The capacitance of a capacitor is one farad when one coulomb of charge changes the potential between the plates by one volt.[1][2] Equally, one farad can be described as the capacitance which stores a one-coulomb charge across a potential difference of one volt.[3]
The relationship between capacitance, charge, and potential difference is linear. For example, if the potential difference across a capacitor is halved, the quantity of charge stored by that capacitor will also be halved.
For most applications, the farad is an impractically large unit of capacitance. Most electrical and electronic applications are covered by the following SI prefixes:
1mF (millifarad, one thousandth (10−3) of a farad) = 0.001F = 1000μF = 1000000000pF
1μF (microfarad, one millionth (10−6) of a farad) = 0.000001F = 1000nF = 1000000pF
1nF (nanofarad, one billionth (10−9) of a farad) = 0.000000001F = 0.001μF = 1000pF
1pF (picofarad, one trillionth (10−12) of a farad) = 0.000000000001F = 0.001nF
The term "farad" was originally coined by Latimer Clark and Charles Bright in 1861,[5] in honor of Michael Faraday, for a unit of quantity of charge, but by 1873, the farad had become a unit of capacitance.[6] In 1881 at the International Congress of Electricians in Paris, the name farad was officially used for the unit of electrical capacitance.[7][8]
Explanation
Examples of different types of capacitors
A capacitor generally 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 terms of SI prefix#List of SI prefixes of farads (F), microfarads (μF), nanofarads (nF) and picofarads (pF).[9] The millifarad (mF) is rarely used in practice; a capacitance of 4.7mF (0.0047F), for example, is instead written as 4700μF. The nanofarad (nF) is uncommon in North America.[10] The size of commercially available capacitors ranges from around 0.1pF to 5000F (5kF) supercapacitors. Parasitic capacitance in high-performance integrated circuits can be measured in femtofarads (1 fF = 0.001pF = 10−15F), while high-performance test equipment can detect changes in capacitance on the order of tens of attofarads (1aF = 10−18F).[11]
A value of 0.1pF 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 1pF or lower can be achieved by twisting two short lengths of insulated wire together.[12][13]
The capacitance of the Earth's ionosphere with respect to the ground is calculated to be about 1F.[14]
Informal and deprecated terminology
The picofarad (pF) is sometimes colloquially pronounced as "puff" or "pic", as in "a ten-puff capacitor".[15] Similarly, "mic" (pronounced "mike") is sometimes used informally to signify microfarads.
Nonstandard abbreviations were and are often used. Farad has been abbreviated "f", "fd", and "Fd". For the prefix "micro-", when the Greek small letter "μ" or the legacy micro sign "μ" is not available (as on typewriters) or inconvenient to enter, it is often substituted with the similar-appearing "u" or "U", with little risk of confusion. It was also substituted with the similar-sounding "M" or "m", which can be confusing because M officially stands for 1,000,000, and m preferably stands for 1/1000. In texts prior to 1960, and on capacitor packages until more recently, "microfarad(s)" was abbreviated "mf" or "MFD" rather than the modern "μF". A 1940 Radio Shack catalog listed every capacitor's rating in "Mfd.", from 0.000005 Mfd. (5 pF) to 50 Mfd. (50 μF).[16]
"Micromicrofarad" or "micro-microfarad" is an obsolete unit found in some older texts and labels, contains a nonstandard metric double prefix. It is exactly equivalent to a picofarad (pF). It is abbreviated μμF, uuF, or (confusingly) "mmf", "MMF", or "MMFD".
Summary of obsolete capacitance units: (upper/lower case variations are not shown)
μF (microfarad) = mf, mfd
pF (picofarad) = mmf, mmfd, pfd, μμF
Related concepts
The reciprocal of capacitance is called electrical elastance, the (non-standard, non-SI) unit of which is the daraf.[17]
CGS units
The abfarad (abbreviated abF) is an obsolete CGS unit of capacitance, which corresponds to 109 farads (1 gigafarad, GF).[18]
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. More commonly, the centimeter (cm) is used, which is equal to the statfarad.
↑ As names for units of various electrical quantities, Bright and Clark suggested "ohma" for voltage, "farad" for charge, "galvat" for current, and "volt" for resistance. See:
Latimer Clark and Sir Charles Bright (1861) "On the formation of standards of electrical quantity and resistance,"Report of the Thirty-first Meeting of the British Association for the Advancement of Science (Manchester, England: September 1861), section: Mathematics and Physics, pp. 37-38.
↑ Sir W. Thomson, etc. (1873) "First report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units,"Report of the 43rd Meeting of the British Association for the Advancement of Science (Bradford, September 1873), pp. 222-225. From p. 223: "The "ohm," as represented by the original standard coil, is approximately 109 C.G.S. units of resistance: the "volt" is approximately 108 C.G.S. units of electromotive force: and the "farad" is approximately 1/109 of the C.G.S. unit of capacity."
