Proximity effect (electromagnetism)

In electromagnetics, proximity effect is a redistribution of electric current occurring in nearby parallel electrical conductors carrying alternating current (AC), caused by magnetic effects.

The concentration of current on the side of the conductor gets larger with increasing frequency, so proximity effect causes adjacent wires carrying the same current to have more resistance at higher frequencies.

The cause of proximity effect can be seen from the accompanying drawings of two parallel wires next to each other carrying alternating current (AC).

[1][2]: p.142-143  The righthand wire in each drawing has the top part transparent to show the currents inside the metal.

From the right hand rule the field lines pass through the wire in an upward direction.

From Faraday's law of induction, when the time-varying magnetic field is increasing, it creates a circular current (E, red loops) within the wire around the magnetic field lines in a clockwise direction.

This is the case in AC electrical power cables, which have two wires in which the current direction is always opposite.

In contrast to the previous case, the net effect is to redistribute the current into a thin strip on the side adjacent to the other wire.

Proximity and skin effect significantly complicate the design of efficient transformers and inductors operating at high frequencies, used for example in switched-mode power supplies.

The winding is usually limited to a single layer, and often the turns are spaced apart to separate the conductors.

Since the current flows on the surface of the conductor, high frequency coils are sometimes silver-plated, or made of litz wire.

It also assumes the magnetic field is perpendicular to the axis of the wire, which is the case in most designs.