Capacitance

Capacitance is the ability of a system to store an electric charge per unit potential difference between two conductors separated by an insulator. In the simplest case, a capacitor consists of two conductive plates separated by a dielectric. The capacitance C is defined as C = Q/V, where Q is the stored charge and V the voltage across the plates. For a parallel-plate capacitor with a dielectric material of relative permittivity εr, area A and separation d, C = ε0 εr A / d, where ε0 is the vacuum permittivity.

SI unit is the farad (F). More convenient scales: microfarad (µF), nanofarad (nF), picofarad (pF).

Capacitance depends on geometry, material properties, and temperature. The dielectric constant εr of the insulator determines how much charge can be stored for a given voltage. Real capacitors have leakage current, equivalent series resistance (ESR), and sometimes inductance (ESL).

Energy stored: E = 1/2 C V^2. Applications include filtering, timing, energy storage, coupling and decoupling signals, and smoothing power supplies.

Capacitors in circuits combine in parallel (C_total = Σ Ci) and in series (1/C_total = Σ 1/Ci).

Types include ceramic, electrolytic, film, tantalum, mica, and supercapacitors. Dielectric breakdown and voltage rating limit operation.

Frequency response: capacitance is largely independent of frequency for many applications, but dielectric losses occur and effective capacitance can vary; at high frequencies parasitic inductances and parasitic resistances become important.

Measurement methods include LCR meters and bridge measurements.