What is a farad (F)?
A farad (F) is the standard unit of electrical capacitance (C) in the International System of Units. It indicates the ability of a substance to hold an electrical charge. The value of most electrical capacitors is expressed in farads, microfarads (µF) or nanofarads (nF).
The farad was named after the 18th-century English physicist Michael Faraday. He committed much of his career to the study of electromagnetism, which focuses on the science of electrical charge and the forces and fields associated with it. Faraday middleware, an older method of sending and retrieving requests from web servers, also bears his name, although it is unrelated to Michael Faraday's work.
In computing, farads as capacitance units of measurement measure the ability of an electrical capacitor to store an electrical charge. This is much like how an ohm is used to measure units of amperage or a joule is used to measure units of energy produced by the force of one newton, which is the acceleration of one meter per second up to the mass of one kilogram. Having the capacity to store an electrical charge -- and not lose it -- is critical to avoid information loss during a power failure. To measure the storage capability of an electrical capacitor in a computer and other electronic applications, a mathematical value expressed as F, µF or nF is used. 1 F equals 1 second to the fourth power ampere squared per kilogram meter squared, noted as follows:
s4 x A2 / kg x m2
Farad explained
To understand a farad's role as a unit of electrical charge measurement, it's best to start with the electrical capacitors on computers.
The job of each capacitor in a computer is to store energy in the form of an electrical charge. To do this, each capacitor consists of two parts, one plate -- or conductor -- that attracts electrons and a second conductor or plate that repels them. This dynamic of attraction and repulsion, where one conductor becomes positively charged and the other becomes negatively charged, generates a field of energy in the area between the two conductors that are stored in the capacitor. Since the energy is stored, it can be fed back into an electrical circuit when needed -- for instance, if there is a power outage. As long as capacitors are supplied with voltage, they will recharge themselves. Farad capacitors are also used in surge protectors and uninterruptible power supplies.
Mathematically, a farad is expressed as the ratio of the amount of charge (q) on either conductor to the potential difference (V) between them:
C = q / V
Farad is the unit of capacitance. A capacitor has a capacitance of 1 F when 1 coulomb (C) of electricity changes the potential between the plates by 1 volt (V). Another way of saying this is that when the voltage across a 1 F capacitor changes at a rate of 1 V per second, the result is a current flow of 1 A.
The SI base units of 1 farad are as follows:
s4 x A2 x m-2 x kg-1.
Mathematically, it can be represented as follows:
1 F = 1 s4 x A2 / m2 x kg
Farad, microfarad, nanofarad, picofarad
The farad is an extremely large unit of capacitance. In most electronic and electrical equipment, capacitors with values this large are rare -- but not impossible. Most capacitors are generally rated in microfarads, nanofarads or picofarads (pF). The older, deprecated terminology for picofarad was micromicrofarad (μμF), which is one-millionth of a farad. A picofarad capacitor is sometimes known as a pic or puff, as noted in the following:
1 µF = 10-6 F = 0.000001 F
1 nF = 10-9 F = 0.000000001 F
1 pF = 10-12 F = 0.000000000001 F
The millifarad (mF), which is rarely used in practice, is represented as follows:
1 mF = 10-3 F = 0.001 F
At radio frequencies, capacitances range from about 1 pF to 1,000 pF in tuned circuits and from about 0.001 µF to 0.1 µF for blocking and bypassing. At audio frequencies, capacitances range from about 0.1 µF to 100 µF. In power supply filters, capacitances can be as high as 10,000 µF.
Supercapacitors and kilofarads
Some capacitors with farad values as large as 1,000 F (kilofarad) are also in use. These capacitors are known as supercapacitors or ultracapacitors. The high farad values indicate that these capacitors can store larger amounts of energy per unit volume or mass -- typically 10 to 100 times more than electrolytic capacitors.
In addition, these capacitors can deliver a charge faster than rechargeable batteries. They can also tolerate more charge and discharge cycles than rechargeable lithium-ion batteries that tend to degrade with each charge cycle.
High-farad value supercapacitors are usually used in applications that require short-term energy storage, power delivery in burst mode, and multiple charge and discharge cycles. They are suitable for the following:
- Automobiles.
- Elevators.
- Cranes.
- Wind turbines.
Supercapacitors aren't suitable for applications that require long-term compact energy storage, such as consumer devices like smartphones or tablets. Nonetheless, some consumer devices use supercapacitors due to their quick recharge capability and prolonged lifecycles. These include MP3 players and professional camera flashes.
How larger capacitances are obtained
Most practical capacitors range between 0.001 μF and 10 F. Larger capacitances can be obtained by increasing conductor area, decreasing the space between the conductor plates or using a dielectric medium with a larger permittivity value -- measured in farads per meter.
However, there's a limit to how much the plate spacing can be reduced to achieve a high capacitance or high farad value. This limit depends on the dielectric breakdown strength of the insulating material between the conducting plates. When this insulation space limit is exceeded, a spark jumps between the plates vc. It also leaves a conducting track within the insulating material, damaging the capacitor.
Knowing the electric field strength of the dielectric or insulating material is also essential. This value calculates the voltage that can be safely applied to a capacitor of a given plate or conductor separation. Here, safe means the voltage value that won't cause arcing and damage in the capacitor. This is why capacitors are stamped with both capacitance, or farad, and voltage values as guidance for their proper use.
What is the difference between a farad and Faraday constant?
Both farad and Faraday constants are symbolized by the capital letter F, but they are different. One farad indicates that the capacitor produces 1 V of potential difference for an electric charge of 1 C. In contrast, the Faraday constant measures the amount, or magnitude, of electrical charge carried by a single mole, or amount of substance or elementary entities of a substance -- otherwise known as Avogadro's number and represented as C/mol.
The value of the Faraday constant is obtained by dividing the Avogadro constant by the number of electrons per coulomb or by multiplying the charge of a single electron with the number of electrons in 1 mol as follows:
Faraday constant (F) = charge of a single electron x the number of electrons in 1 mol
= 1.6023 x 10-19 C x 6.02 x 1023
= 96,485 C/mol
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