A
capacitor (originally known as
condenser) is a
passive two-terminal electrical component used to store
energy in an
electric field. The forms of practical capacitors vary widely, but all contain at least two
electrical conductors separated by a
dielectric (insulator); for example, one common construction consists of metal foils separated by a thin layer of insulating film. Capacitors are widely used as parts of
electrical circuits in many common electrical devices.
When there is a
potential difference (voltage) across the conductors, a static
electric field develops across the dielectric, causing 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,
capacitance, measured in
farads. This is the ratio of the
electric charge on each conductor to the potential difference between them.
The capacitance is greatest when there is a narrow separation between large areas of conductor, hence capacitor conductors are often called
plates, referring to an early means of construction. In practice, the dielectric between the plates passes a small amount of
leakage current and also has an electric field strength limit, resulting in a
breakdown voltage, while the conductors and
leads introduce an undesired
inductance and
resistance.
Capacitors are widely used in electronic circuits for blocking
direct current while allowing
alternating current to pass, in filter networks, for smoothing the output of
power supplies, in the
resonant circuits that tune radios to particular
frequencies, in electric power transmission systems for stabilizing voltage and power flow, and for many other purposes.