# Superconductivity and magnetic fields

1. Jan 4, 2005

### mios76

Hello.

Do some materials have a magnetic field when electricity runs through them and they are in the superconductive state?

Do they lose energy because of that field?

Thank you.

Miro

Last edited: Jan 4, 2005
2. Jan 4, 2005

### marlon

The Meissner-effect states that magnetic field lines will be pushed out of a specimen in the superconductive state. Yet suppose you were looking at two magnetic monopoles which ofcourse are "connected " via a magnetic field. In the superconductive state the magnetic field lines must be pushed out due to Meissner but then again some field lines must remain because of the two present magnetic "charges". The result will be that some of the remaining field lines will be pushed together and thus forming the socalled fluxtube.

Keep in mind that this is a theoretical picture because no magnetic monopole has yet been observed in experiments. If you were to replace the electric field by the magnetic field you have a very interesting picture though : the magnetic monopoles are now two electrical charges, say two quarks. They are connected via an electrical field that must bring these charges over due to Gauss' law. But the dual Meissner now pushed out electrical fields in stead of magnetic fields and the superconducting state is now represented by dual Cooper pairs (you know : not two electrons in a pair but two magnetic monopoles). The two quarks will again have an electrical fluxtube between them along which we find a linear potential. This means, the closer the quarks, the more stable this state...This could form an explanation for the quarkconfinement to some level.

Check out my journal if you wanna know more...

regards
marlon

3. Jan 4, 2005

### Gokul43201

Staff Emeritus
A current running through any material will produce a mgnetic field. With a superconductor, you get to pass a lot more current than in a conventional metal, so you can make bigger magnetic fields. This is why high-field magnets are wound from superconducting wire.

And when you say "lose energy", what are you comparing to ? Are you suggesting that they radiate power, or do you mean something else ?

4. Jan 4, 2005

### mios76

I am sorry. I meant to write: “Do they lose energy because of that field?”
I wanted to know whether magnetic field being emitted by a current in the wire inhibits this wire to reach a superconductive state.

Miro

5. Jan 4, 2005

### Gokul43201

Staff Emeritus
Not in the sense that there is a loss of energy due to an "emitted field". A field is not emitted. But I suspect what you are really looking for is the critical magnetic field (Hc) for a superconductor.

When a superconductor is subjected to fields greater than some critical value, (which usually has a roughly $H_c ~ \alpha~1-(T/Tc)^2$ dependence on temperature) it loses its ability to exclude the applied field; or it loses its superconductivity. And since a current produces a magnetic field, you can reach this critical field simply by ramping up to a corresponding critical current (Ic).

6. Jan 5, 2005

### mios76

I suggest this conclusion:
A wire doesn’t lose its magnetic field produced by a current when it reaches a superconductive state.
However, this superconductor excludes all external magnetic fields. If we use magnetic fields, which exceed a critical value, then we lose superconductivity.
Of course, there are exemptions with some materials. If we use strong external magnetic field, which overcomes a critical value of the superconductor, then the superconductor interact with the external field without losing its own superconductivity.
I hope I’m not wrong.

Miro