Re: [Phys-l] Poynting Vector

[John Denker <jsd@av8n.com>]

On 06/28/2008 09:48 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?

The energy is flowing around and around, chasing its tail.  It does not 
accumulate anywhere, as you can easily verify by showing that ∇•S = 0.

Since it is not particularly easy to visualize the fringing fields of
a parallel-plate capacitor, it may help to consider (as a warm-up
exercise at the very least) a concentric coaxial capacitor, i.e. 
where the field is in the gap between two tubes.  The situation is
nicely symmetric, and the energy just flows around and around in
circles.