Is electrical current purely skin effect? Or does it travel through the wire Ac ,Dc, Rf?

Question

Reason I ask for the 3 different types is I understand that RF has skin effect. But if Ac and Dc is same why is it we need such heavy cables? And not simply a weaved cable over a much cheaper core.

Answer 1

The skin depth is proportional to frequency. At 0 frequency (DC) the current circulates through the whole wire evenly. At 60 hz (normal AC) there is more circulation near the surface, with the circulation at the center of the wire only about 70% of the current density at the surface. Because of this and to provide greater strength, high voltage transmission lines have a steel core (which only conducts about 10% as well as copper) with outer conductors of aluminum. It is at frequencies of hundreds of thousands of hertz that the current only circulates in the very outer layer of the conductor.

Answer 2

The skin effect is defined for ac, however if you use the skin effect equations to calculate the skin depth of dc you will get infinity. (angular frequency is 0)

As far as ac is concerned, I am assuming you mean 60 Hz. At that frequency a copper cable thicker than 8.5mm or so is typically a waste of copper, though thicker cable could be used for other reasons. Keep in mind that a thicker cable will have a lower resistance per distance than a thinner one. This is good for a power company distributing power as less power is wasted in transmission.

If you are wondering about overhead power lines, they are not always copper, and are often aluminum with a steel core.

Check out wikipedia for the skin effect and the other link provided is helpful for distributed power calculations.

Answer 3

Current is confined to a skin depth only for high frequencies. The depth is inversely proportional to the square root of the frequency and is due to resistive diffusion.