"Timo A. Nieminen" <timo@[EMAIL PROTECTED]
> wrote in message
news:Pine.WNT.4.64.0806260615270.660@[EMAIL PROTECTED]
> On Wed, 25 Jun 2008, Bill Miller wrote:
>
>>
>> "Timo A. Nieminen" <timo@[EMAIL PROTECTED]
> wrote in message
>> news:Pine.WNT.4.64.0806240535450.1160@[EMAIL PROTECTED]
>>> On Mon, 23 Jun 2008, Bill Miller wrote:
>>>
>>>>
>>>> "Timo A. Nieminen" <timo@[EMAIL PROTECTED]
> wrote in message
>>>> news:Pine.WNT.4.64.0806221102300.656@[EMAIL PROTECTED]
>>>>> On Sat, 21 Jun 2008, Bill Miller wrote:
>>
>> <snip>
>>
>
>
>> Recognition of this simple concept would, as one example, have stopped
>> generations of instructors from teaching their students that E causes H
>> and
>> H causes E. It would have also stopped generations of mathematical
>> physicists from publi****ng articles in peer-reviewed publications that
>> "proved" that E causes H or vice versa. And it would have stopped
>> generations of experimental physicists from spending countless hours
and
>> dollars in building apparatus to measure the non-existent H between the
>> plates of capacitors
>
> Not so many papers, hours, or dollars.
I concur with most of what you said -- until here!
Two examples: One experiment used a superconductor based setup to try and
measure H "caused" by Displacement Current. Not an inexpensive exercise.
Worse still is a set of patents issued for a variety of Crossed Field
Antennas (CFA). The patent holder is a professor in Scotland. The basic
"operating principle" is the formation of separate E and H fields
using capacitor plates and inductors that are phased at 90 degrees to form
an EM wave. The claim is that the device is as efficient as a full-sized
antenna, but an order of magnitude smaller than conventional 1/4 wave
monopoles against ground.
Numerous antennas were sold. None worked. The most salient was on the Isle
Of Man where an entire multi-million dollar radio transmitter project was
scrapped -- with substantial losses to the investors since it was
physically
impossible to substitute a conventional antenna.
So, Timo, the E causes H error is not just an academic exercise gone bad.
It
has cost time and money.
>The existence of electromagnetic waves, transverse (i.e., div(E) = 0,
>div(H) = 0) solutions of the vector Helmholtz equation, is pretty good
>evidence that the dD/dt term really belongs in the Maxwell equations, so
>perhaps a more direct "proof" isn't seen as essential.
>
>> If I have understood your position on this, Timo, it is that the
teaching
>> of
>> this is expeditious & that once one gets to the PG leve, the "true
facts"
>> are revealed and everything is fine.
>
> Well, for better or for worse, what is taught is necessarily simplified.
> Alas, while a small number of misconceptions might be corrected in later
> undergrad or postgrad education, many are not. With limited time, and
> limited initial knowledge, there are limits to what can be taught. I
> woulnd't say that everything is fine, just that it isn't easy to do much
> better. One particular problem is that many (most?) courses lead to just
> cramming for an exam, followed by forgetting. Emphasis on techniques and
> proofs over any real understanding. One problem is that understanding
> comes with experience, and a 1 semester course allows very little time
for
> that.
>
> I don't think that the current issue (E, H, and causality) is such a big
> deal. It's mostly harmless as far as beliefs go,
If you consider millions of dollars wasted in craossed field antennas as
"harmless." There are others "using" similar techniques. They do not work
"as advertised" either.
and only one of very many
> various misconceptions that students pick up.
Herein is the proble: Students are not "pickibg up" misconceptions. They
are
being TAUGHT them.
The participants on this lis are not dumb. But look at some of the replies
in this thread. More than a few folks still harbor the misconception that
H
causes E and vice versa.
They learn far worse ones
> along the way. It also isn't enough to just tell them in lectures -
> students don't learn from lectures, they learn from *****sment and
> preparing for *****sment. It's hard to *****s understanding, so
> understanding tends to not be *****sed strongly, so students don't get
> really motivated to understand.
>
>> That leaves hordes of non PG students still believing this rubbish. And
>> it
>> begs the question of why, in UG cl*****, the instructors don't say
>> *something* about how theses two parameters don't cause each other, but
>> that
>> they always appear simultaneously. I suspect its because the
instructors
>> don't know it!
>
> Likely enough.
Strong agreement here!
>> Is there a single UG EM textbook that correctly categorizes the
>> relation****p
>> between E and H? Even ONE?
>
> Probably. Very likely. But specialised EM texts are the stuff of
advanced
> (or at least intermediate) undergrad courses. Those who do just 1 or 2
> physics courses ay uni, or only do high school physics never get to see
> them. There might well even be generic introductory textbooks that don't
> get it wrong, by not mentioning it at all. If I have time in the near
> future, I will look.
