Electric generator

2014-02-03T07:42:23Z

202.186.56.42: /* History */

{{About|the unit of measurement|the former settlement in California|Farad, California}}
{{Distinguish|faraday (unit)}}
{{lead too short|date=December 2012}}

{{Infobox Unit
| bgcolour =
| name = Farad
| image = [[File:OneFarad5.5Velectrolyticcapacitor.jpg|300px]]
| caption = A comparatively small 1 farad capacitor, for low voltages and current transfers.
| standard = [[SI derived unit]]
| quantity = [[Capacitance]]
| symbol = F
| dimension = M−1⋅L−2⋅T4⋅I2
| namedafter = [[Michael Faraday]]
| extralabel = In [[SI base unit]]s:
| extradata = 1 F = 1 [[second|s]]{{Smallsup|4}}·[[ampere|A]]{{Smallsup|2}}·[[meter|m]]{{Smallsup|−2}}·[[kilogram|kg]]{{Smallsup|−1}}
}}

[[File:Capacitors Various.jpg|thumb|right|Examples of various types of capacitors.]]

The '''farad''' (symbol: F) is the [[SI derived unit]] of electrical [[capacitance]]. It is named after the English physicist [[Michael Faraday]].

== Definition ==
One farad is the value of [[capacitance]] that produces a potential difference of one [[volt]] when it has been [[electric charge|charged]] by one [[coulomb]]. A coulomb is equal to the amount of charge (electrons) produced by a current of one [[ampere]] flowing for one [[second]]. For example, the voltage across the two terminals of a 47 nF [[capacitor]] will increase linearly by 1 V when a current of 47 nA flows through it for 1 s.

For most applications, the farad is an impractically large unit of capacitance, although capacitors measured in farads are now used, especially for backing up memory. The most commonly used [[SI prefix]]es for electrical and electronic applications are:

* 1 millifarad (mF) = one thousandth ({{10^|−3}}) of a farad or {{gaps|1|000}} μF
* 1 microfarad (μF, or MFD in industrial use) = one millionth ({{10^|−6}}) of a farad, or {{gaps|1|000|000}} pF, or {{gaps|1|000}} nF
* 1 nanofarad (nF) = one billionth ({{10^|−9}}) of a farad, or {{gaps|1|000}} pF
* 1 picofarad (pF) = one trillionth ({{10^|−12}}) of a farad

=== Equalities ===
A farad has the base SI representation of:
[[Second|s]]{{sup|4}} × [[ampere|A]]{{sup|2}} × [[meter|m]]{{sup|−2}} × [[kilogram|kg]]{{sup|−1}}

It can further be expressed as:

:<math>\mbox{F}
= \,\mathrm{\frac{A \cdot s}{V}
= \dfrac{\mbox{J}}{\mbox{V}^2}
= \dfrac{\mbox{W} \cdot \mbox{s}}{\mbox{V}^2}
= \dfrac{\mbox{C}}{\mbox{V}}
= \dfrac{\mbox{C}^2}{\mbox{J}}
= \dfrac{\mbox{C}^2}{\mbox{N} \cdot \mbox{m}}
= \dfrac{\mbox{s}^2 \cdot \mbox{C}^2}{\mbox{m}^{2} \cdot \mbox{kg}}
= \dfrac{\mbox{s}^4 \cdot \mbox{A}^2}{\mbox{m}^{2} \cdot \mbox{kg}}
= \dfrac{\mbox{s}}{\Omega}}
</math>

where A=[[ampere]], V=[[volt]], C=[[coulomb]], J=[[joule]], m=[[metre]], N=[[newton (unit)|newton]], s=[[second]], W=[[watt]], kg=[[kilogram]], Ω=[[ohm]].

== History ==
The term "farad" was coined by [[Josiah Latimer Clark]] in the year of 1861, in honor of [[Michael Faraday]], but it was for a unit of quantity of charge.

