Re: trying to understand/visualize EM waves. please help

On Thu, 17 Jun 2010 23:05:47 -0400, "Bill Miller" <kt4ye@xxxxxxxxx>
wrote:


"Salmon Egg" <SalmonEgg@xxxxxxxxxxxxx> wrote in message
news:SalmonEgg-BBCE6E.19202717062010@xxxxxxxxxxxxxxxxxxxxxx
<big snip>>
So to the bottom line: Why does displacement current not produce
magnetic field?
if you look through the literature you will see that there are a
number of citations in which they calculate a magnetic field from
displacement current. It would help your case if you cited an
experiment that tested this proposition.
Bill

Yes... this is the bottom line and an excellent question.

The answer has to do with the basic idea of causality: If A causes B, then A
*must* occur before B.

You may want to think about that for a while. Try to come up with any
exceptions -- ever.

In Maxwell's equation delcrossH = J +dD/dt, H, J and D are all occurring
at the same time. Therefore, according to the basic tenet of causality,
*none* of those three "ingredients" can cause any or all of the other two.

Put another way, Maxwell's Equations are descriptive but not causal.

Once you accept that concept, then it is possible to start looking for what
really is the cause. Jefimenko took one approach. Panofsky and Phillips took
another. Both arrived at the same set of equations showing that E and H are
caused by the same "stuff." That stuff is charges and the motion of charges.

In an EM wave, why do time-varying E and H always occur together? Why are
they always phase locked? Because they are both caused simultaneously by the
same sources.
No, it's because space has the impedance Z that E and H share:
Z = sqrt(mu/eps),
just as space has the velocity
c = sqrt(1/eps*mu).

The two are related by a sort of Ohms Law:
H amp turns/m = (E volts/m) / Z ohms.

In the case of a resistor with current flowing through it, is it the
voltage that is the cause, or is it the current? Without the resistor
there would be no action.

If this does not make you uncomfortable , then I would be surprised. For me,
it took several readings -- with lots of thought in between -- for it to
sink in. Then I said, "OH S**T!" (awakening my wife at 3 AM) as I realized
that the basic concept of electromagnetism; the concept that I first learned
att age 15 when I built my first radio, was just plain wrong.

All the best...

Bill


An old man would be better off never having been born.

John Polasek
.

Relevant Pages

  • Re: trying to understand/visualize EM waves. please help
    ... Therefore, according to the basic tenet of causality, ... att age 15 when I built my first radio, ... Equations including delcrossH = J +dD/dt. ... what the magnetic field is when I know what J and D are as functions of ...
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  • Re: trying to understand/visualize EM waves. please help
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