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Re: [Phys-l] Poynting Vector

I wrote:

It's coming from one side and going to the other. ;-)

But I probably should have added that the fringing fields guide it in the other direction on the outside back around to the start so that it isn't really "coming from" or "going to" anywhere.

This would seem to be a special case of what I think must be a perfectly general statement about the flow of EM energy (and, thus, momentum) around a static charge distribution immersed in a uniform magnetic field. In such a case, the magnitude of the integral of the Poynting flux over all space is proportional to the magnitude of the integral of the electric field vector over all space. Since every infinitesimal charge produces a spherically symmetric electric field with a vanishing integral over all space, the superposition principle guarantees that the integral of any arbitrary electrostatic field over all space is also zero.

Thus, there can be no net energy or momentum flow in the case under consideration or any other involving an electrostatic E field in a uniform B field.

Of course, as Curtis and Brian have intimated, none of this necessarily applies to an electrodynamic situation.

John Mallinckrodt
Cal Poly Pomona

On Jun 28, 2008, at 9:48 AM, 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?