Named for scientist Michael Faraday of England, the farad is a unit of electrical capacitance. If a device called a capacitor stores a charge of 1 coulomb at a potential difference of 1 volt across its plates, it is a one-farad capacitor. This unit is actually too large for most practical applications. Typical-use capacitors fall within the millifarad (10^{-3}) to microfarad (10^{-6}) to picofarad (10^{-12}) ranges. The farad as a unit is so large because the coulomb, used in defining the farad, is itself large — 1 coulomb=1 ampere-second.
Units of measurement that include the farad are called the International System of Units or SI units, and are metric-based. Basic SI units include the meter, the kilogram and the second. Other units used in this system include the joule for energy and work, the ohm for resistance, the newton for force and the henry for inductance. Along with the farad, all these electrical quantities are used in important mathematical expressions. It is impossible to discuss what a farad is without discussing the nature of and applications for the capacitor.
A parallel-plate capacitor is the simplest design to illustrate the basic parameters of capacitance. This property relates directly to the surface area of the plates and is inversely proportional to the distance between those plates. Proportionality depends upon the medium between the plates and a quality it possesses called "permittivity," which is the measure of resistance to an electric field that the material between the plates supports or permits.
The mathematical relationship is C=kε_{0}A/d. In this equation, the permittivity of space is "ε_{0}" which equals 8.854*10^{-12} farads/meter. The plates' area in square meters is represented by "A" and "d" is the number of meters between them. Constant k is the relative permittivity of the actual material between the plates, whether it is air, a ceramic, oil or some other substance. As an example, if a parallel-plate capacitor has plates with an area of 1 square meter each, and the distance between them is 0.001 meters, and the permittivity, k, is 1.07, then the capacitance is 9,474 picofarads.
Applications for capacitors include direct current blockage, signal filtration, for electrical discharge, to limit transient magnitude and to couple two circuits. For this last application, capacitors may be used in conjunction with coils to set up a specific inductance. This is called an LC circuit, sometimes a tuned or resonant circuit, and when combined with a resistance or impedance, it is called an "RLC" circuit. It limits signal transfer from a more complex signal to a particular frequency. Such circuits are used in television reception.