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The farad (symbol: F) is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.

## Contents

### Definition

A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second. Example: A 47 mA current causes the voltage across a capacitor to increase by 1 volt per second. It therefore has a capacitance of 47 mF. It has the base SI representation of s4·A2·m−2·kg−1. Further equalities follow:

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

A=ampere, V=volt, C=coulomb, J=joule, m=meter, N=newton, s=second, W=watt, kg=kilogram, Ω=ohm

One farad is a fairly large amount of capacitance. The most commonly used submultiples in electrical and electronic usage are the microfarad, nanofarad and picofarad.

### History

The 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

The size of commercially available capacitors ranges from 100 fF to 5 kF.[1]

Values of capacitors are usually specified in ranges of farads (F), microfarads (μF or MFD, one millionth or 10^-6 of a farad), nanofarads (nF, 10^-9 farad), or picofarads (pF, 10^-12 farad).[2] When speaking of capacitor values a picofarad is sometimes referred to as a "puff" or "pic", as in "a ten puff capacitor".[3] If 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), 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 4.7×10−3 F, for example, is sometimes written as 4,700 µF. North American usage also avoids nanofarads: a capacitance of 1×10−9 F will frequently be indicated as 1000 pF; and a capacitance of 1×10−7 F as 0.1 μF. Very small capacitance values, such as those used in integrated circuits, may also be expressed in femtofarads (fF), one femtofarad being equal to 1×10−15 F.