A Question on Skin Effect and Eddy Currents

[Narayanan KR]

TL;DR

I have a doubt on how eddy currents create skin effect for alternating magnetic fields in solid metals, as in NDT (Non Destructive Testing) Processes.

When you try to create time varying magnetic fields in solid metals, there is severe heating due to eddy currents, when you increase the frequency, just like in NDT(non destructive testing) the magnetic field is pushed away from the core to the periphery due to eddy currents opposing the field, this is called the skin effect.

My Question is as soon as there is no magnetic field present in the Core due to skin effect, Once again the eddy currents in the core region must go down to zero, which will pave way for the magnetic field to occupy the core, which should now induce eddy currents in the core which then will push apart the field back to periphery starting the whole process again in an oscillatory fashion, but why do we see the inability of alternating magnetic field to occupy the core, even though we have no eddy currents in the core at high frequencies ?

 

[artis]

EM fields travel at c in vacuum and less than c in other mediums.
EM fields (AC) have half cycles where the field starts then reaches its peak and goes back to 0 on each half cycle.
So the field has a limited time to enter the metal. In this limited time the surface currents (since metal is conductive and the field starts from outside of it) created by the very field oppose the field, this decreases the field strength on the metal surface and in the thin layers beneath the surface and limits the debt to which the field can reach in this limited time it has
The lower the frequency the longer the wavelength meaning that the field changes more slowly and exists for a longer period of time, this allows it to penetrate deeper into the metal.
A static field penetrates the metal fully after a certain amount of time from it's beginning.

 

[Narayanan KR]

EM fields travel at c in vacuum and less than c in other mediums.
EM fields (AC) have half cycles where the field starts then reaches its peak and goes back to 0 on each half cycle.
So the field has a limited time to enter the metal. In this limited time the surface currents (since metal is conductive and the field starts from outside of it) created by the very field oppose the field, this decreases the field strength on the metal surface and in the thin layers beneath the surface and limits the debt to which the field can reach in this limited time it has
The lower the frequency the longer the wavelength meaning that the field changes more slowly and exists for a longer period of time, this allows it to penetrate deeper into the metal.
A static field penetrates the metal fully after a certain amount of time from it's beginning.

hmmm...I might be wrong, but is there any chance that the copper electrons which have magnetism(due to spin) all align in direction opposite to that of applied field there by making B=0 in the core, also more importantaly, why does this effect look very similar to Meissner Effect where the metallic body seems to not allow magnetic field to pass through it ?

 

[artis]

No it's not due to electron spin, electron spin doesn't cancel B field within a conductor.
It's also not the Meissner effect but superconductors do have surface current and that is indeed somewhat similar to eddy currents produced within conductor surface in normal conductors exposed to AC magnetic field, but for the comparison to work you have to imagine higher frequency AC fields , low ones like 50hz penetrate deep into conductor.

This surface current within the superconductor is working similarly to the eddy currents in that it doesn't allow external applied fields to penetrate the superconductor. Note that unlike eddy currents this superconducting surface current can be broken if the applied external field strength exceeds some value.

I suggest read these links, start with this
https://web.pdx.edu/~pmoeck/lectures/312/supercon.pdf

https://physics.stackexchange.com/questions/197102/is-an-electron-a-superconductor/197129#197129

 

[Narayanan KR]

No it's not due to electron spin, electron spin doesn't cancel B field within a conductor.
It's also not the Meissner effect but superconductors do have surface current and that is indeed somewhat similar to eddy currents produced within conductor surface in normal conductors exposed to AC magnetic field, but for the comparison to work you have to imagine higher frequency AC fields , low ones like 50hz penetrate deep into conductor.

This surface current within the superconductor is working similarly to the eddy currents in that it doesn't allow external applied fields to penetrate the superconductor. Note that unlike eddy currents this superconducting surface current can be broken if the applied external field strength exceeds some value.

I suggest read these links, start with this
https://web.pdx.edu/~pmoeck/lectures/312/supercon.pdf

https://physics.stackexchange.com/questions/197102/is-an-electron-a-superconductor/197129#197129

informative ...

 

[artis]

informative ...

Make sure you understand the first link, it is very good and simple and explains the basics very well.
If you have any further questions be sure to ask them
Also make sure to read this
https://en.wikipedia.org/wiki/Cooper_pair

 

[Narayanan KR]

yes sir, i am a slow learner, i am reading your first link again and again, i will ask if any doubt araises...