The farad (symbolized F) is the standard unit of capacitance in the International System of Units (SI). Reduced to base SI units, one farad is the equivalent of one second to the fourth power ampere squared per kilogram per meter squared (s4 · A2 · kg-1 · m-2).
When the voltage across a 1 F capacitor changes at a rate of one volt per second (1 V/s), a current flow of 1 A results. A capacitance of 1 F produces 1 V of potential difference for an electric charge of one coulomb (1 C). The farad is an extremely large unit of capacitance. In practice, capacitors with values this large are almost never seen.
In common electrical and electronic circuits, units of microfarads (µF), where 1 µF = 10-6 F, and picofarads (pF), where 1 pF = 10-12 F, are used. At radio frequencies (RF), capacitances range from about 1 pF to 1,000 pF in tuned circuits, and from about 0.001 µF to 0.1 µF for blocking and bypassing. At audio frequencies (AF), capacitances range from about 0.1 µF to 100 µF. In power-supply filters, capacitances can be as high as 10,000 µF.
Also see capacitor, inductor, reactance, and henry.
This was last updated in March 2010
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