We all know very well that due to eddy current (back EMF) generation within an AC carrying conductor, "skin effect" occurs.

Eddy current density:
Highest at center of conductor core
Lowest at surface of conductor core

Thus source current density:
Highest at surface of conductor core
Lowest at center of conductor core

My question is:
According to Ampere's law, magnetic field intensity increases as we go far from center of conductor to surface, so eddy should be highest at surface instead of core, right?

Megnetic Field Distribution Within Conductor


1 Answer 1


Yes, the magnetic field is densest at the surface, but the eddy currents are not.

The main current induces the magnetic field, H. The H field forms rings with the greatest intensity near the surface, and induces eddy currents. The eddy currents flow in rings around the magnetic field lines. It is impossible to make a ring around a field line on the very surface of the conductor, but more importantly, there is less space inside a ring-shaped field line than outside - the density of the eddy currents is greater inside a magnetic field "ring" than outside it. Furthermore, currents from more of these hypothetical rings overlap in the central part of the conductor. Thus, the eddy currents are strongest away from the surface. They do not cluster in the center as strongly - there is plenty even a short way from the surface, which is why the source current is reduced to traveling through the skin.

This image explains it:

DIagram of current I in cylinder, with H rings perpendicular to the current direction and I rings perpendicular to the H rings; https://en.wikipedia.org/wiki/Skin_effect

In short, we have a series of reversals: Source current densest in the center induces a magnetic field strongest near the surface, which induces eddy currents stronger away from the surface, which cancel most of the source current, leaving most near the surface, even though the magnetic field is strongest there.

  • \$\begingroup\$ I addressed it with the H field rather than the B. This may be a simplification; you can get substantially more detail on the fields, which change with time. \$\endgroup\$
    – MBer
    Commented Jun 28, 2016 at 5:55
  • \$\begingroup\$ Hello MBerka, you mean to say "Strongest Eddy" produced by "Strongest B" of surface found center as least opposing area for their flow, so strongest eddy are founded @ center.!! Right ? \$\endgroup\$
    – vasant
    Commented Jun 28, 2016 at 8:20
  • \$\begingroup\$ Do you mean replacing "densest" with "strongest"? Can do, but I was trying to match your phrasing in the question. \$\endgroup\$
    – MBer
    Commented Jun 28, 2016 at 14:47

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