Capacitans

Capacitans, usually referred to in modern terminology as capacitance, is a property of a system that quantifies its ability to store electric charge when a potential difference is applied. It is defined as C = Q/V, where Q is the stored charge and V is the voltage across the system. The SI unit is the farad (F), defined as one coulomb per volt.

In a simple capacitor consisting of two conductors separated by a dielectric, the stored energy is E = 1/2 C V^2, or equivalently E = Q^2/(2C). Capacitance depends on geometry and the dielectric material between the conductors: for a parallel-plate capacitor, C = ε0 εr A/d, where A is plate area, d is separation, ε0 is vacuum permittivity, and εr is the relative permittivity of the dielectric. More complex shapes yield different capacitances, and in circuits capacitance can be distributed among components and wiring.

In alternating current circuits, an ideal capacitor presents a reactance Xc = 1/(2πfC), which decreases with higher frequency. Real capacitors exhibit parasitic effects such as equivalent series resistance and leakage.

Applications of capacitance include storage of electrical energy, filtering and smoothing in power supplies, timing and frequency selection in oscillators, and coupling or decoupling signals in electronic circuits.

In some historical or physiological contexts, the term capacitans is used to refer to capacitance, or to venous capacitance in the circulatory system, describing the ability to store a volume of blood analogous to charge storage.

See also: dielectric, permittivity, capacitor.