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Farad

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The farad (symbol F) is the SI unit of capacitance (named after Michael Faraday). A capacitor has a value of one farad when one coulomb of charge causes a potential difference of one volt across it. Its equivalent expressions in other SI units are:

C ⋅ V − 1 = m − 2 ⋅ kg − 1 ⋅ s 4 ⋅ A 2 {\displaystyle {\mbox{C}}\cdot {\mbox{V}}^{-1}={\mbox{m}}^{-2}\cdot {\mbox{kg}}^{-1}\cdot {\mbox{s}}^{4}\cdot {\mbox{A}}^{2}}

Since the farad is a very large unit, values of capacitors are usually expressed in microfarads (μF), nanofarads (nF), or picofarads (pF). Multiples higher than the microfarad are rarely used in practice, even for large capacitances, so that a capacitance of 4.7×10 F, for example, is usually written as 4700 μF.

The farad should not be confused with the faraday, an old unit of charge nowadays superseded by the coulomb.

The reciprocal of capacitance is called elastance, the (non-standard, non-SI) unit of which is the daraf.

SI electricity units

SI electromagnetism units
Symbol Name of quantity Unit name Symbol Base units
E energy joule J = C⋅V = W⋅s kg⋅m⋅s
Q electric charge coulomb C A⋅s
I electric current ampere A = C/s = W/V A
J electric current density ampere per square metre A/m A⋅m
U, ΔV; Δϕ; E, ξ potential difference; voltage; electromotive force volt V = J/C kg⋅m⋅s⋅A
R; Z; X electric resistance; impedance; reactance ohm Ω = V/A kg⋅m⋅s⋅A
ρ resistivity ohm metre Ω⋅m kg⋅m⋅s⋅A
P electric power watt W = V⋅A kg⋅m⋅s
C capacitance farad F = C/V kg⋅m⋅A⋅s
ΦE electric flux volt metre V⋅m kg⋅m⋅s⋅A
E electric field strength volt per metre V/m = N/C kg⋅m⋅A⋅s
D electric displacement field coulomb per square metre C/m A⋅s⋅m
ε permittivity farad per metre F/m kg⋅m⋅A⋅s
χe electric susceptibility (dimensionless) 1 1
p electric dipole moment coulomb metre C⋅m A⋅s⋅m
G; Y; B conductance; admittance; susceptance siemens S = Ω kg⋅m⋅s⋅A
κ, γ, σ conductivity siemens per metre S/m kg⋅m⋅s⋅A
B magnetic flux density, magnetic induction tesla T = Wb/m = N⋅A⋅m kg⋅s⋅A
Φ, ΦM, ΦB magnetic flux weber Wb = V⋅s kg⋅m⋅s⋅A
H magnetic field strength ampere per metre A/m A⋅m
F magnetomotive force ampere A = Wb/H A
R magnetic reluctance inverse henry H = A/Wb kg⋅m⋅s⋅A
P magnetic permeance henry H = Wb/A kg⋅m⋅s⋅A
L, M inductance henry H = Wb/A = V⋅s/A kg⋅m⋅s⋅A
μ permeability henry per metre H/m kg⋅m⋅s⋅A
χ magnetic susceptibility (dimensionless) 1 1
m magnetic dipole moment ampere square meter A⋅m = J⋅T A⋅m
σ mass magnetization ampere square meter per kilogram A⋅m/kg A⋅m⋅kg

See also

References

  1. International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry, 2nd edition, Oxford: Blackwell Science. ISBN 0-632-03583-8. pp. 14–15. Electronic version.

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