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Re: a frequency-variable resistor Re: request for braid data (was Another grounding question)
Tesla List wrote:
>
> Original Poster: Wells Campbell <wellscampbell-at-erols-dot-com>
>
> > Ok, but the skin effect is independent of the voltage. The reason is
> > more magnetic than electric. Parallel wires with current in the same
> > direction tend to move apart.
> >
> > Antonio Carlos M. de Queiroz
>
> Different skin effects, maybe? I have never heard of magnetic field
> repulsion as the reason for the skin effect, but that doesn't mean it's
> not the case :-) I am not as familiar with higher frequency phenomena,
> but the skin effect which I studied was in electrostatic context (hair
> standing on end, Van de Graaf generator) which would not produce
> magnetic fields at all. There is no use arguing about facts, can anyone
> set us straight? See my post on similar topic just a few minutes ago...
>
> Wells
Wells and all,
Antonio is indeed correct. Skin effect is an electromagnetic phenomenon
that only shows up when you try to change current flowing in a
conductor. It is NOT related to the electrostatic effect where charges
concentrate at the surface of a conductor, nor does it show up during
steady-state DC current flow. Skin depth is a function of the material's
electrical conductivity, magnetic permeability, and the frequency of the
current. Skin effect is much more pronounced in a magnetic conductor
than a non-magnetic conductor - the more magnetic the material (the
greater it's permeability), the thinner the "skin". Because of this
effect, you definately want to avoid using iron or steel in the
conduction path for any high-current RF applications.
For example, the electrical conductivity of copper is about 5.8X better
than that of cold-rolled steel. However, cold-rolled steel has a
magnetic permeability of about 180 (at 20 Gauss) versus 1.0 for copper,
which significantly reduces the skin depth (the portion of the material
that's actually conducting the current). Let's compare them at 150 kHz:
Resistivity Relative Skin Depth
(Ohm-cm) Permeability -at-150 kHz (mils)
Copper 1.724e-6 1.00 6.72
Steel 1.000e-5 180.00 1.21
In effect, we get hit with a double-whammy! The steel already starts out
being 5.8 times more resistive than copper even for DC. Because of its
higher permeability, RF current then is forced to flow through a much
thinner "skin" of this poorly-conducting material (the skin depth of
steel is only about 17% that of copper). The combination of these two
effects means that a steel conductor will be 32 TIMES lossier than an
identically-sized copper conductor at RF frequencies.
-- Bert --