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Re: a frequency-variable resistor Re: request for braid data (was Another grounding question)
Tesla List wrote:
>
> Original Poster: "Bill Noble" <william_b_noble-at-email.msn-dot-com>
>
> aaah, wouldn't a frequency variable resistor be an inductor? (if you wanted
> more ohms with increasing freq) Or perhaps a capacitor??? (for the opposite
> effect)
>
Bill,
I have just had this explained to me fairly well, so maybe I can help--
I think it is the other way around--an inductor is in some ways a
frequency variable resistor. To be more accurate, you should call it
impedance instead of resistance, although they are both measured in
ohms. Inductance tends to resist -changes- in current. A DC current will
eventusally flow through an inductor just as easily as it will flow
through the same amount of straight wire--only affected by the
resistance of the wire. To reach this flow after it is turned on, or
changes from zero to it's potential flow rate, it must first build a
magnetic field around the inductor, whose size is directly related to
the inductance of the coil. The more inductance, the longer it takes a
current to build the magnetic field, and begin flowing.
AC current, or any source with a constantly changing current, must
constantly build and re-build this magnetic field--every time the
current changes direction, a field must be built with opposite polarity
as the previous one. Once again, the higher the inductance, the longer
it takes to build this field. Therefore, a low frequency source has
plenty of time to build the field and then flow, rebuild and flow, etc.
as the frequency gets higher, the current spends more time building and
less time flowing. Therefore the impedance. This Impedance limits the
flow, but does -not- dissipate power. Resistors dissipate power by
turning it into heat. (corrections anyone? am I right to assume that the
magnetic field will return the energy to the circuit??)
Capacitors work in opposite fashion. As the same DC current is turned
on, the capacitor will at first freely take in current to charge.
However, as the voltage of each plate charges, less and less current
flows in, until it reaches the max. charge as defined by the voltage of
the power source. AC current therefore will flow at first, then stop,
and flow in the opposite direction, then stop, etc. The higher the
frequency, the less likely it is that the capacitor has time to charge
fully, and current can flow more freely. So while low frequencies are
inhibited, higher frequencies can move more freely.
Copper braid works in a different way, by utilizing skin effect. A DC
current from a low voltage source will flow through the entire
cross-section of a given wire. However, as voltage increases, the
charges that make up the current will repel itself (like charges repel)
and be foreced to flow closer and closer to the surface of a given
conductor. This is called the "skin effect" and explains why lightning
blows clothes off, but rarely flows through the body. It also explains
why a metal cage protects the occupant from high voltage (car in
thunderstorm). The electrons literally push each other out of the middle
of the car, to the outside.
Braid is constructed of many small wires that dive in and out of the
shape of the braid. Low voltage DC will follow these wires in and out of
the shape easily. But as the current begins to repel itself at higher
voltages, It has a harder time following the wires back into the center
of the braid, and is forced to jump from wire to wire along the outside.
At higher frequencies, higher peak voltages are necessary to keep the
same current. Therefore, the braid will pass low frequencies better than
higher ones. Instead of the energy going into building and re-building a
magnetic field, it goes into resistance caused by having to jump in and
out of the wires, which leads to dissipation by heating. Inductors do
other things for an AC circuit, such as forming resonant conditions with
a capacitor, etc. Braid will have no such effect.
I hope this helps, and glaring errors welcome correction.
Wells