Re: The Joy Of Poynting Vectors.

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Jim Lux" <jimlux@xxxxxxxxxxxxx>


----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Saturday, April 23, 2005 7:27 AM
Subject: Re: The Joy Of Poynting Vectors.


> Original poster: stork <stork@xxxxxxxxxxxxxxxxxxx>
>
> Tesla list wrote:
> >Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>
>
> >The Poynting vector is the cross product of the electric and magnetic
> >fields. It shows the direction that power is flowing in a system of EM
> >fields. It's the same as the "direction of propagation" of a radio wave
> >that you get taught in your ham radio licence classes.
>
> Careful friend.  Power oscillating in the same direction as the "direction
> of propagation" of a radio wave is a dangerous concept on this board.
Some
> may confuse it with longitudinal waves.  A concept not accepted and banned
> on this the Tesla list.
>
> stork

The instantaneous power flow (i.e. the direction of the Poynting vector) can
oscillate towards and away from the overall outward flow (i.e. the outs are
bigger than the ins).. this is what happens in the nearfield of an antenna.
In fact, the definition of the boundary between near and far field (for this
purpose) is where the power that radiates away (never to return) is equal to
that returning back to the antenna.

This is very different from somewhere a "long way" from the source where the
Poynting vector would always point the same direction.

The longitudinal wave folks postulate that you can have this back and forth
forever (as in acoustic wave propagation, which uses a totally different
propagation method, albeit still nicely described by second order
differential equations).