The *charge* (Q) stored on the plates of a capacitor is related to the *capacitance* (C) multiplied by the voltage (V) across the capacitor plates.

Charge is measured in *coulombs* (C) while capacitance is given in *farads* (F), and voltage in *volts* (V).

General purpose capacitor values are commonly given in picofarads (pF), microfarads (µF) or millifarads (mF), although supercapacitors are available as energy storage devices with values in excess of 1 farad. Polarised types, such as electrolytic or tantalum capacitors, typically offer larger storage capacities than their non-polarised equivalents.

#### Worked Example 1

Find the charge stored by a 4.7 µF capacitor charged to a voltage of 50 V.

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Q = C × V

= 4.7 × 10^{−6} × 50

= 235 µC

#### Worked Example 2

Find the potential difference across a 220 nF capacitor which holds a charge of 5 µC.

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Transposing to make V the subject gives:

V = Q / C

= 5 × 10^{−6} / 220 × 10^{−9}

= 22.73 V

#### Worked Example 3

Find the capacitance of an unknown capacitor if a stored charge of 20 µC causes a potential difference of 100 V to be developed across the plates of the capacitor.

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Transposing to make C the subject gives:

C = Q / V

= 20 × 10^{−6} / 100

= 200 pF