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The farad (symbol: F) is the unit of electrical capacitance, the ability of a body to store an electrical charge, in the International System of Units (SI), equivalent to 1 coulomb per volt (C/V). It is named after the English physicist Michael Faraday (1791–1867). In SI base units 1 F = 1 kilogram−1⋅meter−2⋅second4⋅ampere2.
The relationship between capacitance, charge, and potential difference is linear. For example, if the potential difference across a capacitor is halved, the quantity of charge stored by that capacitor will also be halved.
For most applications, the farad is an impractically large unit of capacitance. Most electrical and electronic applications are covered by the following :
Expressed in combinations of SI units, the farad is:
where , , , , , , , , Hz = hertz, , , .
Values of capacitors are usually specified in terms of SI prefixes of farads (F), microfarads ( μF), nanofarads ( nF) and picofarads ( pF). The millifarad ( mF) is rarely used in practice; a capacitance of 4.7 mF (0.0047 F), for example, is instead written as . The nanofarad ( nF) is used more often in Europe than in the United States. The size of commercially available capacitors ranges from around 0.1 pF to (5 kF) . Parasitic capacitance in high-performance integrated circuits can be measured in femtofarads (1 fF = 0.001 pF = F), while high-performance test equipment can detect changes in capacitance on the order of tens of attofarads (1 aF = F).
A value of 0.1 pF is about the smallest available in capacitors for general use in electronic design, since smaller ones would be dominated by the parasitic capacitances of other components, wiring or printed circuit boards. Capacitance values of 1 pF or lower can be achieved by twisting two short lengths of insulated wire together.
The capacitance of the Earth's ionosphere with respect to the ground is calculated to be about 1 F.
Nonstandard abbreviations were and are often used. Farad has been abbreviated "f", "fd", and "Fd". For the prefix "micro-", when the Greek small letter "μ" or the legacy micro sign "μ" is not available (as on typewriters) or inconvenient to enter, it is often substituted with the similar-appearing "u" or "U", with little risk of confusion. It was also substituted with the similar-sounding "M" or "m", which can be confusing because M officially stands for 1,000,000, and m preferably stands for 1/1000. In texts prior to 1960, and on capacitor packages until more recently, "microfarad(s)" was abbreviated "mf" or "MFD" rather than the modern "μF". A 1940 Radio Shack catalog listed every capacitor's rating in "Mfd.", from 0.000005 Mfd. (5 pF) to 50 Mfd. (50 μF).
"Micromicrofarad" or "micro-microfarad" is an obsolete unit found in some older texts and labels, contains a nonstandard metric double prefix. It is exactly equivalent to a picofarad (pF). It is abbreviated μμF, uuF, or (confusingly) "mmf", "MMF", or "MMFD".
Summary of obsolete or deprecated capacitance units or abbreviations: (upper/lower case variations are not shown)
is a square version of ファラッド (, the Japanese word for "farad") intended for Japanese [[vertical text]].
It is included in Unicode for compatibility with earlier character sets.
The statfarad (abbreviated statF) is a rarely used CGS unit equivalent to the capacitance of a capacitor with a charge of 1 statcoulomb across a potential difference of 1 statvolt. It is 1/(10−5 c2) farad, approximately 1.1126 picofarads. More commonly, the centimeter (cm) is used, which is equal to the statfarad.
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