Not to be confused with faraday (unit). For the former settlement in California, see Farad, California.

The farad (symbol: F) is the SI derived unit of electrical capacitance, the ability of a body to store an electrical charge. It is named after the English physicist Michael Faraday.

## Definition

One farad is defined as the capacitance of a capacitor across which, when charged with one coulomb of electricity, there is a potential difference of one volt. Conversely, it is the capacitance which, when charged to a potential difference of one volt, carries a charge of one coulomb. A coulomb is equal to the amount of charge (electrons) produced by a current of one ampere (A) flowing for one second. For example, the voltage across the two terminals of a 2 F capacitor will increase linearly by 1 V when a current of 2 A flows through it for 1 second.

For most applications, the farad is an impractically large unit of capacitance. Most electrical and electronic applications are covered by the following SI prefixes:

### Equalities

A farad has the base SI representation of: s4 × A2 × m−2 × kg−1

It can further be expressed as:

${\mbox{F}}=\,\mathrm {{\frac {A\cdot s}{V}}={\dfrac {\mbox{J}}{{\mbox{V}}^{2}}}={\dfrac {{\mbox{W}}\cdot {\mbox{s}}}{{\mbox{V}}^{2}}}={\dfrac {\mbox{C}}{\mbox{V}}}={\dfrac {{\mbox{C}}^{2}}{\mbox{J}}}={\dfrac {{\mbox{C}}^{2}}{{\mbox{N}}\cdot {\mbox{m}}}}={\dfrac {{\mbox{s}}^{2}\cdot {\mbox{C}}^{2}}{{\mbox{m}}^{2}\cdot {\mbox{kg}}}}={\dfrac {{\mbox{s}}^{4}\cdot {\mbox{A}}^{2}}{{\mbox{m}}^{2}\cdot {\mbox{kg}}}}={\dfrac {\mbox{s}}{\Omega }}} ={\dfrac {{\mbox{s}}^{2}}{\mbox{H}}}$ where A=ampere, V=volt, C=coulomb, J=joule, m=metre, N=newton, s=second, W=watt, kg=kilogram, Ω=ohm, H=henry.

## History

The term "farad" was coined by Josiah Latimer Clark in 1861, in honor of Michael Faraday, but it was for a unit of quantity of charge.Template:Elucidate

## Explanation

A capacitor consists of two conducting surfaces, frequently referred to as plates, separated by an insulating layer usually referred to as a dielectric. The original capacitor was the Leyden jar developed in the 18th century. It is the accumulation of electric charge on the plates that results in capacitance. Modern capacitors are constructed using a range of manufacturing techniques and materials to provide the extraordinarily wide range of capacitance values used in electronics applications from femtofarads to farads, with maximum-voltage ratings ranging from a few volts to several kilovolts.

Values of capacitors are usually specified in farads (F), microfarads (μF), nanofarads (nF) and picofarads (pF). The millifarad is rarely used in practice (a capacitance of 4.7 mF (0.0047 F), for example, is instead written as Template:Gaps), while the nanofarad is uncommon in North America. The size of commercially available capacitors ranges from around 0.1 pF to Template:Gaps (5 kF) supercapacitors. Parasitic capacitance in high-performance integrated circuits can be measured in femtofarads (1 fF = 0.001 pF = Template:10^ F), while high-performance test equipment can detect changes in capacitance on the order of tens of attofarads (1 aF = Template:Gaps pF = Template:10^ F).

A value of 0.1 pF is about the smallest available in capacitors for general use in electronic design, since smaller ones would be dominated by the parasitic capacitances of other components, wiring or printed circuit boards. Capacitance values of 1 pF or lower can be achieved by twisting two short lengths of insulated wire together.

The capacitance of the Earth's ionosphere with respect to the ground is calculated to be about 1 F.

### Informal terminology

The picofarad is sometimes colloquially pronounced as "puff" or "pic", as in "a ten-puff capacitor". 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, and confusingly often mmf or MMF), an obsolete unit sometimes found in older texts, is the equivalent of a picofarad. Industrial parts (e.g. automotive, lighting) sometimes use the abbreviation MFD instead of µF.

### Related concepts

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

## CGS units

The abfarad (abbreviated abF) is an obsolete CGS unit of capacitance equal to Template:10^ farads (1 gigafarad, GF). This very large unit is used in medical terminology only.

The statfarad (abbreviated statF) is a rarely used CGS unit equivalent to the capacitance of a capacitor with a charge of 1 statcoulomb across a potential difference of 1 statvolt. It is 1/(10-5c2) farad, approximately 1.1126 picofarads.