== Explanation ==
Values of capacitors are usually [[SI prefix#List of SI prefixes|specified in ranges]] of '''farads''' (F), '''millifarads''' (mF), '''microfarads''' (μF, or MFD in industrial use<ref>In texts prior to 1960, '''mf''' rather than the modern '''µF''' frequently represented microfarads. Similarly, '''mmf''' represented picofarads.</ref>), '''nanofarads''' (nF) or '''picofarads''' (pF).<ref name="Braga">{{cite book |title=Robotics, Mechatronics, and Artificial Intelligence |last=Braga |first=Newton C. |year=2002 |accessdate=2008-09-17 |publisher=Newnes |page=21 |url=http://books.google.com/books?id=yqb-f-HKem0C&pg=PA21&q=microfarad+common+measurement |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.}}</ref>

The size of commercially available capacitors ranges from around 100 fF (femtofarads, {{10^|−15}} F) to 5 kF (kilofarads, {{10^|3}} F) [[supercapacitor]]s. Designers of high performance integrated circuits are concerned about [[parasitic capacitance]] measured in femtofarads, while makers of high performance test equipment are able to detect changes in capacitance on the order of tens of attofarads ({{10^|−18}}).<ref>{{cite book |title=Analog MOS Integrated Circuits for Signal Processing |last=Gregorian |first=Roubik |year=1976 |publisher=John Wiley & Sons |page=78}}</ref>

A ''picofarad'' is sometimes referred to as a ''"puff"'' or ''"pic"'', as in "a ten puff capacitor".<ref>{{cite web |title=Puff |url=http://scienceworld.wolfram.com/physics/Puff.html |publisher=Wolfram Research |accessdate=2009-06-09}}</ref> If [[Mu (letter)|the Greek letter μ]] is not available, the notation ''uF'' is often used as a substitute for ''μF'' in electronics literature. A ''micro-microfarad'' (μμF, and confusingly often mmf or MMF), an obsolete unit sometimes found in older texts, is the equivalent of a ''picofarad''. The ''millifarad'' is less used in practice, so that a capacitance of 4.7{{e|−3}} F, for example, is sometimes written as {{gaps|4|700}} µF; industrial parts at times use the abbreviation MFD instead of µF.<ref>{{cite web |title=Microfarad |url=http://www.thefreedictionary.com/Microfarad |publisher=The Free Dictionary |accessdate=2012-08-13}}</ref> North American usage also avoids ''nanofarads'': a capacitance of 1{{e|−9}} F will frequently be indicated as {{gaps|1|000}} pF; and a capacitance of 1{{e|−7}} F as 0.1 μF.

The reciprocal of capacitance is called [[electrical elastance]], the (non-standard, non-SI) unit of which is the [[daraf]].<ref>{{cite web |title=Daraf |url=http://www.websters-dictionary-online.com/definitions/daraf |publisher=Webster's Online Dictionary |accessdate=2009-06-19}}</ref>

A capacitor 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 [[volt]]s to several kilovolts.

One picofarad is about the smallest value of capacitor available for general use in electronic design, since smaller capacitors would be dominated by the parasitic capacitances ([[stray capacitance]]) of other components, wiring or [[printed circuit board]]s. When capacitance values of 1 pF or lower are required, engineers sometimes create their own capacitors by twisting two short lengths of insulated wire together.<ref>{{cite web |title=What's All This Femtoampere Stuff, Anyhow? |url=http://electronicdesign.com/test-amp-measurement/whats-all-femtoampere-stuff-anyhow |last=Pease |first=Bob |authorlink=Bob Pease |publisher=Electronic Design |date=2 September 1993 |accessdate=2013-03-09}}</ref><ref>{{cite web |title=What's All This Best Stuff, Anyhow? |url=http://electronicdesign.com/analog/whats-all-best-stuff-anyhow |last=Pease |first=Bob |publisher=Electronic Design |date=1 December 2006 |accessdate=2013-03-09}}</ref>

The capacitance of the Earth's [[ionosphere]] with respect to the ground is calculated to be about 1 F.<ref>{{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 |accessdate=2012-08-13}}</ref>

==CGS units==
The '''abfarad''' (abbreviated abF) is an obsolete [[CGS]] unit of capacitance equal to {{10^|9}} farads (1 gigafarad, GF). This very large unit is used in medical terminology only.

The '''statfarad''' (abbreviated statF) is a different and also rarely used CGS unit of capacitance that corresponds to approximately 1.1126 picofarads. It is equivalent to the capacitance of a capacitor with a charge of 1 statcoulomb across a potential difference of 1 statvolt.

== See also ==
*[[Supercapacitor]]

== Notes ==
{{Reflist|33em}}

== External links ==
* [http://www.translatorscafe.com/cafe/units-converter/electrostatic-capacitance/c/ Farad unit conversion tool]

{{SI units}}

[[Category:SI derived units]]
[[Category:Units of electrical capacitance]]
[[Category:Michael Faraday]]

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