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Re: a frequency-variable resistor Re: request for braid data (was Another grounding question)
Tesla List wrote:
> 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...
There are really "two different skin effects". The electrostatic
one, because similar charges repel each other, causes charges to
locate at the outer surface of a charged (round) conductor, as in a
Van de Graaff generator. The electromagnetic one says that an AC
current can only penetrate the surface of a conductor if its
intensity decays exponentially with the distance inside the
conductor. The depth where most of the current is concentrated
the "skin depth" for that conductor, that varies in a way
inversely proportional to the square root of the frequency.
Both effects are independent. The charges that move to the
surface are just charging the "body capacitance" of the wire,
and do not cross to the other end of the wire.
The current that crosses the wire is subject to the electromagnetic
skin effect (the charging current is subject to this effect too).
Both effects can be taken into account simultaneously if the
wire is treated as a transmission line.
But as a normal conductor stores insignificant charge, compared to the
charges that move when it conducts current (ex: 10 pF of capacitance
at 10 kV means 1e-7 Coulombs at the surface of the wire, while a modest
1A current moves 1 Coulomb/second), the first effect is
insignificant when we are interested in the electrical resistance
of a wire.
Antonio Carlos M. de Queiroz