The capacitance of a linear two-terminal capacitor is the constant relating its separated electric charge to its terminal electric potential difference:Here is the electric charge on one electrical conductor, the other carries , and is their electric potential difference with the same ordering. Reversing the terminal ordering reverses both and and leaves capacitance unchanged. An ideal passive capacitor has ; an ordinary pair of separated electrical conductors with positive permittivity has . At , use rather than the undefined ratio .
For example, electric charge magnitude at terminal electric potential difference gives capacitance . At , the same linear capacitor has and the same capacitance. The plots in capacitor energy show that capacitance is the slope of the electric charge versus terminal electric potential difference graph.
For fixed geometry and linear material response, capacitance depends on dimensions and permittivity, rather than the source's terminal electric potential difference. This is why capacitance is a useful property of a component: once it is known, predicts the separated electric charge for each operating terminal electric potential difference. The SI unit is the farad, . [ref].
For combinations of capacitors, the same terminal definition gives an equivalent capacitance. Two identical capacitors of capacitance connected in parallel share the same terminal electric potential difference, and their electric charges add, giving . Connected in series with an initially neutral isolated junction, they carry equal plate electric charge magnitudes and their terminal electric potential differences add, giving . [ref].
Capacitance is a measure of a capacitor's ability to store electric charge. It is defined as the amount of electric charge \( Q \) stored per unit voltage \( V \) across the capacitor.
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