DIELECTRICS
One method of keeping the plates of a capacitor apart, which is necessary to
maintain charge separation and store potential energy, is to insert an insulator
(called a dielectric) between the plates.
A dielectric always increases the capacitance of a capacitor.Let’s see why this is true. Imagine charging a capacitor to a potential difference of
∆V with charge +Q on one plate and −Q on the other. Now disconnect the capacitor
from the charging source and insert a dielectric. What happens? Although the
dielectric is not a conductor, the electric field that existed between the plates
causes the molecules within the dielectric material to polarize; there is more
electron density on the side of the molecule near the positive plate.