Farad

The farad or farad (symbol: F) is the unit of electrical capacity of the International System of Units (SI). It is named in honor of Michael Faraday.

A farad is the capacity of a capacitor between whose plates there is a difference of electric potential of 1 volt (1 V) when it is charged with an amount of electricity equal to one coulomb (1 C).

In electrical engineering, it measures more specifically the capacity of a capacitor or a system of conductors, that is, the charge it can store when a voltage is applied to it.

Where:

• F = Faradio
• A = Amperio
• V = Volt
• C = Culombio
• J = July
• m = metro
• N = Newton
• S = Second
• W = Watt
• kg = Kilogram
• Ω = Ohmio
• H = Henrio

Not to be confused with the faraday (unit), which is an old unit of electrical charge equivalent to Faraday's constant.

Definition

A farad is defined as the capacitance across which, when charged with one coulomb, there is a potential difference of one volt. Likewise, a farad can be described as the capacitance that stores a charge of one coulomb at through a potential difference of one volt.

The relationship between capacitance, charge, and potential difference is linear. For example, if the potential difference across a capacitor is halved, the amount of charge stored by that capacitor will also be halved.

For most applications, the farad is an impractical unit of capacity. Most electrical and electronic applications are covered by the following SI prefixes:

• 1 mF (milifaradio, a thousandth (10)^-3) of a pharaoh = 0.001 F = 1000μF = 1000000nF
• 1 μF (micropharyadium, one millionth (10)^-6) of a lighthouse = 0.000 001 F = 1000NF = 1000000pF
• 1 nF (nanofaradio, one billionth (10)^-9) of a faradium = 0.000 000 001 F = 0.001 μF = 1000pF
• 1 pF (picfaradium, a trillionthony^-12) of a lighthouse = 0.000 000 001 F = 0.001 nF

Equals

A farad is a derived unit based on four of the seven base units of the International System of Units: kilogram (kg), meter (m), second (s), and ampere (A).

Expressed in combinations of SI units, the farad is:

F=s4⋅ ⋅ A2m2⋅ ⋅ kg=s2⋅ ⋅ C2m2⋅ ⋅ kg=CV=A⋅ ⋅ sV=W⋅ ⋅ sV2=JV2=N⋅ ⋅ mV2=C2J=C2N⋅ ⋅ m=sΩ Ω =1Ω Ω ⋅ ⋅ Hz=SHz=s2H,♪

where F = farad, C = coulomb, V = volt, W = watt, J = joule, N =newton.

In SI base units, the farad is expressed as follows:

Ф = А²·s⁴·kg⁻¹·m⁻².

History

The term "farad" was originally coined by Latimer Clark and Charles Bright in 1861, in honor of Michael Faraday, for a unit of quantity of charge, but by 1873, the farad had become a unit of capacitance. In 1881, at the International Congress of Electricians of Paris, the name farad was officially used for the unit of electrical capacity.

Explanation

Examples of different types of capacitors

A capacitor is usually made up of two conductive surfaces, often called plates, separated by an insulating layer often called a dielectric. The original condenser was the Leyden jar developed in the 18th century. The accumulation of electrical charge on the plates is what gives rise to capacitance. Modern capacitors are constructed using a variety of manufacturing techniques and materials to provide the extraordinarily wide range of capacitance values used in electronic applications, from femtofarads to farads, with maximum voltage values ranging from a few volts to several kilovolts..

Capacitor values are usually specified in farads (F), microfarads (μF), nanofarads (nF), and picofarads (pF). The millifarad is rarely used in practice (a capacitance of 4.7 mF (0.0047 F), for example, is instead written as 4700 μF), while the nanofarad is uncommon in North America. The size of the capacitors available in the The market ranges from about 0.1 pF to 5000F (5 kF) supercapacitors. Stray capacitance in high-performance ICs can be measured in femtofarads (1 fF = 0.001 pF = 10¹⁵ F), while high-performance test sets can detect changes in capacitance on the order of of tens of attofarads (1 aF = 10¹⁸ F).

A value of 0.1 pF is about the smallest available for capacitors for general use in electronic design, since the smallest would be dominated by stray capacitance from other components, wiring, or printed circuit boards. Capacitance values of 1 pF or less can be achieved by twisting together two short lengths of insulated wire.

The capacitance of Earth's ionosphere with respect to ground is estimated to be about 1 F.

Informal and outdated terminology

The picofarad (pF) is sometimes colloquially pronounced as "puff" or "pic", as in "a ten-beanbag condenser". Similarly, "mic" (pronounced "mike") is sometimes used informally to mean microfarads.

Non-standard abbreviations have been and are often used. Farads have been abbreviated as "f", "fd" and "Fd". For the prefix "micro-", when the lowercase Greek letter "μ" or the micro legacy sign "μ" is not available (as on typewriters) or inconvenient to enter, it is often replaced by the "u" or "U" similar in appearance, with little risk of confusion. It was also replaced by the "M" or "m", similar in appearance, which can be confusing because M officially stands for 1,000,000, and m, preferably, 1/1000. In texts before 1960, and on capacitor packages until recently, "microfarad(s)" was abbreviated "mf" or "MFD" instead of modern "μF". A 1940 Radio Shack catalog listed the rating of each capacitor in "Mfd.", starting at 0.000005 Mfd. (5 pF) up to 50 Mfd. (50μF).

"Micromicrofarad" or "micromicrofarad" is an obsolete unit found in some old texts and labels, it contains a non-standard double metric prefix. It is exactly equivalent to one picofarad (pF). It is abbreviated μμF, uuF, or (confusingly) "mmf", "MMF", or "MMFD".

Summary of obsolete capacitance units: (case variations not shown)

• μF (micropharyadium) = mf, mfd
• pF (picfaradium) = mmf, mmfd, pfd, μF

Related concepts

The reciprocal of capacitance is called electrical elastance, whose (non-standard, non-IS) unit is the daraf.

Multiples of the SI

The following is a table of the multiples and submultiples of the International System of Units.

This unity of the International System is named in honor of Michael Faraday. In the units of the SI whose name comes from a person's own name, the first letter of the symbol is written with capital (F), while his name always starts with a tiny letter (Faradio), except in case you start a sentence or a title.

Based on The International System of UnitsSection 5.2.

In addition, in technical fields, the Scythian micro-microfarad unit is used as MMFD, mmfd, MMF or uuF, which is equivalent to the picofarad (pF).