At DC, current density is distributed throughout a conductive material. As frequency increases, the depth of current density decreases. The current density depth is called “Skin Depth”.
Skin depth is the depth, from the surface, where the current density is equal to 1/e of the surface value.
e = Euler’s Number ≈ 2.718
Where:
= Material Resistivity (Ω/m)
π = 3.14…
= Frequency (Hz)
= Permeability (H/m)
H = Henries (Ω·s)