Even in introductory texts, it wouldn't be terribly difficult to say
something like, "Maxwell's equations demonstrate a clear relation****p
between Electric(E) and Magnetic (H) fields. This is sometimes erroneously
interpreted as meaning that E causes H and H causes E. However, a careful
analysis shows that E fields and H fields are sololy caused by charges and
the movement of charges.""
Not too hard.
>>> Newton 1 and 2 are pretty much just definititions of inertial motion
and
>>> force, while Newton 3 is (as above) conservation of momentum. How can
>>> these
>>> be modified sensibly?
>>
>> I'd say that looking at Causality and incor****ating what we learn into
>> new
>> expressions that include factors tor time variation of position and for
>> timevariation of mass would get the job done.
>
> New expressions for what? My point is that it's easy enough to modify,
> e.g., a force law (as Heaviside did Newton's law of universal
> gravitation), or Coulomb's law (as per Maxwell), but a very different
> thing to try to modify Newton's laws of motion, which are of a very
> different fundamental nature.
>
>> Naturally, any such additions
>> must obey conservation of momentum AND must reduce to the original form
>> when
>> time dependancy is absent.
>
> Well, if you want conservation of momentum, then you're _not_ going to
be
> modifying Newton 3. Also, if you want retarded interaction at a distance
> along with conservation of momentum, then you're going to need fields
that
> can carry the momentum from point A to point B (or some other "thing" to
> have the momentum when the "interacting" objects don't have it).
Yep. Gravitational fields do that.
>
> Or we change our ideas about what conservation of momentum means.
No need to do that.
>
>>>>> Yes, if we consider two m***** as above (or two electric charges) to
>>>>> be
>>>>> interacting with each other, we have exactly the problem you point
>>>>> out.
>>>>>
>>>>> Does this mean Newton 3 is wrong? Does this mean momentum is not
>>>>> conserved? I would say that it means that we simply don't have a
>>>>> situation
>>>>> where there are two objects interacting at a distance, but two
objects
>>>>> each interacting with a field. Perhaps the interaction really is as
>>>>> described by field theory, a local interaction between field and
body,
>>>>> and
>>>>> we should throw away the classical mechanics idea of interaction at
a
>>>>> distance rather than Newton 3.
>>>> That's one way of dealing with it. I believe that a better way would
be
>>>> to
>>>> recognize that Newton's work is not applicable to time dependency.
>>>> That's
>>>> no
>>>> different, conceptually, from Maxwell's expansion of Ampere's Law to
>>>> include
>>>> The rate of Change Of E field in order to deal with magnetic fields
in
>>>> a
>>>> time-dependent environment.
>>>
>>> I think it is different conceptually. One is a conservation law, and
the
>>> other describes a limited set of experimental observations.
>>
>> It is true that gravity is a whole bunch of orders of magnitude more
>> difficult to deal with experimentally than EM. Building planetary
>> objects,
>> setting them rotating, and zapping them past one another is way beyond
>> our
>> experimental capability.
>>
>> BUT, we have a handy set of "toys" in our own night sky. And there we
>> have
>> an interesting set of pre-made experiments already in process. Like,
>> maybe,
>> does our model explain why the rotational speed of the Sun at the
equator
>> is
>> different from the speed at the poles? Or does it provide some insights
>> into
>> Mercury's (supposedly) residual precession? Does it sup****t or deny the
>> existence of black holes? Does it provide some insight into the
"missing
>> mass of the universe" question? What might it say about gravitational
>> waves?
>> Does it provide a definitive explanation of the process wherein
potential
>> energy is converted into kinetic energy by a falling body? Does it
>> explain
>> why EM beams are deflected by gravity?
>>
>> But that's a lotta work. Doing it wrong is a lot easier!
>
> Apart from the rotational speed of the sun, which at first glance is
more
> a matter of fluid dynamics and plasma physics rather than gravitation,
Try a second glance.
and
> perhaps the missing mass stuff, yes, our best models of gravity do
answer
> these questions. Alas, they don't reduce to Heaviside's model in the
> appropriate limit,
As I mentioned earlier, Heaviside didn't take it all the way. And that may
be because he was stuck in the "action at a distance" mode and did not
seem
to thoroughy grasp Causality. Or perhaps he resolved this in his V4 of
"Electromagnetic Theory" (unfinished) that was destroyed by thieves and
vandals in 1925?
>There was a nice paper by Robert Forward in Proc. IEEE
> (or Proc. IRE, if before the name change of the journal) in the 1960s
Is this the one in his "Guidelines to Antigravity"?
Bill
> about the weak-field limit of GR.
>
> Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
> E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
> Shrine to Spirits:
http://www.users.bigpond.com/timo_nieminen/spirits.html
>
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