Date: Wed, 12 Aug 1998 12:04:37 +0200 (DFT)
From: Konrad Hinsen <hinsen@cnrs-orleans.fr>
CC: chemistry@infomeister.osc.edu
Subject: CCL:Re: CCL:permittivity


> In the expression for Coulombic potential
>
>                    e^2
>      V(r) = -  -----------
>               4  pi  Eo  r
>
> where Eo the permittivity of free space; e charge; r distance between
> electron and proton. Could someone enlighten me the significance
> of permittivity, what actually it means and why does the coulombic
> potential decreases with the increase in permittivity.

The "permittivity of free space" is a universal physical constant,
like the speed of light or the gravitational constant. In fact, you
can make it disappear by choosing different units of measurement for
the electrostatic quantities.

The concept of permittivity comes from the theory of electrostatics in
matter. This theory was developed at a time when the structure of
matter was not yet understood, and so "free space" was considered a
material just like any other. It was found empirically that the
influence of simple materials on electrostatic fields could be described
by a single parameter, called "permittivity".

If you put charges into some material (meaning a macroscopic sample,
not just a few atoms), the field generated by the charges will cause
polarization in the material, and this polarization will weaken the
effective electrostatic field. The permittivity indicates how strong
this induced polarization is in a given material. This explains why a
higher permittivity leads to a smaller effective potential.

In vacuum there is no polarization, and the permittivity of vacuum is
simply a universal constant whose name says more about the history
of electrostatics than about the quantity itself.

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