A capacitor is a device for storing charge. The simplest type of capacitor is made up of two conductors separated by either empty space or by an insulating material known as the dielectric. For a capacitor storing charge Q, one conductor has a charge of +Q and the other has a charge of -Q.

The amount of charge a capacitor can store for a given potential difference is given by its capacitance, C. The unit of capacitance is the farad (F).

For a capacitor with a charge of +Q on one plate and -Q on the other:

Q = C DV

For a parallel-plate capacitor   C

=

eoA d

The energy stored in a capacitor is U

=

Q DV 2

=

C DV2 2

=

Q2 2C