# Capacitor Theory

A capacitor basically consists of
two plates with an insulator in between, although in practice the 'plates' are normally
rolled up in a can to save space. It can be used in a circuit to store charge for small
periods of time.

#### Charge stored in a capacitor:

Charge Q = CV where C is the capacitance in Farads

charge Q is measured in coulombs (C)

#### Energy stored in a capacitor:

Energy stored, W = ½ QV = ½ CV^{2} joules

#### Capacitance:

If the dielectric (the material between the plates)
is a vacuum, Capacitance C = e_{0} (A / l) where A is
the area of the capacitor plates, and l is the distance between them.

e_{0} is the
permittivity of free space (8.85X10-12)

If the dielectric is another material, capacitance is given by:

C = e_{r}e_{0} (A / l) where e_{r}
is the relative permittivity, which varies between materials.

#### Capacitors in Series:

Putting capacitors in series reduces the overall capacitance:

(1/C) = (1/C_{1}) + (1/C_{2}) + (1/C_{3})
.....

#### Capacitors in parallel:

Putting capacitors in parallel increases the total capacitance:

C = C1 + C2 + C3 .....

Note that the series and parallel capacitance formulae are the opposite of those for resistance.

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