Capacitance definition and charge equation

Definition: Capacitance is the ability of a capacitor to store charge. It is defined as the ratio of the charge stored on one plate to the potential difference between the plates:

C = Q / V

The unit is the farad (F). One farad is the capacitance of a capacitor that stores 1 coulomb of charge when 1 volt is applied across it.

Charge equation

Rearranging the definition:

Q = C V

where Q = charge in coulombs, C = capacitance in farads, V = potential difference in volts.

The charge is directly proportional to the applied voltage for a given capacitor. For a parallel plate capacitor, C = ε A / d, so capacitance depends on plate area A, plate separation d and the dielectric permittivity ε, not on Q or V.

Example: a 100 μF capacitor connected to 12 V stores Q = 100 × 10⁻⁶ × 12 = 1.2 × 10⁻³ C = 1.2 mC.