Capacitor

Capacitor, or condenser, electrical component used to store electric charge and to provide reactance in alternating current circuits. In essence, a capacitor consists of 2 conducting plates separated by a thin layer of insulator. When the plates are connected to the terminals of a battery, a current flows until the capacitor is “charged,” with 1 plate positive and the other negative. The ability of a capacitor to hold charge, its capacitance C, is the ratio of the quantity of electricity on its plates, Q, to the potential difference between the plates, V. The electric energy stored in a capacitor is given by CV². The capacitance of a capacitor depends on the area of its plates, their separation, and the dielectric constant of the insulator. Small fixed capacitors are commonly made with metal-foil plates and paraffin-paper insulation; to save space, the plates and paper are rolled up into a tight cylinder. Some small capacitors have a mica dielectric. Variable capacitors used in radio tuners consist of intermeshing metal vanes separated by an air gap. In electrolytic capacitors, the dielectric is an oxide film formed on the plates by the action of a solid electrolyte.