Gene Fuller wrote:
The Poynting vector, generally described as ExH, is the energy flow
density. It has units of energy/area/time.

Rate of energy flow through an area? Sure sounds like joules/sec
(power) to me. The IEEE Dictionary agrees.

In practical terms the Poynting vector ExH and the Poynting theorem have
little utility for radio amateurs.

I particularly like Johnson's treatment where the forward power
and reflected power have separate Poynting Vectors. :-)

A quote from Ramo & Whinnery's, _Fields_and_Waves_:

"... it is often convenient to think of the Poynting Vector as
the vector giving direction and magnitude of energy flow at any
point in space."
--
73, Cecil http://www.qsl.net/w5dxp

On Thu, 02 Dec 2004 14:46:34 -0600, Cecil Moore
wrote:
Gene Fuller wrote:
...energy/area/time.
sounds like joules/sec (power) to me. The IEEE Dictionary agrees.
If so, then a strange dictionary indeed (or strange reader),
Gene's term reduces to energy·time·area^-1 not energy·time^-1

What kind of sound was that anyway?

Richard Clark wrote:

On Thu, 02 Dec 2004 14:46:34 -0600, Cecil Moore
wrote:

Gene Fuller wrote:

...energy/area/time.

sounds like joules/sec (power) to me. The IEEE Dictionary agrees.

If so, then a strange dictionary indeed (or strange reader),
Gene's term reduces to energy·time·area^-1 not energy·time^-1

Every thorough discussion of the Poynting Theorem stresses the caveat
that Gene poynted out.

Born and Wolf does observe that the Poynting vector is adequately
defined as the "density of the energy flow", "the amount of energy which
crosses a boundary surface per second a unit area normal to the
directions of E and H." They add however:
"It should be noted that the interpretation of S as energy flow (more
precisely as the density of the flow) is an abstraction which introduces
a certain degree of arbitrariness. For the quantity which is physically
significant is, according to (41) [an expression for the rate of change
of energy within a volume], not S itself, but the integral of S . n
taken over a _closed_ surface." Emphasis on 'closed' is mine.

They also point out that the integral of the Poynting vector over an
arbitrary volume which contains no radiator or absorber of energy, or
where no mechanical work is done, is equal to zero. They cite
conservation of energy as the directive.

73, Jim AC6XG

Jim Kelley wrote:
They also point out that the integral of the Poynting vector over an
arbitrary volume which contains no radiator or absorber of energy, or
where no mechanical work is done, is equal to zero. They cite
conservation of energy as the directive.

All that says is: if the flowing energy doesn't change, it hasn't
been dissipated or radiated. The energy is in the process of being
losslessly transferred from one place to another with joules/sec
passing an infinite number of points in space. From _Optics_,
by Hecht: "The energy streaming through space in the form of an
electromagnetic wave, is shared equally between the constituent
electric and magnetic fields. ... We now make the reasonable
assumption (for isotropic media) that the energy flows in the
direction of the propagation of the wave."
--
73, Cecil http://www.qsl.net/w5dxp

On Thu, 02 Dec 2004 14:28:26 -0800, Jim Kelley
wrote:
Born and Wolf does observe that the Poynting vector is adequately
defined as the "density of the energy flow", "the amount of energy which
crosses a boundary surface per second a unit area normal to the
directions of E and H."

Hi Jim,

You were desirous of "value added" writing, I believe. The
explanation and Gene's observation that this vector is not about power
has more correlatives in radiation, of the observable kind. We may as
well tread into the optics side of the family as long as we are here.

The same area bounded expression for light is Lux whose definition is
Lumens (power or energy/second) per square Meter (area).

Lumens are printed (mandate of law) on every box of light bulbs.

