sci.physics.electromag

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On Sat, 15 Sep 2007, Bill Miller wrote:

> Hello Timo... Thanks for the thoughts. Please see below.
>
> "Timo A. Nieminen" wrote:
> On Fri, 14 Sep 2007, Bill Miller wrote:
>> "Szczepan Bia=B3ek" < @ > 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 i=
n
>> 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 end=
s,
>> the "wire" doubles back toward the center -- spaced a short distance fro=
m
>> 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 zer=
o,
> 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.

Two anti-phased dipoles =3D two capacitors, each plate of each capacitor=20
belonging to a different antenna. Well, for centre-fed dipoles.

>> 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.

Well, the usual kink in the field due to sudden motion of a point charge=20
is not that bad a picture. But this makes it very much a case of the=20
external field being responsible for the radiation, rather than the=20
charge. Which is not necessarily a wrong or bad thing, and it provides a=20
pretty good answer for the two dipoles above.

One can say that each plate of the capacitor radiates, but the radiation=20
is cancelled by the radiation from the other plate. Mathematically, sound=
=20
enough, but I don't find it to be very satisfying. I'd be interested in=20
seeing what happens at the level of individual charges (something also not=
=20
looked at in the usual antenna problem). Another nice case would be the=20
example of an expanding spherically symmetric cloud of charge,=20
accelerating. Classically, no radiation. But what would an electron see?

Given that electrons are fundamentally problematic in classical EM,=20
perhaps it'd just be a nice mathematical exercise, but I think it might be=
=20
interesting anyway.

>> (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 tau=
ght
> that 1. An E field causes an H field, 2. An H field causes an E field, an=
d
> 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 -- ar=
e
> integral to his theory. They and other "stuff" were discarded over a hund=
red
> years ago, for reasons that may or may not have been valid at the time.

Care to elaborate?

Anyway, if you haven't read it already, J. Roche, The present status of=20
Maxwell's displacement current, Eur. J. Phys. 19, 155-166 (1998) is=20
worthwhile.

> 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.

The physicists teaching EM mostly don't do EM professionally. The=20
engineers teaching EM are interesting in teaching students how to extract=
=20
numbers from the theory. OK, those are overgeneralisations, and I know=20
exceptions to both.

Students also (often) like to learn things by rote. Thinking is hard.

I like to wave the unsolved problems of classical EM in front of students,=
=20
encourage them to tackle aspects of them for assignments. Thinking might=20
be hard, but it's good for them.

--=20
Timo Nieminen - Home page: http://www. /people/nieminen/
E-prints: /view/person/Nieminen,_Timo_A..html
Shrine to Spirits: /timo_nieminen/

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