Re: [Phys-l] Poynting Vector

[John Denker <jsd@av8n.com>]

On 06/30/2008 10:31 AM, chuck britton wrote:

> 	The Poynting vector is non-zero only within the capacitor		nicht whar?

Yes and no.
 -- In the version that started this thread, fringing fields were
  unavoidably important, so there were Poynting vectors all over
  the place.
 -- In the modified version with the coaxial capacitor, with suitable
  engineering you can make the fringing fields unimportant.

> 	But the space filling Vector  Potential (A?) doesn't give a hoot  
> about the charged capacitor	

Trying to separate magnetism (B and A) from electricity (E and Φ)
is a risky business.  You can do it in the _static (DC) limit_ in
_one particular frame_ ... but if the fields change or if you have
any non-stationary observers, there is no longer any separation.

In some deep sense, there is no "E" or "B";  there is just the EM
field /F/.  This F is well behaved with respect to special relativity.

/Derivatives/ of A act directly on the charges.

> 	Would Feynmann's solenoid rotate if there were no charge in the  
> picture?

No, it would not rotate (leading order, excluding irreproducible junk 
effects).   The  whole point of the device is to demonstrate the angular 
momentum stored in the EM field.