Farad

The farad (symbol: F) is the unit of electrical capacitance, the ability of a body to store an electrical charge, in the International System of Units (SI), equivalent to 1 coulomb per volt (C/V). It is named after the English physicist Michael Faraday (1791–1867). In SI base units 1 F = 1 kg⁻¹⋅m⁻²⋅s⁴⋅A².

The capacitance of a capacitor is one farad when one coulomb of charge changes the potential between the plates by one volt. Equally, one farad can be described as the capacitance which stores a one-coulomb charge across a potential difference of one volt.

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:

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: $${\displaystyle {\text{F}}={\dfrac {{\text{s}}^{4}{\cdot }{\text{A}}^{2}}{{\text{m}}^{2}{\cdot }{\text{kg}}}}={\dfrac {{\text{s}}^{2}{\cdot }{\text{C}}^{2}}{{\text{m}}^{2}{\cdot }{\text{kg}}}}={\dfrac {\text{C}}{\text{V}}}={\dfrac {{\text{A}}{\cdot }{\text{s}}}{\text{V}}}={\dfrac {{\text{W}}{\cdot }{\text{s}}}{{\text{V}}^{2}}}={\dfrac {\text{J}}{{\text{V}}^{2}}}={\dfrac {{\text{N}}{\cdot }{\text{m}}}{{\text{V}}^{2}}}={\dfrac {{\text{C}}^{2}}{\text{J}}}={\dfrac {{\text{C}}^{2}}{{\text{N}}{\cdot }{\text{m}}}}={\dfrac {\text{s}}{\Omega }}={\dfrac {1}{\Omega {\cdot }{\text{Hz}}}}={\dfrac {\text{S}}{\text{Hz}}}={\dfrac {{\text{s}}^{2}}{\text{H}}},}$$

where F = farad, s = second, C = coulomb, V = volt, W = watt, J = joule, N = newton, Ω = ohm, Hz = hertz, S = siemens, H = henry, A = ampere.

The term "farad" was originally coined by Latimer Clark and Charles Bright in 1861, in honor of Michael Faraday, for a unit of quantity of charge, and by 1873, the farad had become a unit of capacitance. In 1881, at the International Congress of Electricians in Paris, the name farad was officially used for the unit of electrical capacitance.