Skin Effect

Skin effect

When steady current flows through any wire of uniform length current flows per unit area, i.e. current density (J) remains constant. If alternating voltage is applied across the wire then the magnetic flux associated with wire changes. The magnetic flux present inside the wire is greater than the flux present outside. i.e. \(\begin{align*}\end{align*}B\alpha \frac{1}{r^{2}}\) . so, change of flux results in induced emf and the effective resistance is very high at the centre so, concentration of current is found to be high at the outer layers. Induced current are produced resulting in dissipation of energy in form of heat and this current are eddy current. When the frequency of AC is high, eddy current reduces value of dB/dt so, it limits the penetration of current into a conductor resulting to stay currents around surface of wire. This phenomenon is Skin effect.

Thus, skin effect is the tendency of an alternative electric current (AC) to become distributed within a conductor such that J (current density) is largest near surface of conductor and decreases with greater depth in conductor. Current flows at skin i.e. between outer layer and a level called skin depth.

The effective resistance discussed above, is back Emf which is found highly concentrate at centre resulting lower current concentration.

Skin depth:

Depth below the surface of conductor at which current density has fallen to 1/e of Js (where, Js= value at surface).

References

Adhikari, Pitri Bhakta. A Textbook of Physics Volume-I. Kathmandu: Sukunda Pustak Bhawan, 2015.

Feynman, Richard P. The Feynman Lectures on Physics Volume 1. Noida: Dorling Kindersley (India) Pvt. Ltd., 2014.

Mathur, D S. Mechanics. New Delhi: S. Chand & Company Pvt. Ltd., 2015.

Young, Hugh D, Roger A Freedman and A Lewis Ford. University Physics. Noida: Dorling Kindersley (India) Pvt. Ltd., 2014