Farad

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

Contents 1 Definition 2 SI multiples 3 Explanation 4 SI electricity units

Definition

A capacitor has a value of one farad when one coulomb of charge causes a potential difference of one volt across it. Its equivalent expression in SI base units is:

<math>F = C\cdot V^{-1} = m^{-2}\cdot kg^{-1}\cdot s^4\cdot A^2</math>.

SI multiples

Multiple	Name	Symbol		Multiple	Name	Symbol
100	farad	F
101	decafarad	daF		10–1	decifarad	dF
102	hectofarad	hF		10–2	centifarad	cF
103	kilofarad	kF		10–3	millifarad	mF
106	megafarad	MF		10–6	microfarad	µF
109	gigafarad	GF		10–9	nanofarad	nF
1012	terafarad	TF		10–12	picofarad	pF
1015	petafarad	PF		10–15	femtofarad	fF
1018	exafarad	EF		10–18	attofarad	aF
1021	zettafarad	ZF		10–21	zeptofarad	zF
1024	yottafarad	YF		10–24	yoctofarad	yF

Explanation

Since the farad is a very large unit, values of capacitors are usually in range of microfarads (μF), nanofarads (nF), or picofarads (pF). The picofarad is comically called a "puff" in laboratory usage. The millifarad is rarely used in practice, so that a capacitance of 4.7×10^-3 F, for example, is usually written as 4700 μF. Very small capacitance values, such as those used in integrated circuits may also be expressed in femtofarads, one femtofarad being equal to 1×10^-15 F. A new technology called supercapacitors offers devices up to the kilofarad range.

The farad should not be confused with the faraday, an old unit of charge nowadays superseded by the coulomb.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 electrical elastance, the (non-standard, non-SI) unit of which is the daraf.

SI electricity units

SI electromagnetic units edit
Quantity	Name	Symbol	Dimensions
Current	ampere (SI base unit)	A	A
Electric charge, Quantity of electricity	coulomb	C	A·s
Potential difference	volt	V	J/C = kg·m2·s−3·A−1
Resistance, Impedance, Reactance	ohm	Ω	V/A = kg·m2·s−3·A−2
Resistivity	ohm metre	Ω·m	kg·m3·s−3·A−2
Electrical power	watt	W	V·A = kg·m2·s−3
Capacitance	farad	F	C/V = kg−1·m−2·A2·s4
Elastance	reciprocal farad	F−1	kg·m2·A−2·s−4
Permittivity	farad per metre	F/m	kg−1·m−3·A2·s4
Conductance, Admittance, Susceptance	siemens	S	Ω−1 = kg−1·m−2·s3·A2
Conductivity	siemens per metre	S/m	kg−1·m−3·s3·A2
Magnetic flux	weber	Wb	V·s = kg·m2·s−2·A−1
Magnetic flux density	tesla	T	Wb/m2 = kg·s−2·A−1
Magnetic induction	ampere per metre	A/m	A·m−1
Reluctance	ampere-turns per weber	A/Wb	kg−1·m−2·s2·A2
Inductance	henry	H	Wb/A = V·s/A = kg·m2·s−2·A−2
Permeability	henry per metre	H/m	kg·m·s−2·A−2
Magnetic susceptibility	(dimensionless)	χ	-