The farad (symbol: F) is the SI derived unit of electrical capacitance, the ability of a body to store an electrical charge. It is named after the English physicist Michael Faraday.
SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI). They are either dimensionless or can be expressed as a product of one or more of the base units, possibly scaled by an appropriate power of exponentiation.
Capacitance is the ratio of the change in an electric charge in a system to the corresponding change in its electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. A material with a large self capacitance holds more electric charge at a given voltage than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components.
Michael Faraday FRS was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic induction, diamagnetism and electrolysis.
One farad is defined as the capacitance across which, when charged with one coulomb, there is a potential difference of one volt.[1] Equally, one farad can be described as the capacitance which stores a one-coulomb charge across a potential difference of one volt.[2]
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two-types of electric charges; positive and negative. Like charges repel and unlike attract. An object with an absence of net charge is referred to as neutral. Early knowledge of how charged substances interact is now called classical electrodynamics, and is still accurate for problems that do not require consideration of quantum effects.
The coulomb is the International System of Units (SI) unit of electric charge. It is the charge transported by a constant current of one ampere in one second:
The volt is the derived unit for electric potential, electric potential difference (voltage), and electromotive force. It is named after the Italian physicist Alessandro Volta (1745–1827).
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.
A capacitor is a passive two-terminal electronic component that stores electrical energy in an electric field. The effect of a capacitor is known as capacitance. While some capacitance exists between any two electrical conductors in proximity in a circuit, a capacitor is a component designed to add capacitance to a circuit. The capacitor was originally known as a condenser or condensator. The original name is still widely used in many languages, but not commonly in English.
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) = 1000μF = 1000000nF
1μF (microfarad, one millionth (10−6) of a farad) = 0.000001F = 1000nF = 1000000pF
1nF (nanofarad, one billionth (10−9) of a farad) = 0.001μF = 1000pF
1pF (picofarad, one trillionth (10−12) of a farad)
A billion is a number with two distinct definitions:
Equalities
A farad is represented in terms of SI base units as s4⋅A2⋅m−2⋅kg−1
The second is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. Analog clocks and watches often have sixty tick marks on their faces, representing seconds, and a "second hand" to mark the passage of time in seconds. Digital clocks and watches often have a two-digit seconds counter. The second is also part of several other units of measurement like meters per second for velocity, meters per second per second for acceleration, and per second for frequency.
The ampere, often shortened to "amp", is the base unit of electric current in the International System of Units (SI). It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.
The kilogram or kilogramme is the base unit of mass in the International System of Units (SI). Until 20 May 2019, it remains defined by a platinum alloy cylinder, the International Prototype Kilogram, manufactured in 1889, and carefully stored in Saint-Cloud, a suburb of Paris. After 20 May, it will be defined in terms of fundamental physical constants.
The term "farad" was originally coined by Latimer Clark and Charles Bright in 1861,[3] in honor of Michael Faraday, for a unit of quantity of charge, but by 1873, the farad had become a unit of capacitance.[4] In 1881 at the International Congress of Electricians in Paris, the name farad was officially used for the unit of electrical capacitance.[5][6]
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 infarads (F), microfarads (μF), nanofarads (nF) and picofarads (pF).[7] The millifarad is rarely used in practice (a capacitance of 4.7mF (0.0047F), for example, is instead written as 4700µF), while the nanofarad is uncommon in North America.[8] 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 (1 aF = 10−18F).[9]
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.[10][11]
The capacitance of the Earth's ionosphere with respect to the ground is calculated to be about 1F.[12]
Informal and deprecated terminology
The picofarad (pF) is sometimes colloquially pronounced as "puff" or "pic", as in "a ten-puff capacitor".[13] 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 1000000 (or 1000), 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).[14]
"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".
Related concepts
The reciprocal of capacitance is called electrical elastance, the (non-standard, non-SI) unit of which is the daraf.[15]
CGS units
The abfarad (abbreviated abF) is an obsolete CGS unit of capacitance equal to 109 farads (1 gigafarad, GF).[16]
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.
↑ Peter M B Walker, ed. (1995). Dictionary of Science and Technology. Larousse. ISBN0752300105.
↑ 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 of extending the CGS system to cover electromagnetism.
The joule is a derived unit of energy in the International System of Units. It is equal to the energy transferred to an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre. It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).
The Faraday constant, denoted by the symbol F and sometimes stylized as ℱ, is named after Michael Faraday. In physics and chemistry, this constant represents the magnitude of electric charge per mole of electrons. It has the currently accepted value
The henry is the SI derived unit of electrical inductance. If a current of 1 ampere flowing through the coil produces flux linkage of 1 weber turn, the 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.
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.
A photoflash capacitor is an electrolytic capacitor used in flash cameras, professional flashes, and also in solid-state laser power supplies. Their usual purpose is to briefly power a high-voltage flash tube, used to illuminate a photographic subject or optically pump a laser rod. As flash tubes require very high current for a very short time to operate, photoflash capacitors are designed to supply high discharge current pulses without excessive internal heating.
In physics, the weber is the SI derived unit of magnetic flux. A flux density of one Wb/m2 is one tesla.
The tesla is a derived unit of the magnetic induction in the International System of Units.
Capacitors are manufactured in many forms, styles, lengths, girths, and from many materials. They all contain at least two electrical conductors separated by an insulating layer. Capacitors are widely used as parts of electrical circuits in many common electrical devices.
The ohm is the SI derived unit of electrical resistance, named after German physicist Georg Simon Ohm. Although several empirically derived standard units for expressing electrical resistance were developed in connection with early telegraphy practice, the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time and of a convenient size for practical work as early as 1861. The definition of the ohm was revised several times. Today, the definition of the ohm is expressed from the quantum Hall effect.
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.
A ceramic capacitor is a fixed-value capacitor where the ceramic material acts as the dielectric. It is constructed of two or more alternating layers of ceramic and a metal layer acting as the electrodes. The composition of the ceramic material defines the electrical behavior and therefore applications. Ceramic capacitors are divided into two application classes:
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 AC induction motor to create a rotating magnetic field. There are two common types of motor capacitors, run capacitors and start capacitors. The units of capacitance are labeled in microfarads (µF). Older capacitors may be labeled with the obsolete terms "mfd" or "MFD" which can be ambiguous but are, especially in this context, used for microfarad as well.
A supercapacitor (SC) is a high-capacity capacitor with capacitance values much higher than other capacitors that bridge the gap between electrolytic capacitors and rechargeable batteries. They typically store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries.
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 the mains voltage to a lower voltage. There are two important limitations: First, the high withstanding voltage required of the capacitor, along with the high-capacitance required for a given output current, mean that this type of supply is only practical for low-power applications. The second is that due to the absence of electrical isolation between input and output, anything connected to the power supply must be reliably insulated so that it is not possible for a person to come into electrical contact with it. By the equation of state for capacitance, where , the current is limited to: 1 amp, per farad, per volt-rms, per radian. Or amps, per farad, per volt-rms, per hertz.
Double-layer capacitance is the important characteristic the electrical double layer which appears, for example, at the interface between a conductive electrode and an adjacent liquid electrolyte. At this boundary two layers of charge with opposing polarity form, one at the surface of the electrode, and one in the electrolyte. These two layers, electrons on the electrode and ions in the electrolyte, are typically separated by a single layer of solvent molecules that adhere to the surface of the electrode and act like a dielectric in a conventional capacitor. The amount of electric charge stored in double-layer capacitor depends on the applied voltage. The unit of capacitance is the farad.
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