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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


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 = ere0 (A / l) where er 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.

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