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When AC is flowing through a conductor, the current is non-uniformly distributed over the cross-section in the manner that the current density is higher at the surface of the conductor compared to the current density at its centre.

It causes larger power loss for a given rms AC than the loss when the same value of DC is flowing through the conductor.

The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. The skin effect is due to opposing eddy currents induced by the changing magnetic field resulting from the alternating current.

Reference: Modern Power system Analysis, 3rd edition by D. P Kothari I J Nagrath page 71

http://en.wikipedia.org/wiki/Skin_effect

When some time varying voltage difference is applied across the conductor, an electric field E wil be present along the conductor, hence the flowing time varying or AC current is related to this voltage through Ohm's law. We can also say it terms of the electric field, thus the current J=conductivity*E. Now because of this current time varying magnetic field H will be inuced in the conductor, by Ampere's law, then this induced magnetic field will induce time varying electric field and current against the original electric field and current, according to Faraday's law. with this arrangement E and H are coupled, and an electromagnetic wave is produced in the conductor. The electromagnetic wave carries its power from the surface towards the core of the conductor, and as it penetrates through the conductor, it suffers ohmic losses due to the conductivity of the conductor. Being  a good conductor means that the conductivity is high, thus ohmic losses are high per unit distance, so as the wave penetrates it decays quickly and dies after very small distances from the surface, which we call the "skin depth". That's why the current flows near the surface of the conductor.

At higher frequencies, the electromagnetic wave varies quicker with time, hence it dies quicker as it flows, which makes the skin depth to decrease.

Following the good explanation of Professor  Zarachy, I add that the at higher frequency the skin depth became more close to the radiofrequency wavelength at high frequency. There is another cause is that at shorter wavelength the conductor became an antenna that could match its size with lambda/2 or multiple of its increasing the attenuation even more.

About the fact of that the current at AC flows at the outer area is due in the inner part  of the conductor it is produced eddy currents generating a magnetic field opposed to the electron moving ones repelling them as two magnets in opposite directions.

becuase  ac current deal with electromagnetic field  which neutralize eachother in the inner area of conductor so we don't have any field and so any force there do to flow current.

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The chapter is as a gift to me. Great thanks dear doctor.

This is due to the Lorentz force. As the current enters the conductor it creates a magnetic field. This magnetic field acts on the continuing current- charge in motion- forcing the motion perpendicular to that of the original current. This force brings the charge towards the surface of the conductor. This increases with frequency. Thus, with increasing frequency the moving charge is constrained to a smaller and smaller cross section area- increasing resistance- the skin effect. The variation of the magnetic permeability of the conductor as well as hysteresis effects will make the variation of the resistance with frequency even more complex

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Ali Arshad God Bless you for the article you shared. Very informative. Thank you