A Question on Skin Effect and Eddy Currents

AI Thread Summary
The discussion centers on the skin effect and eddy currents in metals when exposed to alternating magnetic fields. It highlights that at high frequencies, eddy currents push the magnetic field away from the core, preventing it from penetrating deeply. The limited time for electromagnetic fields to enter the metal due to their oscillatory nature results in decreased field strength at the surface. Comparisons are made to the Meissner effect in superconductors, noting that while both phenomena involve surface currents, they operate under different conditions. The conversation emphasizes the importance of frequency in determining how deeply magnetic fields can penetrate conductive materials.
Narayanan KR
Messages
76
Reaction score
4
TL;DR Summary
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.
eddy1.jpg
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 ?
 
Engineering news on Phys.org
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.
 
  • Informative
Likes Narayanan KR
artis said:
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 ?
 
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
Likes Narayanan KR
artis said:
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 ...
 
Narayanan KR said:
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
 
Last edited:
  • Like
Likes 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...
 
Back
Top