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Farad
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:This is about the capacitance unit of measure. For the charge unit, see Faraday (unit).
The farad (symbol: F) is the SI unit of capacitance. The farad is named after the British physicist Michael Faraday.
Definition
A farad is the charge in coulombs a capacitor will accept for the potential over it to change 1 volt.
Discussion
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Encyclopedia
:This is about the capacitance unit of measure. For the charge unit, see Faraday (unit).
The farad (symbol: F) is the SI unit of capacitance. The farad is named after the British physicist Michael Faraday.
Definition
A farad is the charge in coulombs a capacitor will accept for the potential over it to change 1 volt. A coulomb is 1 ampere second. Example: A 47mA current causes the voltage across a capacitor to increase 1 volt/second. It therefore has a capacitance of 47mF. It has the base SI representation of s4 · A2 · m-2 · kg-1. Further equalities follow:
The most commonly used multiples and submultiples in electrical and electronic usage are the microfarad, nanofarad and picofarad.
Explanation
The size of commercially available capacitors range from 100fF to 5kF.
Values of capacitors are usually specified in range of Farads (F), microfarads (µF or MFD), nanofarads (nF), or picofarads (pF), as one, millionths, billionths or trillionths of a farad.
The picofarad is sometimes comically called a "puff", as in "a ten puff capacitor". Since the Greek letter µ caused printing difficulties, the notation uF (or MFD) is often to be found as a substitute for µF in the electronics literature. A micro-microfarad (µµF) as found in older texts is the same as a picofarad. The millifarad is rarely used in practice, so that a capacitance of 4.7 F, for example, is usually written as 4700 µF. North American usage also avoids nanofarads. A capacitance of 1 F will frequently be indicated as 1000 pF. A capacitance of 1 F will frequently be indicated as 0.1 µF. Very small capacitance values, such as those used in integrated circuits may also be expressed in femtofarads (fF), one femtofarad being equal to 1 F.
The farad should not be confused with the faraday, an old unit of charge nowadays superseded by the coulomb.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 extraordinarily wide range of capacitance values used in practical electronics applications from femtofarads to farads and voltage withstand capabilities from a few volts to several kilovolts.
One picofarad is about the smallest value of capacitor available for general use in electronic design, since smaller capacitors would be dominated by the parasitic capacitances (stray capacitance) of other components, wiring or printed circuit boards. When requiring capacitance values of 1 pF or lower, engineers sometimes create their own capacitors by twisting two short lengths of insulated wire together.
See also
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