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Superconducting coil

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boris
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Post Number: 165
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Posted on Monday, 16 April, 2007 - 03:20 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If a coil/inductor resists the rise of current because the magnetic fields push against each other, wouldn’t that mean that a superconducting coil would never allow any current to flow and act like an insulator?
I love deadlines. I like the whooshing sound they make as they fly by.
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obiwan
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Posted on Monday, 16 April, 2007 - 03:35 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Well, apparently not, because they use them in super coliders. They exist, simple. And they act like super magnets.
Do Not Hit The Fly That Lands On The Tigers Head.
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vlf
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Posted on Monday, 16 April, 2007 - 09:04 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Looks like your going down the Scalar Field area !!

Brian.
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epithumia
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Posted on Monday, 16 April, 2007 - 09:12 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The maths used to explain how current rises in an inductor doesn't actually involve the resistance of the coil.

So in a superconducting coil the maths is unchanged.

I had a quick look at 'inductance' at Wikipedia.com; the maths is there but I didn't work through it all.

The end result is the equation:

V = L di/dt
or
Volts = Inductance * (rate of change of current)

With no resistance in the circuit, an applied voltage causes the current to rise in a straight line.

Rob
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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boris
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Posted on Tuesday, 17 April, 2007 - 04:33 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Perhaps I misunderstand how inductors work.

A current flowing through a wire creates a magnetic field around the wire. Another wire in close proximity will have a current induced into it as the incident field lines expand and pass through it. But the induced current produces its own magnetic field which is opposite to the original (a reflection), thereby causing the two fields to push against one another, resisting the growth of each. It is only the losses that allow the fields to grow at all. If there were no losses and the reflected field was the exact same magnitude as the incident field, then surely neither field could grow?

Everyone has seen the floating magnet demonstration of how superconductors can produce zero loss magnetic interactions. This shows how a magnetic field when moved towards a superconductor will generate an equal and opposite field in the conductor. Like a magnetic mirror.

So my question is how can the fields push through each other in a superconducting coil?
I love deadlines. I like the whooshing sound they make as they fly by.
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armadillo
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Posted on Tuesday, 17 April, 2007 - 06:34 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)


quote:

I had a quick look at 'inductance' at Wikipedia.com; the maths is there but I didn't work through it all.

The end result is the equation:

V = L di/dt
or
Volts = Inductance * (rate of change of current)




I can't remember the maths but I do remember, the correct formula is:-
E = -L df/dt where
E = voltage
L = Inductance
df/dt = rate of change of magnetic flux.

Note the minus sign. A sudden drop in flux causes a +ve voltage.
Of course the flux change can be caused by the current change but it can also be caused by moving a magnet past the coil.

Just thought I stick my oar in the pond

Armadillo (UK)
There's no such thing as gravity..........
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epithumia
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Posted on Tuesday, 17 April, 2007 - 10:17 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

>It is only the losses that allow the fields to grow at all.

I admit to being a bit rusty on superconductors and the maths, but I don't think it's the losses that allow the fields to grow. I don't want to sound authoritative here, because this really is making me think.

Imagine an ideal voltage source connected to an ideal inductor. That is, there is no resistance anywhere in the circuit.

OK, so we have a non-zero voltage across the inductor, which has zero resistance. So the back EMF of the inductor must equal the applied voltage.

But that voltage can only be created by a changing magnetic flux, so the flux must increase as a linear rate so continue opposing the applied voltage. flux is proportional to current, so current increases linearly.

So the classic behaviour of inductors does not appear to rely on losses.

Granted, superconductors have some bizarre properties which may or may not invalidate this argument.

The floating magnet example is very interesting, but not quite the same thing. As a coil is energised, energy is stored in the coil:

Energy = (L * I2)/2

Similarly, when the magnet approaches the superconductor, energy is stored in the coil.

But while the magnet floats, no energy is being used because the magnet isn't moving.

