The farad is a unit of electrical capacity named after Michael Faraday. Capacitors typically fall in the ranges from millifarads to picofarads. The mathematical relationship for capacitance is C=kε0A/d. Capacitors have various applications, including DC blocking, signal filtering, and coupling two circuits. They can also be used in LC and RLC circuits for signal transfer and television reception.
Named for scientist Michael Faraday of England, the farad is a unit of electrical capacity. If a device called a capacitor stores a charge of 1 coulomb with a potential difference of 1 volt across its plates, it is a one farad capacitor. This unit is actually too large for most practical applications. Capacitors in typical use fall in the ranges from millifarads (10-3) to microfarads (10-6) to picofarads (10-12). The farad as a unit is so large because the coulomb, used to define the farad, is itself large — 1 coulomb=1 ampere-second.
Units of measurement that include the farad are called the International System of Units or SI units and are based on the metric. SI base units include the meter, kilogram, and second. Other units used in this system include the joule for energy and work, the ohm for resistance, the newton for force, and the henry for inductance. Together with the farad, all these electrical quantities are used in important mathematical expressions. It is impossible to discuss what a farad is without discussing the nature and applications of the capacitor.
A parallel plate capacitor is the simplest design to illustrate the basic parameters of capacitance. This property is directly related to the surface area of the plates and is inversely proportional to the distance between the plates. Proportionality depends on the medium between the plates and a quality it possesses called “permittivity,” which is the measure of resistance to an electric field that the material between the plates supports or allows.
The mathematical relationship is C=kε0A/d. In this equation, the permittivity of space is “ε0” which equals 8.854*10-12 farads/meter. The area of the plates in square meters is represented by “A” and “d” is the number of meters between them. The constant k is the relative permittivity of the actual material between the plates, whether it be air, ceramic, oil, or some other substance. For example, if a parallel plate capacitor has plates with an area of 1 square meter each and the distance between them is 0.001 meter and the permittivity, k, is 1.07, the capacitance is 9.474 picofarads.
Applications for capacitors include DC blocking, signal filtering, electrical discharge, limiting transient amplitude, and coupling two circuits. For the latter application, capacitors can be used in conjunction with coils to set a specific inductance. This is called an LC circuit, sometimes a tuned or resonant circuit, and when combined with a resistance or impedance, it is called an “RLC” circuit. Limits the signal transfer from a more complex signal to a particular frequency. Such circuits are used in television reception.
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