Capacitor

Capacitor, device for storing an electrical charge, sometimes called a condenser. In its simplest form a capacitor consists of two metal plates separated by a non-conducting layer called the dielectric. The dielectric may be air, plastic, waxed paper, or another substance such as the mineral mica. When one plate of a capacitor is charged using a battery or other source of direct current, the other plate becomes charged with the opposite sign; that is, positive if the original charge is negative, and negative if the original charge is positive.

The electrical size of a capacitor is its capacitance, that is the amount of electric charge it can hold per unit potential difference across its plates—C = Q/V. The SI unit of capacitance is the farad (F). Because this is such a large unit, capacitors commonly have their size expressed in µF (1 microfarad = 10^-6 F) or pF (1 picofarad = 10^-9 F). The capacitance of a parallel plate capacitor can be calculated from the relationship:

where A is the area of the plates, d is the distance between them, ε₀ is the permittivity of free space, and ε_r is the relative permittivity of the dielectric between the two plates.

Capacitors can hold a limited amount of electric charge. As more and more charge is added to the plates of a capacitor, the potential difference between the plates increases. Eventually this potential difference becomes so great that the atomic structure of the dielectric breaks down, and charge “leaks” through it. Capacitors can conduct direct current for only an instant but are able to act as conductors in alternating-current circuits, as they constantly charge and discharge as the direction of the current constantly changes. This property makes them useful when direct current must be prevented from entering some part of an electric circuit. Fixed-capacity and variable-capacity capacitors are used with coils in resonant circuits in radios and other electronic equipment.

Because the dielectric of a capacitor may break down, there is a limit to the potential difference that may be applied across a capacitor. Capacitors are therefore labelled not only with their capacitance but also with their working potential difference in order to prevent breakdown of the dielectric in use.