I find that idea fascinating: that something can be held up without using energy. I know it's true, but have trouble getting my mind around it!

I suppose in a sense it's obvious: stack bricks and the lower bricks hold up the higher bricks without a power source even though the bricks are just atoms which are mostly empty space...

Hi, Armadillo,
You're right about the negative sign, but I think the equation you're thinking of is Faraday's Law:

E = - N df/dt

http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction

Rob
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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armadillo
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Post Number: 155
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Posted on Tuesday, 17 April, 2007 - 02:34 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Could be - my memory isn't what it was and it's 45yrs since I had any use for it as a formula.
The only thing I would worry about is using Wikpedia as a reference - I'm told that it's littered with errors.

I'll bow out at this point my knowledge of super conductors is approximately zero.

Armadillo
There's no such thing as gravity..........
The earth sucks!
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eagre
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Posted on Thursday, 19 April, 2007 - 01:23 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Armadillo's comment about Wikipedia is certainly appropriate. Any idiot can edit it, including those pushing such nonsense as creationism. That said, there is also much useful information there from dedicated contributors.

As far as circuits under superconductivity are concerned, the basic laws of electromagnitism are in no way altered; R is just 0. The math is unchanged. Just don't divide by R.

Ed
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obiwan
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Posted on Thursday, 19 April, 2007 - 02:17 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I should know more about this, but have forgotten much.

In the circuit breakers we designed, we ran into this very problem several times. Not super conducting mind you, but still, close to what you're describing about coils and their interaction with each winding.

If you look at a circuit breaker, take one apart, you'll notice that they actually use this principle. The contacts don't just run straight through a breaker, like in a straight line.

The contacts are actually "folded" back on each other. like taking a straight piece of wire, and giving it a small "z" fold in the middle of it. the actual contacts would be right in the center of that "z" fold.

The reason being that as the current increased, the breaker would actually use the increasing magnetic field to help open the breaker.

And at the higher currents, the magnetic field would actually help keep the breaker FROM opening! As the current increased, it would help keep the breaker closed.

So, the theory being, at high currents, (we worked with thousands of amps), at the current increased so fast and so high, the computer may not be able to control things, so they relied on the magnetic fields to open the breaker.

Worked well too.

But we also ran into with our current transformer design. This is really hard to explain.

But I think the part that concerns you, is the actually close proximity of the windings. if they're close together, as in a "regular" coil, they act as one, one winding is the same as the other. So you don't experience what you're thinking of.

But we used what is called a "split bobbin". Essentially a rectangular torrid coil. Since the coil was so large, it was much easier to build the coil in two sections, much like power transformers using E and I plate sections, and assemble them, instead of trying to build a regular (true) torrid coil.

It consisted of a coil of wire on top of the buss bar, and a coil on the bottom, and they each were connected (magnetically) with rectangular "L" plates.

But! During the power cycle, we would experince VERY high voltages. Without really trying to explain what was going on, it amounted to the top coil fighting the bottom coil.

One was trying to create a HIGH current to match the current it should have been trying to put out, While the lower coil was trying to put out a smaller current to match the curent IT saw.

And since they were fighting each other, and because of the way current transformers operate, they were trying to generate currents not normally seen on the planet earth. (but never got there, because A) it's impossible, and B) they were fighting each other).

But, we didn't see this in the real circular toroids we did manage to wind.

So basically, it had to do with the way the coil was wound. As long as the coils were wound normally, wires next to each other, they worked. But separate the wires even by a little, you got those high current spikes.

So, as long as the wires were next to each other, the acted as one, one big coil, but separate them even a little, and you got trouble, sort of like what you're describing.

I hope that makes sense. Because I don't think I can describe it any further than that.

And you would not believe how long it took us to figure all of this out at work. And what a mad house too, potentially, every breaker we have out there in the field was a time bomb just waiting to go off.
Do Not Hit The Fly That Lands On The Tigers Head.

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