[Daggers fly here]
Unfortunately our resident Optical (sic) wizard here, has never been
able to express ANY answer for his Optical pronouncements in ANY
Optical term, not even Lumens. Bringing such topics as Optics to the
discussion and leaving them adrift demands sneers in response to such
babbling sophistries.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Thu, 02 Dec 2004 14:28:26 -0800, Jim Kelley
wrote:

Born and Wolf does observe that the Poynting vector is adequately
defined as the "density of the energy flow", "the amount of energy which
crosses a boundary surface per second a unit area normal to the
directions of E and H."


Hi Jim,

You were desirous of "value added" writing, I believe. The
explanation and Gene's observation that this vector is not about power
has more correlatives in radiation, of the observable kind.

It's accurate to say that power is something which itself doesn't
propagate in any fashion, at any wavelength.

[Daggers fly here]
Unfortunately our resident Optical (sic) wizard here, has never been
able to express ANY answer for his Optical pronouncements in ANY
Optical term, not even Lumens. Bringing such topics as Optics to the
discussion and leaving them adrift demands sneers in response to such
babbling sophistries.

When Maxwell wanted to draw such distinctions, he included a frequency
dependent term - but allowed the same units throughout.

73, Jim AC6XG

73's
Richard Clark, KB7QHC

Jim Kelley wrote:
It's accurate to say that power is something which itself doesn't
propagate in any fashion, at any wavelength.

I know that's the physicists view, but some engineering views
are slightly different because of a differing definition of
"power". Let's say we have a one second EM pulse containing one
joule of energy traveling along a transmission line with three
joules/sec measuring points. For one second periods in succession,
we will measure one watt at the measuring points. In each case,
the one watt is the same one watt, displaced in time, being
transferred to the load. That one watt appears at the first
measuring point, then at the second measuring point, then at
the third measuring point, and finally heats up the load. It
certainly appears to be associated with the pulse moving from
the source to the load.

That's the way a lot of engineers deal with power, including
all the power company engineers that I know, and that's the way
the IEEE Dictionary deals with power.

Your "tree falling in the forest making no sound" concept
may not be the best approach for the real world.
--
73, Cecil http://www.qsl.net/w5dxp

Jim Kelley wrote:

It's accurate to say that power is something which itself doesn't
propagate in any fashion, at any wavelength.

Will someone please try to explain to Cecil that engineering is an
application of fundamental physical properties, and that the disciplines
of physics and engineering are not at odds with one another?

thanks,

AC6XG

Jim Kelley wrote:
Will someone please try to explain to Cecil that engineering is an
application of fundamental physical properties, and that the disciplines
of physics and engineering are not at odds with one another?

On the contrary, many of the pure-physics-oriented posters to
this newsgroup have alleged that the IEEE Definitions are Bull$hit
and are to be ignored. Check Google if you don't believe it.

The IEEE Definitions are what engineers abide by. Of course,
the disciplines of physics and engineering are certainly at
odds with one another. For instance, engineers have an altogether
different view of what "work" is, compared to the physicists.
Physicists say no work is done if the starting line and the finish
line are the same for a marathon. Engineers will say: "Then why am
I so friggin' tired?"

The POWER engineers at a POWER generating station operated by
a POWER company simply don't buy into your pure physics BS.
They assume that POWER (joules/sec) leaving a generating station
will, after subtracting losses, make money for the company.
--
73, Cecil http://www.qsl.net/w5dxp

Richard,

Your point is correct of course, but I must highlight an ASCII-based
spot of confusion in my post.

Both area and time are in the denominator. I was sloppy in writing the
equation.

energy / (area * time)

Of course the area is still there, and it is an energy flow 'density'.

But hey, what's a few missing units among friends?

73,
Gene
W4Sz

Richard Clark wrote:
On Thu, 02 Dec 2004 14:46:34 -0600, Cecil Moore
wrote:

Gene Fuller wrote:

...energy/area/time.

sounds like joules/sec (power) to me. The IEEE Dictionary agrees.

If so, then a strange dictionary indeed (or strange reader),
Gene's term reduces to energy·time·area^-1 not energy·time^-1

What kind of sound was that anyway?