Re: [Phys-L] charged paralle plates

[John Denker <jsd@av8n.com>]

On 01/22/2014 03:41 PM, I wrote:
> 5) Superpose the two sub-problems.  We now have
>  2 units of gorge and -1 units of charge.
>
> 6) Check the work.  In particular, verify that the
>  two inner faces still have equal-and-opposite
>  charges.  This is consistent with the requirement
>  to ignore fringing fields, together with the basic
>  equations of electrostatics.  Any field line that
>  originates from one inner face must terminate on
>  the other inner face, because there is nowhere
>  else for it to go.  This alone is enough to 
>  guarantee uniqueness of the solution.


I should have continued:

  Check the work.  Check that both plates have the
  specified amount of charge.

This would have revealed that the right answer is
2.5 units of gorge (not 2) ... plus 1 unit of charge.

Sorry.

==========

Lame excuse #1:  It's basically just a typo.  I solved
it correctly;  I just didn't write it up correctly.
Either way, it's amazingly dumb, given that other
folks had already posted the correct numbers.

Lame excuse #2:  I've got the flu.  My temperature 
is 4.3° F above normal.  This makes me even more
incoherent than usual.  I'm going back to bed now.


==========================

There are a lot of situations where it really helps
to be able to think clearly about the distinction
between /gorge/ on a capacitor and plain old net
/charge/ on a capacitor.  Ditto for batteries.
For details, see:
  http://www.av8n.com/physics/electrophorus.htm#sec-gorge