Farad
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2008-07-13T19:32:50Z
AkhtaBot
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robot Adding: [[ar:فاراد]]
[[Image:Capacitors Various.jpg|thumb|right|Examples of various types of capacitors.]]
[[Image:OneFarad5.5Velectrolyticcapacitor.jpg|thumb|right|a 1-farad capacitor.]]
:''This is about the capacitance unit of measure. For the charge unit, see [[Faraday_(unit)]].''
The '''farad''' (symbol: F) is the [[SI]] unit of [[capacitance]]. It is named after the British physicist [[Michael Faraday]].
== Definition ==
The farad is defined as the amount of [[capacitance]] for which a [[potential difference]] of one [[volt]] results in a [[electric charge|static charge]] of one [[coulomb]]. It has the base SI representation of [[Second|s]]<sup>4</sup> · [[Ampere|A]]<sup>2</sup> · m<sup>-2</sup> · kg<sup>-1</sup>. Since an [[ampere]] is the rate of electrical flow ([[Electric current|current]]) of one coulomb per second, an alternate definition is that a farad is the amount of capacitance that requires one second for a one ampere flow of charge to change the voltage by one [[volt]]. Further equalities follow:
:<math>\mbox{F}
= \,\mathrm \frac{A \cdot s}{V}
= \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}}
</math>
F stands for Farad, A for Ampere, s for second, V for Volt, C for Coloumb, J for [[Joule]], N for [[Newton]], m for [[meter]], kg for [[kilogram]].
== Explanation ==
Since the farad is a very large unit compared to typical requirements in electronic devices, values of [[capacitor]]s are usually in range of microfarads (μF), nanofarads (nF), or picofarads (pF). The picofarad is sometimes comically called a "puff", as in "a ten puff capacitor". Since the Greek letter μ caused printing difficulties, the notation uF is often to be found as a substitute for μF in the electronics literature. A micro-microfarad (μμF) as found in older texts is the same as a picofarad. The millifarad is rarely used in practice, so that a capacitance of 4.7{{e|−3}} F, for example, is usually written as 4700 µF. North American usage also avoids nanofarads. A capacitance of 1{{e|−9}} F will frequently be indicated as 1000 pF. A capacitance of 1{{e|−7}} F will frequently be indicated as 0.1 μF. Very small capacitance values, such as those used in [[integrated circuit]]s may also be expressed in femtofarads, one femtofarad being equal to 1{{e|−15}} F. A new technology called [[supercapacitor]]s offers devices up to the kilofarad range.
The farad should not be confused with the [[faraday (unit)|faraday]], an old unit of charge nowadays superseded by the [[coulomb]].
The reciprocal of capacitance is called [[electrical elastance]], the (non-standard, non-SI) unit of which is the [[daraf]].
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 charge on the plates that results in [[capacitance]]. Modern [[capacitor]]s are constructed using a range of manufacturing techniques and materials to provide the extraordinarily wide range of capacitance values used in practical [[electronics]] applications from femtofarads to farads and voltage withstand capabilities 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 requiring capacitance values of 1 pF or lower, engineers sometimes create their own capacitors by twisting two short lengths of insulated wire together.<ref>[http://www.national.com/rap/Story/0,1562,5,00.html What's All This Femtoampere Stuff, Anyhow?<!-- Bot generated title -->]</ref><ref>[http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=14109 Electronic Design Welcome<!-- Bot generated title -->]</ref>
== See also ==
*[[Capacitance]]
*[[Capacitor]]
== References ==
{{reflist}}
== External links ==
* [http://www.translatorscafe.com/cafe/units-converter/electrostatic-capacitance/c/ Farad unit conversion tool]
[[Category:SI derived units]]
[[Category:Units of electrical capacitance]]
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