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farad (F)

The farad (symbolized F) is the standard unit of capacitance in the International System of Units (SI). Reduced to base SI units, one farad is the equivalent of one second to the fourth power ampere squared per kilogram per meter squared (s4 · A2 · kg-1 · m-2).

When the voltage across a 1 F capacitor changes at a rate of one volt per second (1 V/s), a current flow of 1 A results. A capacitance of 1 F produces 1 V of potential difference for an electric charge of one coulomb (1 C). The farad is an extremely large unit of capacitance. In practice, capacitors with values this large are almost never seen.

In common electrical and electronic circuits, units of microfarads (µF), where 1 µF = 10-6 F, and picofarads (pF), where 1 pF = 10-12 F, are used. At radio frequencies (RF), 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 (AF), capacitances range from about 0.1 µF to 100 µF. In power-supply filters, capacitances can be as high as 10,000 µF.

Both the farad and Faraday's constant are symbolized by the capital letter F; however they are different. Faraday's constant is a measure of the amount of electrical charge in a single mole.

Also see: faraday, capacitor, inductor, reactance, and henry.

This was last updated in January 2015

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Capacitors of one Farad and more are now readily available, and more applications for these large values are growing.
In English-Opposit charges attract - even in metals like copper. Two conductors with different polarity separated by an insulator so they can not short circuit or equalize voltage difference form a capacitor.
The opposite charges, like north and south pole magnets, start attracting each other and accumulate or bunch where the opposite charges or wires cross.
When wanted, this capacitance helps smooth out the flow of electricity or will store a charge. Example changing 120 V 60Hz AC to smooth 2.3 v DC.
When not wanted parasitic capacitance interferes with the circuit. Static shock is one example.
Capacitance is also used as a way to measure fuel in a fuel tank with no moving parts.


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