Page 148 - MODERN ELECTROCHEMISTRY

88 CHAPTER 2

Fig. 2.25. The charge on the capacitor plates induces dipoles in
the molecules of a dielectric.

charge on them but separated by a material medium, a so-called dielectric. The model
by which dielectrics affect fields is easy to understand at a qualitative level. The
molecules of the medium either contain a permanent dipole moment in their structure
or have one induced by the field between the plates. Of course, when the field on the
condenser plates is switched on, the dipoles orient against it (Fig. 2.25) and cause a
counter electric field. The result is that the net electric field between the plates is less
than it is when there is no medium between them (Fig. 2.26).
The counter field and the resulting net field can be calculated in mathematical
form, but historically a more empirical way has been used; the field in the presence of
a dielectric is simply expressed by dividing the field in its absence by an empirical
“dielectric constant.” The greater the counter field set up by the medium between the
plates (Fig. 2.26), the greater the dielectric constant and the less the net electric field.
With this simple background model, then, it is easy to see that there will be a
decrease of the dielectric constant of solutions (compared with that of the original

Fig. 2.26. The orientation of dipoles in the dielec-
tric sets up an internal field that is directed counter
to the external field produced by the charges on
the plates.