capacitance


capacitance
/keuh pas"i teuhns/, n. Elect.
1. the ratio of an impressed charge on a conductor to the corresponding change in potential.
2. the ratio of the charge on either conductor of a capacitor to the potential difference between the conductors.
3. the property of being able to collect a charge of electricity. Symbol: C
[1905-10; CAPACIT(Y) + -ANCE]

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Property of a pair of electric conductors separated by a nonconducting material (such as air) that permits storage of electric energy by the separation of electric charge, and that is measured by the amount of separated charge that can be stored per unit of electric potential between the conductors.

If electric charge is transferred between two initially uncharged conductors, they become equally (but oppositely) charged. A potential difference is set up between them and some of the electricity can be stored. Capacitance C is the ratio of the amount of charge q on one of the conductors to the potential difference V between the conductors, or C = q/V. The unit of capacitance is coulombs per volt (C/V), or farads (F).

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      property of an electric conductor, or set of conductors, that is measured by the amount of separated electric charge that can be stored on it per unit change in electrical potential. Capacitance also implies an associated storage of electrical energy. If electric charge is transferred between two initially uncharged conductors, both become equally charged, one positively, the other negatively, and a potential difference is established between them. The capacitance C is the ratio of the amount of charge q on either conductor to the potential difference V between the conductors, or simply C = q/V.

      In both the practical and the metre–kilogram–second scientific systems, the unit of electric charge is the coulomb and the unit of potential difference is the volt, so that the unit of capacitance—named the farad (symbolized F)—is one coulomb per volt. One farad is an extremely large capacitance. Convenient subdivisions in common use are one-millionth of a farad, called a microfarad (μF), and one-millionth of a microfarad, called a picofarad (pF; older term, micromicrofarad, μμF). In the electrostatic system of units, capacitance has dimensions of distance.

      Capacitance in electric circuits (electric circuit) is deliberately introduced by a device called a capacitor. It was discovered by the Prussian scientist Ewald Georg von Kleist (Kleist, E Georg von) in 1745 and independently by the Dutch physicist Pieter van Musschenbroek (Musschenbroek, Pieter van) at about the same time, while in the process of investigating electrostatic phenomena. They discovered that electricity obtained from an electrostatic machine could be stored for a period of time and then released. The device, which came to be known as the Leyden jar, consisted of a stoppered glass vial or jar filled with water, with a nail piercing the stopper and dipping into the water. By holding the jar in the hand and touching the nail to the conductor of an electrostatic machine, they found that a shock could be obtained from the nail after disconnecting it, by touching it with the free hand. This reaction showed that some of the electricity from the machine had been stored.

      A simple but fundamental step in the evolution of the capacitor was taken by the English astronomer John Bevis in 1747 when he replaced the water by metal foil forming a lining on the inside surface of the glass and another covering the outside surface. This form of the capacitor with a conductor projecting from the mouth of the jar and touching the lining had, as its principal physical features, two conductors of extended area kept nearly equally separated by an insulating, or dielectric, layer made as thin as practicable. These features have been retained in every modern form of capacitor.

      A capacitor, also called a condenser, is thus essentially a sandwich of two plates of conducting material separated by an insulating material, or dielectric. Its primary function is to store electrical energy. Capacitors differ in the size and geometrical arrangement of the plates and in the kind of dielectric material used. Hence, they have such names as mica, paper, ceramic, air, and electrolytic capacitors. Their capacitance may be fixed or adjustable over a range of values for use in tuning circuits.

      The energy stored by a capacitor corresponds to the work performed (by a battery, for example) in creating opposite charges on the two plates at the applied voltage. The amount of charge that can be stored depends on the area of the plates, the spacing between them, the dielectric material in the space, and the applied voltage.

      A capacitor incorporated in an alternating-current (AC (alternating current)) circuit is alternately charged and discharged each half cycle. The time available for charging or discharging thus depends on the frequency of the current, and if the time required is greater than the length of the half cycle, the polarization (separation of charge) is not complete. Under such conditions, the dielectric constant appears to be less than that observed in a direct-current circuit and to vary with frequency, becoming lower at higher frequencies. During the alternation of polarity of the plates, the charges must be displaced through the dielectric first in one direction and then in the other, and overcoming the opposition that they encounter leads to a production of heat known as dielectric loss, a characteristic that must be considered when applying capacitors to electrical circuits, such as those in radio and television receivers. Dielectric losses depend on frequency and the dielectric material.

      Except for the leakage (usually small) through the dielectric, no current flows through a capacitor when it is subject to a constant voltage. Alternating current will pass readily, however, and is called a displacement current (q.v.).

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Universalium. 2010.

Look at other dictionaries:

  • capacitance — [ kapasitɑ̃s ] n. f. • 1927; mot angl. ♦ Phys. Impédance d un circuit qui se comporte comme un condensateur. ● capacitance nom féminin En électricité, réactance opposée par une capacité au passage d un courant alternatif sinusoïdal. capacitance… …   Encyclopédie Universelle

  • Capacitance — Cette page d’homonymie répertorie les différents sujets et articles partageant un même nom. La capacitance peut désigner : l impédance d un condensateur en électricité, elle s exprime en F (Farad). (voir : Capacité électrique) Le… …   Wikipédia en Français

  • capacitance — ca*pac i*tance (k[.a]*p[a^]s [i^]*tans), n. 1. an electrical phenomenon whereby an electric charge is stored. Syn: electrical capacity, capacity. [WordNet 1.5] 2. (Electronics) a measure of the ability of a capacitor to store electrical charge;… …   The Collaborative International Dictionary of English

  • Capacitance —   [engl.], Kapazität …   Universal-Lexikon

  • capacitance — 1893, from CAPACITY (Cf. capacity) (q.v.) + ANCE (Cf. ance) …   Etymology dictionary

  • capacitance — ► NOUN Physics ▪ the ability of a circuit or object to store electric charge, equivalent to the ratio of the change in electric charge to the corresponding change in electric potential …   English terms dictionary

  • capacitance — [kə pas′i təns] n. [ CAPACIT(Y) + ANCE] Elec. that property of a capacitor which determines how much charge can be stored in it for a given potential difference between its terminals, equal to the ratio of the charge stored to the potential… …   English World dictionary

  • Capacitance — Electromagnetism Electricity · …   Wikipedia

  • capacitance — The quantity of electric charge that may be stored upon a body per unit electric potential; expressed in farads, abfarads, or statfarads. * * * ca·pac·i·tance kə pas ət ən(t)s n 1 a) the property of an electric nonconductor that permits the… …   Medical dictionary

  • capacitance — talpinė varža statusas T sritis fizika atitikmenys: angl. capacitance; capacity reactance; capacitive reactance vok. kapazitiver Widerstand, m rus. ёмкостное сопротивление, n pranc. capacitance, f; réactance capacitive, f …   Fizikos terminų žodynas


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