The frequency-dependent redistribution of alternating current within conductors caused by time-varying magnetic fields from nearby conductors, which concentrates current and increases effective AC resistance relative to the DC case.

Principle

Magnetic flux produced by neighboring conductors induces eddy currents and alters local current density; at increasing frequency the induced currents force the usable current toward portions of the conductor cross-section determined by geometry and neighbor fields, raising I²R losses and changing impedance.

Demonstration

Illustrative scenario — Situation: Two parallel conductors in a high-frequency winding carry currents in the same direction. Recognition: Each conductor’s time-varying field influences the other. Action: Current distribution shifts toward surfaces facing away from the neighbor (or toward it depending on relative phasing), increasing measured AC resistance of the winding at operating frequency. Consequence: The winding runs hotter and its impedance deviates from DC predictions.

Misapplication

Confusing proximity effect with skin effect: both alter current distribution with frequency, but skin effect is caused by a conductor’s self-induced fields, while proximity effect is caused by external nearby conductors’ fields. The mistake is to attribute geometry-dependent neighbor-induced losses to self-induced skin alone.

Consequence

Higher AC resistance increases dielectric and copper losses, reduces Q in inductors, changes impedance of transmission structures, raises heating and can limit power-handling or efficiency in high-frequency transformers and busbars.

Reversal

At low frequencies the proximity effect is negligible and DC resistance approximations hold. It can be substantially reduced by design choices such as increased spacing, transposed conductors, using litz wire, or alternate winding geometries.

Boundary

Clearly within: tightly spaced high-frequency transformer windings showing AC resistance much larger than DC resistance. Boundary case: moderate spacing where proximity losses are significant only above a certain frequency. Clearly outside: bulk ferromagnetic hysteresis losses or dielectric losses, which are distinct phenomena.

Semantic Tension

Conflict between compact, high-density conductor placement (for size and coupling) and the need to minimize proximity-induced losses; optimizing both simultaneously is often impossible without added complexity or cost.

Synthesis

Proximity effect is a neighbor-induced current redistribution that, together with skin effect, controls high-frequency conductor losses; effective mitigation requires geometry changes, specialized conductors (e.g., litz), or transposition rather than relying solely on material selection.