skin effect
When current flows through a conductor, it seeks the path of least resistance. When current flows, magnetic fields are generated within a conductor, which generate a countervoltage by induction. The magnitude of this countervoltage depends on the frequency and decreases exponentially from the center of the conductor to the surface of the conductor and is practically non-existent at the surface of the conductor.
As the frequency increases, the current is displaced from the center of the conductor toward the surface by the self-induced magnetic fields. This displacement effect, which becomes more pronounced as the frequency increases, is called the skin effect and, at high frequency, has the effect that the electrons in the center of the conductor practically no longer move and the current density increases outward, to the extent that the entire current flows only at the conductor surface.
The skin effect already occurs at low frequencies and is reflected in the penetration depth. The reduction of the penetration depth is calculated from a quotient whose denominator is formed from the square root of the frequency difference. According to this, the penetration depth is halved at four times the frequency. For example, the penetration depth of a copper wire is 0.1 mm at 500 kHz and only 0.01 mm at 50 MHz. The skin effect means that the effective conductor cross-section decreases and the conductor resistance increases at the same time.
The skin effect can be compensated for by increasing the conductor surface area, which is done in the case of high-frequency cables and loudspeaker cables by using stranded wires whose surface area is many times greater than that of a comparable compact core.
Another current displacement effect is the proximity effect, which occurs with closely spaced conductors.