When a piece of insulator is inserted into a capacitor, we call the insulator a dielectric.

The dielectric is polarized by the field, and the electric field from the dielectric will partially cancel the electric field from the charge on the capacitor plates.

Adding a dielectric allows the capacitor to store more charge for a given potential difference.

Every material has a dielectric constant k that tells you how effective the dielectric is at increasing the amount of charge stored.

k

=

Eo E

> 1

E_o is the field without the dielectric and E is the field with the dielectric.

For a parallel-plate capacitor containing a dielectric, the capacitance is:

C

=

k eo A d