Secondly superconductors are not diamagnetic

Thirdly, you have discovered why the syllabus does explain the cause of the magnetic effect, rather why the magnet hovers.

You might want to and read up on the London Effect as one possibility.
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/chrlen.html
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html

 

[serge]

There is a magnetic field strength that will stop the meisner effect
(ie. will stop superconductivity as well) It's called critical flux density

BUT IF WERE TALKING ABOUT SUPERCONDUCTORS WHICH ARE DIAMAGNETIC (magnetic feild excluding) MATERIAL WHEN THEIR TEMPERATURE IS COOLED TO NEAR ABSOLUTE ZERO

it doesnt have to be near absolute zero,
thats just crappy old superconductors
(new one's need about 150K)

 

[codey dog]

The term is contradictory. I was in macquarie uni the otherday and even asked a professor. They said that impurities in the superconductor that are not super conducting but are none the less in contact with it get "pinned down" and they start the current when the magnetic field passes through the impurity but not the superconductor. The guy couldn't give me a straight answer. Though if you stick to the sylabus and don't actively contradict yourself when answering a question on the miesner affect you can't go wrong. Hope that helps.

 

[rama_v]

I have a feeling this is one of those thigns that needs super high level mathematics to explain

 

[Haku]

after reading all that i really don;t see a contradiction. when they say the superconductor exclude the B field it mean that the flux lines don;t cut it. well as "helper" stated, that the eddy currents induced in the superconductor fully oppose the external applied B field is also wrong. How can current oppose a field? u gotta state that the induced current will also induce an B field that exactly equal and opposite to the external applied B field, this result in next B field being zero. ie, support ur first point, superconductor exclude B fields.

so ur contradiction so doesn;t contradict.

 

[Captain Gh3y]

I like how in that guide from the SMH, in the physics section, it says specifically NOT to EVER give information beyond the scope of the syllabus because you risk contradicting yourself and losing marks.

Why do they teach us this stuff it it's not right?

 

[richz]

well my teacher said that the meisner effect isnt due to lenz's law...

 

[helper]

Nosadness, what he was trying to say was what you said, but to induce the currents a magnetic field is needed to penetrate the surface.
As Codey has said and what the London effect is about, is trying to explain how this active exclusion works. The one thing they are sure about it is not a plain application of Lenz's law as in a normal zero resistance diamagnet.

 

[helper]

Why do they teach us this stuff it it's not right?

Science is not a fixed set of facts, you are taught to what is considered to be an appropriate model for what you are looking at.
Eg Parabolic motion and Newton's Laws are contradicted by Einstein's theory of general relativity. But if your teacher tried to teach relativity and quantum mechanices to year 7 or 8, when motion and energy are first taught, they wouldn't have a clue what you are talking about.

 

[Captain Gh3y]

Yeah I know, it's just, to paraphrase our Italian friend,

"Everything for the Syllabus; nothing outside the Syllabus; nothing against the Syllabus,"

 

[helper]

well my teacher said that the meisner effect isnt due to lenz's law...

This could mean two things:
Lenz's law is a law. Scientific laws describe things, but do not explain them, so Lenz's law is describing a different phenomenom, while the syllabus asks you to explain them. That is it after the theory. Same problem occured in 2002, q23.b.ii., when people tried to explain an effect in terms of lenz's law, when lenz's law can only be used to describe what occurs.

OR
As mentioned above, the mechanism of the active eclusion of magnetic fields is still under scientific debate. The one thing that it is clear, it is not just a simple application of Lenz's law.
So yes you are talking about a magnetic field being brought near the surface of a supermagnet. The Meissner effect then talks about the active exclusion of magnetic fields from penetrating the surface. Induced currents are set up in the superconductor, which produce a magnetic field, that results in a magnetic force upwards, that equals the gravitational force downwards, resulting in levitation.
This is then explained further at:
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html

 

[richz]

yes i wud agree with ur second statement.

 

[helper]

More everything for the markers as it is only them who will decide which is right.
And remember just because it is in a text book, it won't mean it is marked correct by the markers.

 

[richz]

lol, thats true, the markers rely on the text books too

 

[helper]

There has been cases in the past, where even though it is in a text book, the examination commitee has stated that the text book is wrong, so that answer is not acceptable.

 

[richz]

has there?? cuz thats kinda stupid, its not the students fault the book is incorrect is it....

 

[helper]

I know of it happening in Biology. The BOS answer was that they do not prescribe a text book, so they can't be held responsible for one that is wrong.

 

[Haku]

so basically saying, in an exam if they ask u the meiseners effect u can no say that the superconductor exclude the magnetic field and at the same time the external magnet induce a current that creats a field that oppose the external magnetic field and gravitational force so that it cause levitation?

as if u say a current is induced that basically mean that the flux lines do cut the conductor...strange.

oh the other hand if u just say it excludes it, thats like a 2 sentence answer...

so helper, what should we say if they ask us to explain the meisenner effect?

 

[helper]

No, you can say what your saying, it is going beyond the syllabus to try and explain why thereis an induced current, so ignore the apparent contradiction.