Farad#Definition

The capacitance of a capacitor is one farad when one coulomb of charge changes the potential between the plates by one volt.{{cite book|title=The International System of Units (SI)|date=2006|publisher=Bureau International des Poids et Mesures (International Committee for Weights and Measures)|page=144|url=https://www.bipm.org/en/publications/si-brochure/|edition=8th}}{{Cite web |title=farad {{!}} Definition, Symbol, & Facts {{!}} Britannica |url=https://www.britannica.com/science/farad |access-date=2022-07-25 |website=www.britannica.com |language=en}} Equally, one farad can be described as the capacitance which stores a one-coulomb charge across a potential difference of one volt.{{cite book|title=Dictionary of Science and Technology|date=1995|publisher=Larousse|isbn=0752300105|editor=Peter M B Walker}}

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 SI prefixes:

• 1 mF (millifarad, one thousandth ({{10^|−3}}) of a farad) = 0.001 F = {{gaps|1|000}} μF = {{gaps|1|000|000|000}} pF
• 1 μF (microfarad, one millionth ({{10^|−6}}) of a farad) = 0.000 001 F = {{gaps|1|000}} nF = {{gaps|1|000|000}} pF
• 1 nF (nanofarad, one billionth ({{10^|−9}}) of a farad) = 0.000 000 001 F = 0.001 μF = {{gaps|1|000}} pF
• 1 pF (picofarad, one trillionth ({{10^|−12}}) of a farad) = 0.000 000 000 001 F = 0.001 nF

A farad is a derived unit based on four of the seven base units of the International System of Units: kilogram (kg), metre (m), second (s), and ampere (A).

Expressed in combinations of SI units, the farad is:

$$\backslash text\{F\}$$

= \dfrac{\text{s}^4 {\cdot} \text{A}^2}{\text{m}^{2} {\cdot} \text{kg}}

= \dfrac{\text{s}^2 {\cdot} \text{C}^2}{\text{m}^{2} {\cdot} \text{kg}}

= \dfrac{\text{C}}{\text{V}}

= \dfrac{\text{A} {\cdot} \text{s}}{\text{V}}

= \dfrac{\text{W} {\cdot} \text{s}} {{\text{V}^2} }

= \dfrac{\text{J}}{{\text{V}^2} }

= \dfrac{\text{N} {\cdot} \text{m}} {{\text{V}^2} }

= \dfrac{\text{C}^2}{\text{J}}

= \dfrac{\text{C}^2}{\text{N} {\cdot} \text{m}}

= \dfrac{\text{s}}{\Omega}

= \dfrac{1}{\Omega {\cdot} \text{Hz}}

= \dfrac{\text{S}}{\text{Hz}}

= \dfrac{\text{s}^{2}}{\text{H}},

where {{nowrap|1=F = farad}}, {{nowrap|1=s = second}}, {{nowrap|1=C = coulomb}}, {{nowrap|1=V = volt}}, {{nowrap|1=W = watt}}, {{nowrap|1=J = joule}}, {{nowrap|1=N = newton}}, {{nowrap|1=Ω = ohm}}, Hz = hertz, {{nowrap|1=S = siemens}}, {{nowrap|1=H = henry}}, {{nowrap|1=A = ampere}}.{{Cite book |url=https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507 |title=The International System of Units (SI) |publisher=Bureau International des Poids et Mesures |year=2019 |edition=9th |pages=138}}

The term "farad" was originally coined by Latimer Clark and Charles Bright in 1861,

As names for units of various electrical quantities, Bright and Clark suggested "ohma" for voltage, "farad" for charge, "galvat" for current, and "volt" for resistance. See:

• Latimer Clark and Sir Charles Bright (1861) [https://www.biodiversitylibrary.org/item/93052#page/483/mode/1up "On the formation of standards of electrical quantity and resistance,"] Report of the Thirty-first Meeting of the British Association for the Advancement of Science (Manchester, England: September 1861), section: Mathematics and Physics, pp. 37-38.
• Latimer Clark and Sir Charles Bright (November 9, 1861) [https://babel.hathitrust.org/cgi/pt?id=nyp.33433090837166;view=1up;seq=15 "Measurement of electrical quantities and resistance,"] The Electrician, 1 (1): 3–4. in honor of Michael Faraday, for a unit of quantity of charge, and by 1873, the farad had become a unit of capacitance.Sir W. Thomson, etc. (1873) [https://www.biodiversitylibrary.org/page/29853513#page/324/mode/1up "First report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units,"] Report of the 43rd Meeting of the British Association for the Advancement of Science (Bradford, September 1873), pp. 222-225. From p. 223: "The "ohm," as represented by the original standard coil, is approximately 10⁹ C.G.S. units of resistance: the "volt" is approximately 10⁸ C.G.S. units of electromotive force: and the "farad" is approximately 1/10⁹ of the C.G.S. unit of capacity." In 1881, at the International Congress of Electricians in Paris, the name farad was officially used for the unit of electrical capacitance.(Anon.) (September 24, 1881) [https://babel.hathitrust.org/cgi/pt?id=nyp.33433090837489;view=1up;seq=309 "The Electrical Congress,"] The Electrician, 7: 297. From p. 297: "7. The name farad will be given to the capacity defined by the condition that a coulomb in a farad gives a volt."{{cite book|last1=Tunbridge|first1=Paul|title=Lord Kelvin: his influence on electrical measurements and units|date=1992|publisher=Peregrinus|location=London|isbn=9780863412370|pages=26, 39–40|url=https://books.google.com/books?id=bZUK624LZBMC|access-date=5 May 2015}}

File:Capacitors Various.jpg

A capacitor generally consists of two conducting surfaces, frequently referred to as plates, separated by an insulating layer usually referred to as a dielectric. The original capacitor was the Leyden jar developed in the 18th century. It is the accumulation of electric charge on the plates that results in capacitance. Modern capacitors are constructed using a range of manufacturing techniques and materials to provide the extraordinarily wide range of capacitance values used in electronics applications from femtofarads to farads, with maximum-voltage ratings ranging from a few volts to several kilovolts.

