# Why Faraday's law works with superconductors?

1. Aug 29, 2015

### DoobleD

1: Faraday's law states that a changing magnetic flux induces a voltage.

2: Voltage is the integral of E field with distance.

3: In a superconductor, or equivalently an with idealized wires having 0 resistance, there can't be any electric field inside. Thus, voltage in a superconductor should always be 0.

My question: how is it possible that Faraday's law induces a voltage in superconductors? I know it does, but I don't understand how it is possible.

Can there be some E fields during brief instants in superconductors? During the variation of magnetic flux?

2. Aug 29, 2015

### Staff: Mentor

Superconductors are free of internal magnetic fields (in the regions where they are superconducting).
Faraday allows to induce currents that cancel any electric fields that might arise - up to a limit where superconduction breaks down.

3. Aug 30, 2015

### DoobleD

But for the current to start flowing, isn't there at some point at least an E field inside the superconductor? Once current is flowing in it, ok, no need for any E field to make it keep going, but to start the flow?

4. Aug 30, 2015

### ZapperZ

Staff Emeritus
There's nothing that says that you cannot have a transient field. It is just that under equilibrium or steady-state situation, you do not have such fields inside the superconductor.

Besides, these currents reside on the surface, and they are in response to the external field on the surface up to the skin depth.

Zz.

5. Aug 30, 2015

Thanks. :)