↑ (Anon.) (September 24, 1881) "The Electrical Congress,"The Electrician, 7: 297. From p. 297: "7. The name farad will be given to the capacity defined by the condition that a coulomb in a farad gives a volt."
↑ Braga, Newton C. (2002). Robotics, Mechatronics, and Artificial Intelligence. Newnes. p.21. ISBN0-7506-7389-3. Retrieved 2008-09-17. Common measurement units are the microfarad (μF), representing 0.000,001 F; the nanofarad (nF), representing 0.000,000,001 F; and the picofarad (pF), representing 0.000,000,000,001 F.
The centimetre–gram–second system of units is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover electromagnetism.
The volt is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). In the present version of the SI it is equal to the electric charge delivered by a 1 ampere constant current in 1 second and to 5×1018/0.801088317 elementary charges, e, (about 6.241509×1018e).
In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ε (epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in response to an applied electric field than a material with low permittivity, thereby storing more energy in the material. In electrostatics, the permittivity plays an important role in determining the capacitance of a capacitor.
In physical chemistry, the Faraday constant, denoted by the symbol F and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of SI base units, which took effect on 20 May 2019, the Faraday constant has the exactly defined value given by the product of the elementary charge e and Avogadro constant NA:
The henry is the unit of electrical inductance in the International System of Units (SI). If a current of 1 ampere flowing through a coil produces flux linkage of 1 weber turn, that coil has a self inductance of 1 henry. The unit is named after Joseph Henry (1797–1878), the American scientist who discovered electromagnetic induction independently of and at about the same time as Michael Faraday (1791–1867) in England.
Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related notions of capacitance: self capacitance and mutual capacitance. An object that can be electrically charged exhibits self capacitance, for which the electric potential is measured between the object and ground. Mutual capacitance is measured between two components, and is particularly important in the operations of the capacitor, a device designed for this purpose as an elementary linear electronic component.
In physics, a drift velocity is the average velocity attained by charged particles, such as electrons, in a material due to an electric field. In general, an electron in a conductor will propagate randomly at the Fermi velocity, resulting in an average velocity of zero. Applying an electric field adds to this random motion a small net flow in one direction; this is the drift.
In physics, the weber is the unit of magnetic flux in the International System of Units (SI), whose units are volt-second. A magnetic flux density of one Wb/m2 is one tesla.
The tesla is the unit of magnetic flux density in the International System of Units (SI).
Gaussian units constitute a metric system of physical units. This system is the most common of the several electromagnetic unit systems based on cgs (centimetre–gram–second) units. It is also called the Gaussian unit system, Gaussian-cgs units, or often just cgs units. The term "cgs units" is ambiguous and therefore to be avoided if possible: there are several variants of cgs with conflicting definitions of electromagnetic quantities and units.
Capacitors are manufactured in many styles, forms, dimensions, and from a large variety of materials. They all contain at least two electrical conductors, called plates, separated by an insulating layer (dielectric). Capacitors are widely used as parts of electrical circuits in many common electrical devices.
Heaviside–Lorentz units constitute a system of units within CGS, named for Hendrik Antoon Lorentz and Oliver Heaviside. They share with CGS-Gaussian units the property that the electric constant ε0 and magnetic constant µ0 do not appear, having been incorporated implicitly into the electromagnetic quantities by the way they are defined. Heaviside–Lorentz units may be regarded as normalizing ε0 = 1 and µ0 = 1, while at the same time revising Maxwell's equations to use the speed of light c instead.
The ohm is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Simon Ohm. Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time, and of a convenient scale for practical work as early as 1861.
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
A capacitance meter is a piece of electronic test equipment used to measure capacitance, mainly of discrete capacitors. Depending on the sophistication of the meter, it may display the capacitance only, or it may also measure a number of other parameters such as leakage, equivalent series resistance (ESR), and inductance. For most purposes and in most cases the capacitor must be disconnected from circuit; ESR can usually be measured in circuit.
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A motor capacitor, such as a start capacitor or run capacitor is an electrical capacitor that alters the current to one or more windings of a single-phase alternating-current induction motor to create a rotating magnetic field.
The gyrator–capacitor model - sometimes also the capacitor-permeance model - is a lumped-element model for magnetic circuits, that can be used in place of the more common resistance–reluctance model. The model makes permeance elements analogous to electrical capacitance rather than electrical resistance. Windings are represented as gyrators, interfacing between the electrical circuit and the magnetic model.
A capacitive power supply, also called a capacitive dropper, is a type of power supply that uses the capacitive reactance of a capacitor to reduce higher AC mains voltage to a lower DC voltage.
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