Re: IONS/metal pedagogy

["Bob Sciamanda" <trebor@velocity.net>]

Repost of a bounce of 10/13/98.  This taught me that if you travel away
from your local ISP and try to post to PHYS-L from other e-mail addresses
(Hotmail, Juno, etc.) they will bounce unless you subscribe each address
to PHYS-L.  I had assumed anyone could post - not so!
*****************************************************
Hi Brian, What you say below was said before by others and I accepted
the arguments.  My problem is a very simple apparent contradiction:  If
your model is "true", then the predicted net electrostatic force
(including external electrostatic forces) on the conducting surface
element dA (carrying the charge sigma*dA), due to the universe, is zero.
(I am redundant for needed emphasis)

This conflicts with the aforementioned prediction of the classical
model that the NET electrostatic force on dA (due to the universe -
including also all external ELECTROSTATIC forces) is not zero but is
(1/2)*sigma^2*dA/epsilon.  A non-electrostatic force seems to be
required to achieve zero net force on dA.

IT IS THE NET ELECTROSTATIC FORCE OF THE UNIVERSE WHICH IS REDUCED TO
ZERO INSIDE A CONDUCTOR; all possible electrostatic forces on dA (due to
the rest of the conductor and the rest of the universe) are included in
the derivation of the statement that dF=(1/2)(sigma^2)*dA/epsilon.
Please convince me otherwise!
Please straighten me out!

-Bob
trebor@velocity.net

> snip
> What happens if we put a
> charged object on the surface of that metal, say for example another
> electron, so that metal now has a net charge.  The electrons in the
metal
> near that charge will be repelled leaving a positive region in the
metal
> which serves to attract that electron and keep it from drifting off
into
> space.  Now lets add a second electron.  You might think that since the
> metal already carries a negative charge, the next electron would be
> repelled.  It is, as long as the electron is not too near the surface.
> By not too near I mean far enough away not to induce a redistribution
of
> the electrons in the metal.  However when the electron gets close
enough,
> it repels the electrons in the metal just as the first one did, and
> creating a positive region which attracts the second electron.  Even
> though the metal carries a charge, the induced interaction is
> stronger than the coulomb one, and the electron is bound to the
surface.
> This happens because the Coulomb force falls off as the square of the
> distance, so the positive region near charge on the surface has a much
> greater influence than the distributed charge further away.  Of course
> as I add more and more electrons, the coulomb repulsion gets bigger and
> bigger, and eventually other things happen.
> end of snip
>
> So what I see is the conductor in "tension" but also in equilibrium
being
> acted on
> by equal and opposite electrostatic forces, external and internal to
the
> conductor.
>
> This gives an explanation in terms of classical forces and preserves
the
> consequences of Gauss' law.
>
> Brian McInnes