Skin Effect

The current density j remains constant over any given section when a steady current passes through a uniform wire. But, when an alternating current of high frequency flows through a wire, the current density is not uniform at all points across a section. There is a great current density at the surface layers than at the interior layers of the conductor. When the frequency is very large, the current is almost entirely confined to the surface layer. This phenomenon is called Skin Effect. Since the alternating currents of high frequency do not pass through the entire cross-section of the wire, the effective resistance of a wire for A.C. is much greater than that for D.C. Hence the conductor required to carry high-frequency alternating currents consists of a number of strands of fine wire connected in parallel at their ends, and insulted throughout their length from each other. This increases the surface area and thus decreases the resistance. Such a conductor ha s negligible skin effect and it’s A.C. resistance is very nearly equal to its D.C. resistance.

Explanation. When the current passes along the axis of a cylindrical wire, the magnetic flux is finite. When the same current passes through the surface of the wire, the magnetic flux within the wire is zero. The current is changing with time with a fixed frequency over the entire cross-section. Hence the rate of change of flux in the wire is greater due to a current near the axis than that for a current near the periphery.  As the induced e.m.f. opposes the applied e.m.f, the effective resistance or impedance is higher at the centre than in the outer layers. Hence less current passes through the inner layers than thorough the outer ones. Since the reactance is dependent on frequency (XL =ωL = 2πvL), the magnitude of the effect is greater at higher frequencies.


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