🚀 go-pugleaf

I hesitate to jump in on this one. It smells like an encounter with
tar baby. I fear there is no escaping, but here goes anyway.

In article <3988@cruzio.santa-cruz.ca.us>,
snarfy@cruzio.santa-cruz.ca.us writes:
|>In article <3978@cruzio.santa-cruz.ca.us>
snarfy@cruzio.santa-cruz.ca.us writes:
|>
|>>>      Now  , back to current draw.  Again, Kirschoff's law states that
|>>>     at a junction I(in) = I(out)  . 

As has been previously noted, Kirchoff's law applies only when integrated
over times long compared to the transients. For instance, applying Kirchoff's
law with fascist fervor at the base of an antenna, one would proclaim that
since the antenna has no other end that no current can ever flow down it,
therefore it cannot ever radiate radio-waves. Es Ist Verboten. Well we
know that broadcasting is possible. Obviously on short times scales,
Kirchoff's law 
is not valid. Averaged over many periods of oscillation, we find that the 
antenna does obey K's law. Likewise with a capacitor, when can have I(in) <> 
I(out) but only until the capacitor has completely charged/discharged.

The secret here is that you are neglecting Maxwell's unique contribution to
Electrodynamics, the displacement current. He modified Ampere's Law to provide
for a time-varying electric displacement. It is the addition of this term
that makes an E&M wave possible. Otherwise there would be none. You can
intuitiviely think of a changing electric field as a  current similar to a
stream of electrons. Of course this current doesn't persist forever
because sooner or later the field will stop changing (or even reverse).
Moreoever this displacement current doesn't obey some of the untuitive
relationships that you associate with current flow caused by charge
carriers, so check your intuition at the door here. If this is new to you,
let me suggest either looking in Purcell from the Berkeley physics series
or in Ritz/Milford/Christy under displacement current to get a grip on this
idea.

[gedanken about separated transmitters and receivers]

I would sincerely, and without malice, suggest becoming more acquainted
and comfortable with Electricity and magnetism before approaching some
items that are genuinely confusing like the above. Otherwise I fear you
will just tie yourself into a paralyzing knot and we'll have to ask
John Baez to administer some sort of topological treatment. :)

|>
|>
|>> If you are
|>>going to consider the Xmttr-Rcvr pair (or triad) as a current loop
|>>of a circuit, you had better consider the circuit element that
|>>connects the transmitter and the receiver - the electromagnetic
|>>radiation (which fills a lot more space than just between the
|>>antennae, which is where the extra energy goes).  At any individual
|>>point between the two, no violation of Kirchoff's Law occurs -
|>
|>
|>    Is  that so? Darn! Let's see now ... we're counting electrons as they go
|>  whizzing by at some  ``individual  point''  between  the  transmitter  and
|>  receiver with our ``space ammeters'' ... I don't know about yours , but my
|>  electron count is zero!

What do ammeters measure? I would suggest that they are not electron counters.
However they are approximations which are widely applicable where there is
no distinction. This is not one of them.


So let me ask. Are your queries here genuine or are you testing the
collective skill (or lack thereof) of the net to answer questions and
arguments that hinge on erroneous but hidden assumptions? No offense
meant, but I was just wondering.

john w. cobb
jwc@fusion.ph.utexas.edu