Values of capacitors are usually specified in terms of SI prefixes of farads (F), microfarads (μF), nanofarads (nF) and picofarads (pF).{{cite book |title=Robotics, Mechatronics, and Artificial Intelligence |last=Braga |first=Newton C. |year=2002 |access-date=2008-09-17 |publisher=Newnes |page=21 |url=https://books.google.com/books?id=yqb-f-HKem0C&q=microfarad+common+measurement&pg=PA21 |isbn=0-7506-7389-3 |quote=Common measurement units are the microfarad (μF), representing 0.000,001 F; the nanofarad (nF), representing 0.000,000,001 F; and the picofarad (pF), representing 0.000,000,000,001 F.}} The millifarad (mF) is rarely used in practice; a capacitance of 4.7 mF (0.0047 F), for example, is instead written as {{gaps|4|700|u=μF}}. The nanofarad (nF) is used more often in Europe than in the United States.{{cite book |title=Make: Electronics: Learning Through Discovery |last=Platt |first=Charles |year=2009 |access-date=2014-07-22 |publisher=O'Reilly Media |page=61 |url=https://books.google.com/books?id=PQzYdC3BtQkC&q=nanofarad&pg=PA61 |isbn=9781449388799 |quote=Nanofarads are also used, more often in Europe than in the United States.}} The size of commercially available capacitors ranges from around 0.1 pF to {{gaps|5|000|F}} (5 kF) supercapacitors. Parasitic capacitance in high-performance integrated circuits can be measured in femtofarads (1 fF = 0.001 pF = {{10^|-15}} F), while high-performance test equipment can detect changes in capacitance on the order of tens of attofarads (1 aF = {{10^|−18}} F).{{cite book |title=Analog MOS Integrated Circuits for Signal Processing |last=Gregorian |first=Roubik |year=1976 |publisher=John Wiley & Sons |page=78}}

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.{{cite web |title=What's All This Femtoampere Stuff, Anyhow? |url=https://www.electronicdesign.com/test-amp-measurement/whats-all-femtoampere-stuff-anyhow |last=Pease |first=Bob |author-link=Bob Pease |publisher=Electronic Design |date=2 September 1993 |access-date=2013-03-09}}{{cite web |title=What's All This Best Stuff, Anyhow? |url=https://www.electronicdesign.com/analog/whats-all-best-stuff-anyhow |last=Pease |first=Bob |publisher=Electronic Design |date=1 December 2006 |access-date=2013-03-09}}

The capacitance of the Earth's ionosphere with respect to the ground is calculated to be about 1 F.{{cite web |url=http://konfluence.org/efield.pdf |title=Electrical Properties of the Fair-Weather Atmosphere and the Possibility of Observable Discharge on Moving Objects |last=Williams |first=L. L. |date=January 1999 |access-date=2012-08-13 |archive-url=https://web.archive.org/web/20161221015744/http://konfluence.org/efield.pdf |archive-date=2016-12-21 |url-status=dead }}

The picofarad (pF) is sometimes colloquially pronounced as "puff" or "pic", as in "a ten-puff capacitor".{{cite web |title=Puff |url=http://scienceworld.wolfram.com/physics/Puff.html |publisher=Wolfram Research |access-date=2009-06-09}} Similarly, "mic" (pronounced "mike") is sometimes used informally to signify microfarads.

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).{{cite web |title=1940 Radio Shack Catalog - Page 54 - Condensers |url=http://www.radioshackcatalogs.com/html/1940/hr054.html |website=radioshackcatalogs.com |access-date=11 July 2017 |archive-url=https://web.archive.org/web/20170711160428/http://www.radioshackcatalogs.com/html/1940/hr054.html |archive-date=11 July 2017}}

"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)

• μF (microfarad) = mf, mfd, uf
• pF (picofarad) = mmf, mmfd, pfd, μμF

{{unichar|3332}} is a square version of {{lang|ja|ファラッド}} ({{Transliteration|ja|faraddo}}, the Japanese word for "farad") intended for Japanese vertical text.

It is included in Unicode for compatibility with earlier character sets.

The reciprocal of capacitance is called electrical elastance, the (non-standard, non-SI) unit of which is the daraf.{{cite web|title=Daraf |url=http://www.websters-dictionary-online.com/definitions/daraf |publisher=Webster's Online Dictionary |access-date=2009-06-19 |url-status=dead |archive-url=https://web.archive.org/web/20111004054525/http://www.websters-dictionary-online.com/definitions/daraf |archive-date=2011-10-04 }}

The abfarad (abbreviated abF) is an obsolete CGS unit of capacitance, which corresponds to {{10^|9}} farads (1 gigafarad, GF).{{cite book |title=Modern Dictionary of Electronics |last=Graf |first=Rudolf F. |year=1999 |publisher=Newnes |page=1 |url=https://books.google.com/books?id=AYEKAQAAQBAJ |isbn=9780080511986 |access-date=2016-04-15}}

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.

{{Reflist|33em}}

• [https://www.translatorscafe.com/unit-converter/electrostatic-capacitance/ Farad unit conversion tool]

{{SI units}}

{{Michael Faraday}}

Category:SI derived units

Category:Units of electrical capacitance

Category:Michael Faraday