Tendency of AC current flow in a conductor's outer layer
"Skin depth" redirects here. For the depth (layers) of biological/organic skin, see skin.
Distribution of current flow in a cylindrical conductor, shown in cross section. For alternating current, current density decreases exponentially from the surface towards the inside. Skin depth, δ, is defined as the depth where the current density is just 1/e (about 37%) of the value at the surface; it depends on the frequency of the current and the electrical and magnetic properties of the conductor.Induction cookers use stranded coils (Litz wire) to reduce heating of the coil itself due to skin effect. The AC frequencies used in induction cookers are much higher than standard mains frequency ‒ typically around 25–50 kHz.
In electromagnetism, skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the conductor. It is caused by opposing eddy currents induced by the changing magnetic field resulting from the alternating current. The electric current flows mainly at the skin of the conductor, between the outer surface and a level called the skin depth. Skin depth depends on the frequency of the alternating current; as frequency increases, current flow becomes more concentrated near the surface, resulting in less skin depth. Skin effect reduces the effective cross-section of the conductor and thus increases its effective resistance. At 60 Hz in copper, skin depth is about 8.5 mm. At high frequencies, skin depth becomes much smaller.
Increased AC resistance caused by skin effect can be mitigated by using a specialized multistrand wire called litz wire. Because the interior of a large conductor carries little of the current, tubular conductors can be used to save weight and cost. Skin effect has practical consequences in the analysis and design of radio-frequency and microwave circuits, transmission lines (or waveguides), and antennas. It is also important at mains frequencies (50–60 Hz) in AC electric power transmission and distribution systems. It is one of the reasons for preferring high-voltage direct current for long-distance power transmission.
The effect was first described in a paper by Horace Lamb in 1883 for the case of spherical conductors,[1] and was generalized to conductors of any shape by Oliver Heaviside in 1885.
^Lamb, Horace (1883-01-01). "XIII. On electrical motions in a spherical conductor". Philosophical Transactions of the Royal Society of London. 174: 519–549. doi:10.1098/rstl.1883.0013. S2CID 111283238.
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