A capacitor is a passive two-terminal electrical component used to store energy electrostatically in an electric field. By contrast, batteries store energy through chemical reactions. The forms of practical capacitors vary widely but all contain at least two electrical conductors separated by an insulator. For example, a common construction consists of metal foils separated by a thin layer of insulating film. Capacitors are widely used as parts of electrical circuits in electrical devices.
Where there is a voltage across the conductors, a static electric field develops across the insulator causing a positive charge to collect on one plate and negative charge on the other plate. Energy Is stored in the electrostatic field. An ideal capacitor is characterized by a single constant value called the capacitance. This is the ratio of the electric charge on each conductor to the potential difference between them. The SI unit of capacitance is a farad. It is equal to one coulomb per volt.
The capacitance is largest when there is a narrow separation between large areas of conductors. Therefore capacitor conductors are often referred to as plates. The insulation between the plates passes a small amount of leakage current and also has an electric field strength limit, the breakdown voltage.
Capacitors are often used in electronic circuits for blocking direct current while allowing alternating current to pass. In analog filter networks, capacitors smooth the output of the power supply. In resonant circuits, capacitors tune radios to particular frequencies.