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Re: [Phys-l] Poynting Vector



In SI, the Poynting vector is defined to be the vector product E cross H (where H equals B/musubzero in the nonmagnetic dielectric of the capacitor). The Poynting vector has unit W/m^2 and so some incorrectly believe that it always gives the em power transferred per perpendicular area.

To quote Reitz and Milford (2nd ed), "It is tempting to interpret E cross H itself as the energy flow per unit time per unit area. The latter interpretation, however, leads to certain inconsistencies; the only interpretation which survives careful scrutiny is that the integral of E cross H over a CLOSED surface represents the rate at which electromagnetic energy crosses the CLOSED surface." (my emphasis).

I use the example of crossed static E and B fields to show the fallacy of assuming the Poynting vector always gives the em power transferred per perpendicular area.

Tom Sandin

At 12:48 PM -0400 6/28/08, Jeffrey Schnick wrote:
Consider a charged simple parallel plate capacitor in a static uniform
downward-directed magnetic field. The capacitor is oriented so that
from our point of view, the electric field between the plates of the
capacitor is directed rightward. Poynting tells us that between the
plates, energy is flowing away from us at a rate proportional to EB.
Where is that energy coming from and going to?
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