EMC Question of the Week: October 5, 2020

Solid copper wire with red insulation

At low frequencies, the internal inductance of a round copper wire is

  1. proportional to the wire diameter
  2. proportional to the wire radius
  3. proportional to the loop area
  4. about 50 nH/m


The best answer is "d". The internal inductance of a wire is the inductance due to the magnetic flux inside the wire coupling to the current in the wire. For round non-magnetic wires with a uniform current distribution, this inductance is μ0/8π or 50 nH/m.

At high frequencies, where the skin effect forces current to flow on the outer surface of the wire, the internal inductance approaches zero. For round cross-sections, the internal inductance is not a function of wire size.

Note that the first two choices were essentially identical, since any quantity that is proportional to the radius would also be proportional to diameter. The third choice could also be ruled out by noting that the internal inductance of a wire depends only on the current distribution in the wire cross-section. 

EMC engineers are rarely concerned with the internal inductance of signal conductors. At low-frequencies, the impedance associated with the internal inductance tends to be negligible compared to the wire resistance. At high frequencies, the skin effect reduces the internal inductance of the conductor to the point where it is negligible compared to the external inductance of the loop.

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