sci.physics.electromag

Hello Timo... Thanks for the thoughts. Please see below.

"Timo A. Nieminen" wrote in message
news: @ ...
On Fri, 14 Sep 2007, Bill Miller wrote:

> "Szczepan Bia³ek" < @ > wrote:
>>
>> It is sufficiently close. I try to be precise. My claim is that OPEN
>> CIRCUITS radiate off their ends (called antenas).
>> Closed circuits also radiated something.
>
> If by your definition, the open circuits are the "wires" that stick up in
> the air, then we are in agreement. So far...
>
> But this is only a partial explanation by example, since LOOP antennas
> also
> radiate nicely, and these are -- by definition -- CLOSED CIRCUITS. In
> addition, there is a third class (sort of) that can be represented by
> devices like the folded dipole. In this configuration, at the dipole ends,
> the "wire" doubles back toward the center -- spaced a short distance from
> the "outbound wire. If we do this at both ends, the doubled back wires
> meet
> at the centre, and there is now a CLOSED CIRCUIT. But this antenna also
> radiates with high efficiency and with a pattern that is essentially
> identical to that of a concentional dipole.

>Consider two short dipoles next to each other, half-wave out of phase - do
>they radiate?

That depends. If they are identical and the distance between them is zero,
then no, they will not. For any other condition, yes they will.

>One can say that that is effectively two capacitors right next to each
>other (and even if driven by "separate" circuits, that'll couple the two
>of them nicely).

Although a short dipole exhibits capacitive characteristics, it is not a
classical capacitor, since the E fields are not constrained between the
"plates" and the usual stipulation that we can neglect "edge effects"
clearly does not apply.

As an aside, it can be shown that a parallel plate capacitor is nothing but
a special case of an open circuit, parallel plate transmission line. This
model also reveals some interesting side effects. One of these is that
capacitors only charge exponentially when Zsource>>>Zcap. Another is that
under the special case where Zsource = Zcap, the capacitor charges
essentially instantaneously to the EXACT value of the open circuit source
voltage. And if Zsourcearound the steady state value.

>Why doesn't each plate of a parallel plate capacitor
>radiate (unlike, say, a single-sphere capacitor, which we could call a
>monopole antenna instead)?

This is, I believe, the wrong question. The correct question is/should be:
Why does *anything* radiate. Answer that question and the answer to the
above will be apparent.

>Easy to say mathematically, harder to say
>physically.

One hint may be that in a classical capacitor, the E field is constrained to
exist only between the plates. And the H, in stark contrast with common
interpretation of classical EM, does not exist at all in the form taight to
both UG and PG students. That is, there is no H "caused" by Maxwell's
Displacement Current. An electromagnetic wave can't exist without the H.

>And one can ask whether a circuit with a capacitor is a
>"closed" circuit.

Certainly, in the gap between the plates there is no charge, and therefore
no conservation of charge. But to grossly mis-quote Galileo, "They (the
current) still moves."

>(Why does the existence/non-existence of "displacement current" >receive so
>much more attention than the existence/non-existence of >"magnetic
>displacement current" - why should the lack of a magnetic >conduction
>current so affect the status of dB/dt?)

Well, BOTH are equally ignored in classroom settings. Instead, we are taught
that 1. An E field causes an H field, 2. An H field causes an E field, and
3. There is no such thing as magnetic monopoles.

Jefimenko -- preceded to some extent by Panofsky and Phillips's classic
textbook of 1962 -- has already shown us that 1 and 2 are false. As for 3,
a careful examination of Maxwell's original 20 quaternion equations shows
that magnetic monopoles -- and the magnetic current that they imply -- are
integral to his theory. They and other "stuff" were discarded over a hundred
years ago, for reasons that may or may not have been valid at the time.

It seems that the attitude of (essentially) all UG and PG instructors is,
"We know everything there is to know about EM, and here it is." Thus, the
students learn EM by rote, and spit it back for years to come.

Another expression that comes to mind is, "Don't confuse me with facts. My
mind is made up!"

Bill

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Timo Nieminen - Home page: http://www. /people/nieminen/
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