Farad

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

Contents

Definition

A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second. Example: A capacitor with capacitance of 47 nF will increase by 1 volt per second with a 47 nA input current.

1 microfarad (μF) = one millionth (10-6) of a farad, or 1,000,000 pF, or 1,000 nF; 1 nanofarad (nF) = one billionth (10-9) of a farad, or 1,000 pF; 1 picofarad (pF) = one trillionth (10-12) of a farad.

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}

A=ampere, V=volt, C=coulomb, J=joule, m=meter, N=newton, s=second, W=watt, kg=kilogram, Ω=ohm

For electronics, one farad is a fairly large amount of capacitance. The most commonly used submultiples in electrical and electronic usage are the microfarad, nanofarad and picofarad.

History

The farad was coined by Josiah Latimer Clark in the year of 1861, in honor of Michael Faraday, but it was for a unit of quantity of charge.

Explanation

Values of capacitors are usually specified in ranges of farads (F), microfarads (μF[1]), nanofarads (nF) or picofarads (pF).[2]

The size of commercially available capacitors ranges from around 100 fF (femtofarads, 10-15 F) to 5 kF (kilofarads, 103 F) ultracapacitors.

When speaking of capacitor values a picofarad is sometimes referred to as a "puff" or "pic", as in "a ten puff capacitor".[3] 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), an obsolete unit sometimes found in older texts, is the equivalent of a picofarad. The millifarad is less used in practice, so that a capacitance of 4.7×10−3 F, for example, is sometimes 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; and a capacitance of 1×10−7 F as 0.1 μF.

The 'farad' should not be confused with the faraday, which is the electric charge carried by one mole of singly charged ions.

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 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 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]

The capacitance of the Earth's ionosphere with respect to the ground is calculated to be about 1.8 F.

CGS units

The abfarad is an obsolete electromagnetic (CGS) unit of capacitance equal to 109 farads (1,000,000,000 F or 1 GF)). This very large unit is used in medical terminology only.

The statfarad is the comparable obsolete electrostatic (CGS) unit of capacitance, and is equal to 1 microfarad.

See also

Notes

  1. ^ in texts prior to 1960, mf rather than the modern µF frequently represented microfarads. Similarly, mmf represented picofarads.
  2. ^ Newton C. Braga (2001). Robotics, Mechatronics, and Artificial Intelligence. Google Books. pp. 336 pages. http://books.google.com/books?id=yqb-f-HKem0C&pg=PA21&lpg=PA21&dq=microfarad+%22common+measurement&source=web&ots=XKiRXY4zeQ&sig=RrHw5Zbb0ymkDh9dRRul_qjsvcw&hl=en&sa=X&oi=book_result&resnum=1&ct=result. Retrieved 2008-09-17. "page 21: Common measurement units are the Farad, microFarad (uF), representing 0.000,001 F; the nanoFarad (nF), representing 0.000,000001 F; and the picoFarad (pF)" 
  3. ^ http://scienceworld.wolfram.com/physics/Puff.html
  4. ^ http://www.websters-dictionary-online.com/definitions/daraf
  5. ^ What's All This Femtoampere Stuff, Anyhow?
  6. ^ Electronic Design Welcome

References

External links

Base units
Ampere · Candela · Kelvin · Kilogram · Metre · Mole · Second
Derived units
Becquerel · Coulomb · degree Celsius · Farad · Gray · Henry · Hertz · Joule · Katal · Lumen · Lux · Newton · Ohm · Pascal · Radian · Siemens · Sievert · Steradian · Tesla · Volt · Watt · Weber
Accepted for use
with SI
See also
Book:International System of Units  · Category:SI base units