Farad

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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.

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