Impedance in a superconductor

Main Question or Discussion Point

Okay, I know in standard inductors the coil itself causes electrical resistance when AC current is passed through it, but I was wondering if a superconductive coil below its Tc has the same effect? Is there any impedance because the meisser effect does not allow outside magnetic fields to penetrate the substance when in a superconductive state. So am I missing something or what would happen?

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Answers and Replies

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f95toli
Science Advisor
Gold Member
Superconductors at AC behave more or less just as very good conductors; the details are of course different from a normal metal but superconductors do exhibit a frequency dependent ohmic loss.
Moreover, if you make an inductor (or capacitor) out of a superconductor the inductance (capacitance) will be more or less the same as for a normal metal. The main difference comes from the fact that you also have a kinetic inductance (which can account for a few percent of the total L).

Maybe I am slightly confused but what causes the impedance in inductors or basically coils of wire. I thought it was the magnetic field created a resistant force in a brother coil below and above the coil but with the meisser effect it could not do this correct because the external magnetic field is not able to penetrate the coil above or below it. In the same way alternating magnetic fields cause flux on other objects allowing things like transformers and Tesla coils to work. Or is the impedance of a inductor caused by something else?

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mheslep
Gold Member
Superconductors at AC behave more or less just as very good conductors; the details are of course different from a normal metal but superconductors do exhibit a frequency dependent ohmic loss.
Interesting. Is that effect (freq dependence) predicted by BCS theory, or only observed? At a glance I don't see it.

K^2
Science Advisor
Maybe I am slightly confused but what causes the impedance in inductors or basically coils of wire.
Seems that way. Look up Faraday's Law of Induction.

mheslep, it's just classical electrodynamics theory. You don't need quantum stuff for it if you don't want to get into details of how superconductor superconducts.

There are one thing we need to clarify.
For the second kind superconductor, as called high temperature superconductor, the magnetic flux can be enter into the superconducting bulk, and it will be pined at the center of defect. Around the flux, one can image a superconducting circuit current is flowing.
The pined flux can not move (exactly we called it "jumping"), unless if they derived the energy (thermal energy or Lorentz force).

With the magnetic field varies, i.e. AC magnetic field, we can derive the information of the dynamics of the superconductivity.

Much theory has been processed, you may find the famous and simple one ---- Born model.

Answer your question, it does show a impedance in the ac magnetic measurement.

mheslep
Gold Member
Seems that way. Look up Faraday's Law of Induction.

mheslep, it's just classical electrodynamics theory. You don't need quantum stuff for it if you don't want to get into details of how superconductor superconducts.
Well a normal conductor sees a frequency dependency due to the skin effect. I thought the magnetic field was excluded in a superconductor, thus no skin effect. No?

f95toli
Science Advisor
Gold Member
Well a normal conductor sees a frequency dependency due to the skin effect. I thought the magnetic field was excluded in a superconductor, thus no skin effect. No?
No, first of all there is still as penetration depth (known as the London penetration depth) even at DC. .
Secondly, losses at AC can be substantial even for type I superconductors The simplest theory for this is based on the so-called London equations.
The surface resistance of superconductors actually increases as the frequency squared which is why normal metals are actually better than superconductors for very high frequency applications (=several hundred GHz).