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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 English physicist Michael Faraday.

Contents

Definition

A farad is the charge in coulombs a capacitor will accept for the potential across 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:

\mbox{F} = \,\mathrm \frac{A \cdot s}{V} = \dfrac{\mbox{J}}{\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}}

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.[1]

Values of capacitors are usually specified in ranges of Farads (F), microfarads (μF or MFD), nanofarads (nF), or picofarads (pF), as one, millionths, billionths or trillionths of a farad.[2] The picofarad is sometimes comically called a "puff", as in "a ten puff capacitor".[3] Since the Greek letter μ can be poorly printed, the notation uF (or MFD) is often used as a substitute for μF in electronics literature. A micro-microfarad (μμF) that can be found in older texts is the equivalent of a picofarad. The millifarad is rarely used in practice, so that a capacitance of 4.7 × 10−3 F, for example, is usually written as 4,700 µF. North American usage also avoids nanofarads. A capacitance of 1 × 10−9 F will frequently be indicated as 1000 pF. A capacitance of 1 × 10−7 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 × 10−15 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The 'farad' should not be confused with the faraday, an old unit of charge nowadays superseded by the coulomb.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.[4]

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 charges 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 voltage withstand capabilities ranging 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 capacitance values of 1 pF or lower are required, engineers sometimes create their own capacitors by twisting two short lengths of insulated wire together.[5][6]

See also

Notes

References

External links

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Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

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Hungarian

Pronunciation

  • IPA: /ˈfaːrɒd/
  • Hyphenation: fá‧rad

Verb

fárad

  1. tire, get tired (become sleepy or weary)
    A vérszegény ember könnyen fárad. - Someone with anemia may get tired more easily.
  2. take pains to do something
    Kérem, ne fáradjon! - Please don't bother! (don't trouble yourself)
  3. take pains to do something on someone's behalf - followed by -ért
    Nagyapa sokat fárad a családért. - Grandfather works very hard for the family.
  4. take the trouble to go somewhere - followed by -hoz/-hez/-höz or -ra/-re
    Ha fizetni szeretne, kérem fáradjon a kasszához. - When you are ready to pay, will you kindly walk to the cashier.
    Kovács úr, kérem fáradjon a harmadik emeletre. - Mr. Smith, please walk to the third floor.

Derived terms


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