Capacitor

A capacitor is a passive two-terminal electronic component that stores energy in an electric field created by two conductors separated by a dielectric. When a voltage is applied, charge accumulates on the conductors and the device stores energy that can be released later. The capacitance, measured in farads, depends on geometry and the dielectric; for a parallel-plate capacitor, C = ε_r ε0 A / d.

Capacitors are manufactured in many forms. The unit is the farad (F); practical devices are typically in microfarads, nanofarads, or picofarads. Types include ceramic, film, mica, and glass capacitors (generally non-polarized), and electrolytic and supercapacitors (polarized, with higher capacitance). Dielectric materials determine voltage rating, stability, and losses.

Electrical behavior follows Z = 1/(jωC) for ideal capacitors; the reactance Xc = 1/(2πfC) decreases with frequency. Real capacitors have characteristics such as leakage current, equivalent series resistance (ESR), and equivalent series inductance (ESL), and their performance varies with temperature and voltage. Polarized types require correct polarity.

Applications include filtering and smoothing of power supplies, decoupling and coupling in signal paths, timing and frequency selection in filters and oscillators, and energy storage in electronic circuits. Capacitors are also used in power-factor correction and tuning circuits. The concept of capacitance and the unit farad are attributed to studies in the 18th and 19th centuries, notably by Michael Faraday.