Capacitor Theory

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 CV² joules

Capacitance:

If the dielectric (the material between the plates) is a vacuum, Capacitance
C = e₀ (A / l) where A is the area of the
capacitor plates, and l is the distance between them.

e₀ is the permittivity of free space
(8.85X10-12)

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

C = e_re₀
(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/C₁) + (1/C₂) + (1/C₃) …..

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.