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Old April 3rd 06, 05:06 AM posted to rec.radio.amateur.antenna
K7ITM
 
Posts: n/a
Default Current through coils

Cec wrote, "How does one amp at the top and zero amps at the bottom
grab you?
Please see my other postings."

It grabs me that what you wrote in your other postings about
capacitance to the outside world, " I didn't say there was no
capacitance to the outside world. I said
such is a secondary effect, not a primary effect, and for the sake
of the present argument, can be ignored as secondary effects often
are ignored," is all wet. And I still say that your other postings
before that were saying you believed that there was NO capacitance to
the outside world. It was the message they sent to me, loud and clear.

Given any volume, say a volume containing a Texas Bugcatcher coil and
the air inside and immediately around it, if you push more electrons in
than come out _for_ANY_abritrarily_short_time_period_, you have changed
the net charge in that volume; if you pull out more electrons than go
in, you have changed the net charge in that volume. If the current at
the top and bottom, the only two conductors crossing the boundary of
that volume, is different, that represents flow of charge into (and out
of, in a cyclic fashion) that volume. I don't know what to call that
except capacitance to the outside world. Yes, it's _distributed_
capacitance. But the key point is that it is THE reason--the WHOLE
reason--for the difference in current between the top and the bottom,
NOT a "secondary effect."

In fact, when YOU say that the coil "behaves differently" in different
external environments, you are AFFIRMING it as an important effect, for
surely the presence or absence of some American gas guzzler (or is it
Diesel guzzler?) strongly affects the capacitance to the outside world,
and does not significantly affect internal capacitances (which in any
event, being contained entirely within that volume, do NOTHING for
storing net charge within the volume, because for those internal
capacitances to store charge, what goes in one end comes immediately
out the other end which is still inside the same volume and thus there
is not any net change in charge within the volume). But the "other
end" of capacitance to the gas guzzler or whatever is OUTSIDE the
volume of the coil, thus EXACTLY accounting for the difference in
current at the two leads going to the coil. -- I suppose they covered
all that in a sophomore EE circuits class, but I wouldn't know. I
suppose they also might have covered how a pure lumped model using only
i(t)=C*dv(t)/dt and v(t)=L*di(t)/dt, with no time delay elements, can
mimic lossless transmission line behaviour to any arbitrary degree of
accuracy you want, but perhaps they don't try to hit you with that
concept till later. I wouldn't know that, either...I just know it's
true.

I suppose it's a bit too much to ask all at once, but I do wish you
could see that just because the specific value of the capacitance is
different in different environments, it does not mean that I need a
different model. The coil does not behave in some fundamentally
different way. I only need to adjust the value of that capacitance
within the model--or if you will, the parameters of the
transmission-line-like behaviour, though other models may work as well
in practical antenna analysis. The model stays the same; the
parameters in the model change. When I change the value of a resistor,
my model of a resistor doesn't change. It's still fundamentally
v(t)=R*i(t). Only the value used for R changes. On a grander scale,
when I include the parasitic effects of a real inductOR, I have more
things to account for in the model than just inductANCE. Some of them
are affected significantly by the environment in which I place the
inductor. And even small changes in the values can have a profound
effect on the overall system behaviour. That's especially true in a
system operated near resonance where the Q is extremely high, such as a
system in which there is only a standing wave.

My only wish is that these musings will be useful to the lurkers trying
to actually learn something, if there still happen to be any around.

Cheers,
Tom