Capacitors

Capacitors are passive electrical components that store energy in an electric field created by two conductors separated by a dielectric. When a voltage is applied, electric charge accumulates on the conductors, producing a stored energy E = 1/2 C V^2, where C is the capacitance. Capacitance depends on the plate area, the distance between plates, and the dielectric constant of the insulating material: C = ε_r ε_0 A / d.

Capacitance values span many orders of magnitude, from fractions of a picofarad to several farads. Real devices have parasitic properties, including an equivalent series resistance (ESR), an equivalent series inductance (ESL), and leakage current across the dielectric.

Capacitors are categorized by dielectric material and construction. Ceramic capacitors have ceramic dielectrics and are typically non-polar and compact. Film capacitors use plastic films for stable performance. Electrolytic capacitors (aluminum, tantalum) provide high capacitance in a small volume but are polarized and sensitive to voltage polarity and leakage. Supercapacitors offer very large capacitance for energy storage but operate at low voltages. Each type offers different combinations of voltage rating, tolerance, temperature performance, and cost.

Common applications include smoothing and filtering in power supplies, coupling and decoupling in signal paths, timing in RC networks, and energy storage in power electronics. Dielectric properties, voltage rating, temperature dependence, and aging affect performance and reliability.

The concept dates to the Leyden jar of the 18th century, with modern laminated and solid dielectrics developing through the 20th century.