Re: IONS/forces

[Ludwik Kowalski <kowalskiL@Mail.Montclair.edu>]

Bob Sciamanda wrote:

>  There is a typo in the force equation I gave in this post
>  (I corrected it in later posts). The charge density sigma
> should be squared; ie: the correct expression is dF/dA =
>  (1/2)*( sigma^2)/epsilon.

Yes,  I remember. Sorry for quoting your original without correcting
it. But, as you certainly noticed, my calculation of the electric pressure
does not depend on this dF/dA formula. It is just Coulomb's law
integrated over the entire shell. As most people know, the field
inside the shell is zero while the field outside is the same as if the
entire charge Q were in the center. But you must drill a tiny hole
to move a fictitious probe charge from inside to outside a shell.
Once you drill this hole the transition becomes gradual, depending
on the size of the opening. It turns out that the net force on the probe
charge dQ, when it is in the middle of the opening (at d=R from
the center), is exactly the same as if the remaining charge, Q-dQ,
were concentrated at one point 2*R away (on the opposite end of
the diameter). I did not know about this shortcut to calculate
electric pressure. It makes me happy to find something by myself;
others may have known this for a long time.

I was quoting you for the sake of continuity. This subject of
"not escaping from metals" is very intriguing, as you emphasized
several times. I hope somebody will solve the mystery (of the
classical "balancing force") to our mutual satisfaction.

Ludwik Kowalski