Re: IONS/metal pedagogy

["Brian McInnes" <bmcinnes@pnc.com.au>]

Hi Bob, 
I see where you are coming from.  If there is a net electrostatic force on
the surface charge then there has to be a (magic) force to balance this out
or there is a (magic) proscription: thou shalt not move off the edge of the
metal!
Now I accept that as you say "the standard classical line of reasoning
(hinging largely on Gauss' law) says that a conductor (charged or neutral -
in an external field or not) is under a force of tension wherever there is a
surface charge". 
However, I want to check out the details of the argument (none of my
classical electricity books are here at home).
  
In the meantime, one point that really worries me is the reference to "force
of tension".  Tension is what you get when molecules are too far away from
one another: examples are a stretched string or the surface of a liquid. 
When the molecules are too close we have compression as in the body of a
liquid (which leads to the phenomenon of pressure) or in a table leg. 
Forces of tension are attractive and, importantly, leave any particular
molecule along the string or in the surface in equilibrium: NO NET FORCE*. 
Similarly for forces of compression.  Now what the classical line calls a
force of tension surely can't be the same as that, or is it?  Anyway, I want
to follow it up early this week.

*Twenty or so years ago, Warren, in his excellent book, Teaching of Physics,
pointed out how flawed most (all?) textbook diagrams are on this point. 

Brian McInnes  


*******************************
> Hi Brian,
> I apologize - my statement was hurried and vague - but of serious import.
> I was referring to a line of reasoning which I earlier outlined:  the
> standard classical line of reasoning (hinging largely on Gauss' law) says
> that a conductor (charged or neutral - in an external field or not) is
> under a force of tension wherever there is a surface charge.  The net
> electrostatic force per unit area (DUE TO THE REST OF THE CONDUCTOR AND
> THE REST OF THE UNIVERSE) = (1/2) sigma^2 /epsilon - always an outward
> force.  This (says the classical model) is the net electrostatic force
> due to the universe, per unit area of charged conductor surface. IE; the
> NET ELECTROSTATIC FORCE is not zero - it is outward.  If our scheme says
> the conducting surface is in equilibrium under only electrostatic forces,
> this (largely Gauss based conclusion) is in question.  I really hope
> something is wrong with my reasoning here, because it looks to me like
> any classical model has to include some non-electrostatic force to
> achieve equilibrium here (as we feel forced to do within the nucleus).
> Please straighten me out!
>
> Bob Sciamanda
> Physics, Edinboro Univ of PA (ret)
> trebor@velocity.net
> http://www.velocity.net/~trebor