The Capacitor

A capacitor stores equal and opposite charges on two conductors. Its capacitance is the charge per volt:

$$Q = CV$$

in farads ($1\ \mathrm{F}$ = $1\ \mathrm{C/V}$).

For parallel plates:

$$C = \varepsilon_0\,\frac{A}{d}$$

Bigger plates, closer together, store more. C does not depend on Q.

The energy stored in a charged capacitor:

$$U = \tfrac{1}{2}CV^2$$