FilmCapacitor

A capacitor is an electrical/electronic device that can store energy in the electric field between a pair of conductors (called "plates"). The process of storing energy in the capacitor is known as "charging", and involves electric charges of equal magnitude, but opposite polarity, building up on each plate.

Capacitors are often used in electric and electronic circuits as energy-storage devices. They can also be used to differentiate between high-frequency and low-frequency signals. This property makes them useful in electronic filters.

Theory of operation
A capacitor consists of two conductive electrodes, or plates, separated by a dielectric, which prevents charge from moving directly between the plates. Charge may however be moved indirectly by external influences, such as a battery connecting the terminals. After removing the external influences, the charge on the plates persists. The separated charges attract each other, and an electric field is present between the plates. The simplest practical capacitor consists of two wide, flat, parallel plates separated by a thin dielectric layer.

Assuming that the area of the plates A is much greater than their separation d, the instantaneous electric field between the plates E(t) is identical at any location away from the edges. If the instantaneous charge on a plate, − q(t), is spread evenly, then, where  is the permittivity of the dielectric. The voltage v(t) between the plates is given by, where z is a position between the plates.

Capacitance
 
Charge separation in a parallel-plate capacitor causes an internal electric field. A polarized dielectric spacer (orange) reduces the electric field and increase the capacitance.A property called the capacitance C, which is a measure of the charge stored on each plate for a given voltage such that q(t) = Cv(t), wherefor an ideal parallel plate capacitor.

In SI units, a capacitor has a capacitance of one farad when one coulomb of charge storage corresponds to one volt between its plates. Since the farad is a very large unit, capacitance is usually expressed in microfarads (µF), nanofarads (nF), or picofarads (pF). In general, capacitance is greater in devices with large plate areas, separated by small distances. When a dielectric is present between two charged plates, its molecules become polarized and reduce the internal electric field and hence the voltage. The capacitance is therefore strongly dependent on the quality of the dielectric.

Energy storage
Work must be done by an external influence to move charge between the plates in a capacitor. When the external influence is removed, the charge separation persists and energy is stored in the electric field. If charge is later allowed to return to its equilibrium position, the energy is released. The maximum energy that can be stored safely in a capacitor is limited by the breakdown voltage of the dielectric. Therefore, all capacitors made with a particular dielectric have approximately identical maximum energy density if the dielectric dominates their volume. In the presence of dielectric more charge can be stored on capicator.