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.

Equation: Q = C × V

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

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