Re: [Phys-l] another capacitor problem

[John Denker <jsd@av8n.com>]

On 03/28/2008 12:15 PM, curtis osterhoudt wrote:

> One final thought (which may be pedagogically useful for
> instructors): Imagine two disconnected plates, lying fixed in the
> same plane. Then connect them via a battery with some EMF. All one
> can say is that one plate becomes positive relative to the other. 

Quite so.

> Now
> bring in an infinite "ground" plane parallel to the plates' plane.
> Does the total energy of the battery-connected plates change? One
> approach says "no" -- 

Huh?

> there is no change of potentials between the
> plates, 

What's the evidence of that?

> and no net charge flow. 

The energy is in the field, not in the charge.  Lack of charge
flow doesn't tell you what you need to know.

> However, there will obviously be a
> rearrangement of the electrostatic field lines as the ground plane
> comes closer. 

If you do the calculation, or do a simulation, 
  http://www.av8n.com/physics/laplace.htm
or even do a careful qualitative analysis, you will find that
potentials do change if you bring in a third electrode.  

BTW, this is true even if the new electrode is not infinite, and 
even if it is not "grounded".

Useful thing to keep in mind:  The notion of "ground" is widely
and wildly abused.  If you're not careful, it is not even possible
to properly define "ground" ... hence the dreaded "ground loops".
Constructive suggestion:  Draw a diagram that shows what is connected
to what, and /how/.  Sometimes it is possible to use the expression 
"connected to ground" as a shorthand for some type of real, physical 
connection ... but it is up to you to explain what you mean by this, 
and to make sure that it is well-defined.  Beginners should avoid
"ground" entirely.