Re: Charge interactions in media

[Bob LaMontagne <rlamont@POSTOFFICE.PROVIDENCE.EDU>]

pvalev wrote:

> V_metal  =  V_vacuum / D             /1/
>
> where D is the dielectric constant of the intervening material. The
> inspection of the physical picture or an experiment would convince
> them that V_m is practically zero, i.e. D is extremely great. But
> then they may find that, in accordance with Coulomb's law
>
> F = k(q1*q2)/Dr^2                     /2/
>
> the attraction between the plates has also vanished. (After all, the
> plates can be ragarded as two charged bodies that obey Coulomb's
> law). But this would be a wrong conclusion - the same physical
> picture that showed vanishing of the voltage now shows INCREASE of
> the force of attraction:

What is the justification of your last sentence? If the metal totally
fills the region between the plates, it shorts out the capacitor. There
must be an insulating gap next to the plates. Let's assume it's vacuum
for simplicity. The extremely low field in the metal is due to the
canceling of the original field before the metal is inserted by the
induced field produced in the metal because of its induced surface
charges. The field in the vacuum gap between the inserted metal and the
plates is still the same as before the metal was inserted. Since F = qE,
where is this INCREASE in the force of attraction coming from?

Bob at PC