# Capacitance

Start with two parallel plates connected to a battery. A potential is set up between them. If a conductor is placed between them, charge will flow. If there is no conductor, static charge will be stored. If an insulating material called a dielectric (such as paper or paraffin) is placed between them, a higher level of charge will be built up.
We call this a parallel plate capacitor.

When two plates are placed in this configuration, there is a proportional relationship between potential difference and total charge. We call this relationship Capacitance (C). The units for capacitance are Farads (F). C = Q/V. Typical capacitors in use in circuits today have capacitance in the microfarad (10 -6 F) to picofarad ( 10 - 12 F)  range.

Recall that V = Ed. For two parallel plate capacitors, the electric field is uniform. If A is the area of the plates, d
is the distance between the plates, and
e0is the permittivity of free space, then V
= Qd/
e0A and C = e 0A/d.

Capacitance is directly proportional to the area of the plates and inversely proportional to the distance between the plates. In a graph of charge vs. potential, the slope of the line represents our capacitance and the area under the curve represents the energy stored.

In a graph of charge vs. potential, the slope of the line represents our capacitance and the area under the curve represents the energy stored.

The energy stored = 1/2 CV2. Units are Joules.