At high frequencies mainly electrons at the surface of an electric conductor are excited. As a consequence, the ohmic ratio increases because limited parts of the conductor are available for the transport.
Reason: A magnetic field is generated inside an electrical wire as well as around the conductor in response to a direct current flow. For a direct current source the current density is the same on average.
Alternating current behaves differently: With a changing polarity of the electric current, also the magnetic field is affected and generates so called turbulent flows inside the conductor, which are directed opposite to the alternating current source and hence weaken it at the centerline of the conductor. The magnetic field surrounding the electric current leads to the phenomena that electrons in the center of the conductor are enclosed by more field lines compared to electrons closer to the outside. For an alternating current the changing magnetic field induces a higher counter electromotive force inside than at the boundary (eddy currents).« Back