Re: teeny atoms absorb huge EM waves

[William Beaty <billb@ESKIMO.COM>]

On Mon, 2 Aug 1999, Leigh Palmer wrote:

> OK, Bill. I'll accept that challenge. Suppose I think of the potential
> energy of an assemblage of charges as being associated entirely with
> their geometrical relationships. I can calculate that energy entirely
> in terms of their geometry, and Then I may think of the energy of that
> system as being localized in the charges, or at least as being inside
> the smallest convex polyhedron I can draw around them. You, on the
> other hand, think of that energy as being distributed in space; and
> potentially in all of space!
>
> If we can't agree on where this stuff called energy is, how can we
> possibly ascribe reality to it?

I don't quite understand.  Here's where my brain-fuzz is located:

In a short pulse of laser light, energy is located in the propagating EM
radiation, OK?  At lower frequencies, it might be located in a particular
region where a 10GHZ radio wave is travelling, no?  If the wave goes into
a waveguide, are things suddenly different, and now the energy is located
at the surfaces of the charged particles in the walls of the conductive
waveguide, and no longer in the empty space where the EM wave is
travelling?  And at zero frequency in a capacitor or inductor, is the
energy now located somewhere at the surface of the charged particles,
rather than spread out where the (static) fields reside?  I do see your
point about energy location in static situations.  But if the location is
obvious at high frequencies, is the situation totally different in a
waveguide, or below a certain frequency?  It seems reasonable to me that
light beams contain energy, and it follows almost directly that the space
between the plates of a "charged" vacuum-capacitor also contains energy of
the same type.  It's as if capacitors store "frozen light."  If
the observer is moving, then a light beam experiences doppler shift,
while b-fields apparently spring into being between the capacitor plates.
Is one situation contradictory, while the other is not?

I suspect that this is an engineer's viewpoint.  We can argue forever
about capacitors and energy location.  But if we can (conceptually, or via
instruments) watch a microwave pulse be absorbed by a dipole antenna and
end up as an e-field in the capacitor's dielectric, the continuity of that
experience powerfully suggests that the energy is located in the e-field,
and is not stored on the surface of the charge carriers.


I recall seeing arguments about Poynting vector and attendant
contradictions in energy concepts regarding static EM.  We aren't supposed
to trust the predictions of Poynting vector calculations.  But if the
contradictions go away for AC electromagnetism, then it seems obvious to
me that the problem is an illusion, since there is no division between DC
and AC.  The static case is an example of AC of very low frequency.


Unless I can understand how an optical pulse is fundmentally different
from the e-field existing between the plates of a capacitor, I am lead to
believe that the energy in a pattern of charges is stored where the fields
appear.



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