More on the "skin effect"
As previously mentioned, the skin effect
is where alternating current tends to avoid travel through the center of
a solid conductor, limiting itself to conduction near the surface. This
effectively limits the cross-sectional conductor area available to carry
alternating electron flow, increasing the resistance of that conductor
above what it would normally be for direct current:
The electrical resistance of the
conductor with all its cross-sectional area in use is known as the "DC
resistance," the "AC resistance" of the same conductor referring to a
higher figure resulting from the skin effect. As you can see, at high
frequencies the AC current avoids travel through most of the conductor's
cross-sectional area. For the purpose of conducting current, the wire
might as well be hollow!
In some radio applications (antennas,
most notably) this effect is exploited. Since radio-frequency ("RF") AC
currents wouldn't travel through the middle of a conductor anyway, why
not just use hollow metal rods instead of solid metal wires and save
both weight and cost? Most antenna structures and RF power conductors
are made of hollow metal tubes for this reason.
In the following photograph you can see
some large inductors used in a 50 kW radio transmitting circuit. The
inductors are hollow copper tubes coated with silver, for excellent
conductivity at the "skin" of the tube:
The degree to which frequency affects the
effective resistance of a solid wire conductor is impacted by the gauge
of that wire. As a rule, large-gauge wires exhibit a more pronounced
skin effect (change in resistance from DC) than small-gauge wires at any
given frequency. The equation for approximating skin effect at high
frequencies (greater than 1 MHz) is as follows:
The following table gives approximate
values of "k" factor for various round wire sizes:
Gage size k factor
======================
4/0 ---------- 124.5
2/0 ---------- 99.0
1/0 ---------- 88.0
2 ------------ 69.8
4 ------------ 55.5
6 ------------ 47.9
8 ------------ 34.8
10 ----------- 27.6
14 ----------- 17.6
18 ----------- 10.9
22 ----------- 6.86
For example, a length of number 10-gauge
wire with a DC end-to-end resistance of 25 Ω would have an AC
(effective) resistance of 2.182 kΩ at a frequency of 10 MHz:
Please remember that this figure is
not impedance, and it does not consider any reactive effects,
inductive or capacitive. This is simply an estimated figure of pure
resistance for the conductor (that opposition to the AC flow of
electrons which does dissipate power in the form of heat),
corrected for skin effect. Reactance, and the combined effects of
reactance and resistance (impedance), are entirely different matters.
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