Farad

Related Topics:
Faraday,_Michael

The farad (symbol: F) is the SI unit of capacitance.

Definition

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

\mbox{F} = \mbox{C} \cdot \mbox{V}^{-1} = \mbox{m}^{-2} \cdot \mbox{kg}^{-1} \cdot \mbox{s}^4 \cdot \mbox{A}^2 = \mbox{C}^2 \cdot \mbox{J}^{-1}

SI multiples

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The farad should not be confused with the faraday, an old unit of charge nowadays superseded by the coulomb.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

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 charge on the plates that results in capacitance. Modern capacitors are constructed using a range of manufacturing techniques and materials to provide the extraordinary wide range of capacitance values used in practical electronics applications from femtofarads to farads and voltage withstand capabilities from a few volts to a several kV.

SI derived units | Units of electrical capacitance

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Definition

SI multiples

Explanation