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Old December 2nd 04, 10:28 PM
Jim Kelley
 
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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

 
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