EMC Question of the Week: October 24, 2022

An air core inductor on a circuit board

The inductance of a tightly wound coil of wire with N turns is approximately proportional to 

  1. N
  2. N2
  3. wire radius
  4. coil radius


The best answer is “b.” The inductance of a coil of wire is approximately proportional to the number of turns squared. Each additional turn increases the current in the loop and increases the loop area coupled by the magnetic flux. A small amount of flux will wrap each winding without coupling all of the turns, so the total inductance will always be slightly less than N2 time the inductance of a single turn. Inductor wire is often insulated with a thin coat of varnish to provide adequate insulation while minimizing the amount of "leakage flux" wrapping each individual turn.

The inductance of coil of wire is not proportional to the wire radius or the coil radius. Also, contrary to what many believe, it is not proportional to loop area. In most practical configurations, it is approximately proportional to the coil radius times the natural log of the coil radius. Loop area, on the other hand, is approximately proportional to the coil radius squared.

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