Skin effect in electricity

The tendency for an alternating current to concentrate near the outer part or skin of a conductor. The current distribution is uniform over the cross section for a steady unidirectional current through a homogeneous conductor, which means the current density is the same at all points in the cross section. The current is displaced more and more to the surface as the frequency increases, with an alternating current. The conductor's effective cross section is therefore reduced so the resistance and energy dissipation are increased compared with the values for a uniformly distributed current. The effective resistance of a wire rises significantly with frequency, the resistance at a frequency of 1 MHz is almost four times the dc value. A skin depth or penetration depth δ is frequently used in assessing the results of skin effect, skin depth is the depth below the conductor surface at which the current density has decreased to 1/e of its value at the surface. This concept applies only to plane solids, but can be extended to other shapes provided the radius of curvature of the conductor surface is appreciably greater than δ. At a frequency of 60 Hz the penetration depth in copper is 8.5 mm and at 10 GHz it is only 6.6 × 10-7 m. Wave-guide and resonant cavity internal surfaces are therefore frequently plated with a high-conductivity material, such as silver, to reduce the energy losses since nearly all the current is concentrated at the surface. Provided the plating material is thick compared to δ, the conductor is as good as a solid conductor of the coating material.

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