# Current in superconductors

1. Apr 17, 2010

### Razzor7

Say you have a superconducting loop of copper. If there's an increasing magnetic field through it that induces an E field around the loop, how are the electrons moving when the field stops increasing? As I understand it the current is preserved. Is there some way to calculate the energy possessed by the electrons using the magnetic energy density? Could we take it a step further and calculate the speed that the electrons are moving by the kinetic energy equation? I suspect this a quantum mechanical problem that transcends the classical picture of electrons moving.

2. Apr 18, 2010

### Redbelly98

Staff Emeritus
Since a superconductor has zero resistance, one cannot induce an E field around it. If one has a changing externally applied magnetic field Bext, the current in the loop will change so that the total magnetic flux remains constant:

$$EMF = \frac{d}{dt}\int B \cdot dA = \frac{d}{dt}\int (B_{ext} + B_I) \cdot dA = 0$$​

where BI is the field due to the coil current I. I will change so that the integral of BI cancels any changes in the integral of Bext.

3. Apr 18, 2010

### Razzor7

Thanks for that answer. Where are the electrons in a current carrying superconducting loop? It seems like they'd go toward the outer edge, centripetal force supplied by the coulomb attraction.

4. Apr 18, 2010

### Redbelly98

Staff Emeritus
I don't know.

5. Apr 27, 2010

### marlena21

Try to use google. I think I have seen somewhere info about it

6. Apr 27, 2010

### Mu naught

Are you asking where are the electrons with respect to the loop, or with respect to the wire they travel through?

A superconductor not only has zero resistivity, but also expels any magnetic field from it. Therefore, any current running through the super conducting wire, which produces a magnetic field, has to be along the surfaces of the wire, because there can be no magnetic field inside the superconductor. So yes, you are correct that all the electrons will be forced to the surface of the wire, but I think you are incorrect in assuming it is due to the circular motion of the electron. I'm also not sure what you mean by "the coulomb attraction" but the electrons would not like gathering all in one concentrated path, and should spread out evenly over the surface of the loop.

7. Apr 27, 2010

### marcusl

BTW, copper is not a superconductor.