Dictionary Definition
capacitance
Noun
1 an electrical phenomenon whereby an electric
charge is stored [syn: electrical
capacity, capacity]
2 an electrical device characterized by its
capacity to store an electric charge [syn: capacitor, condenser, electrical
condenser]
User Contributed Dictionary
English
Noun
 physics uncountable The property of an element of an electrical circuit that permits it to store charge
 physics countable That part of an electrical circuit exhibiting capacitance
 physics uncountable The ratio C of charge to potential in a conductor
Translations
property of an element of an electrical circuit
 German: Kapazität
 Italian: capacità
 Portuguese: capacitância
Derived terms
Extensive Definition
Capacitance is a measure of the amount of
electric
charge stored (or separated) for a given electric
potential. The most common form of charge storage device is a
twoplate capacitor.
If the charges on the plates are +Q and −Q, and V gives the voltage
difference between the plates, then the capacitance is given
by

 C = \frac
Energy
The energy (measured in joules) stored in a capacitor is equal to the work done to charge it. Consider a capacitance C, holding a charge +q on one plate and q on the other. Moving a small element of charge \mathrmq from one plate to the other against the potential difference V = q/C requires the work \mathrmW: \mathrmW = \frac\,\mathrmq
where
 W is the work measured in joules
 q is the charge measured in coulombs
 C is the capacitance, measured in farads
We can find the energy stored in a capacitance by
integrating this
equation. Starting with an uncharged capacitance (q=0) and moving
charge from one plate to the other until the plates have charge +Q
and Q requires the work W:
 W_ = \int_^ \frac \, \mathrmq = \frac\frac = \fracCV^2 = W_
Combining this with the above equation for the
capacitance of a flatplate capacitor, we get:
 W_ = \frac C V^2 = \frac \epsilon \frac V^2 .
where
 W is the energy measured in joules
 C is the capacitance, measured in farads
 V is the voltage measured in volts
Capacitance and 'displacement current'
The physicist James Clerk Maxwell invented the concept of displacement current, \frac, to make Ampère's law consistent with conservation of charge in cases where charge is accumulating, for example in a capacitor. He interpreted this as a real motion of charges, even in vacuum, where he supposed that it corresponded to motion of dipole charges in the ether. Although this interpretation has been abandoned, Maxwell's correction to Ampère's law remains valid (a changing electric field produces a magnetic field).Maxwell's equation combining Ampère's law with
the displacement current concept is given as \vec \times \vec =
\vec + \frac. (Integrating both sides, the integral of \vec\times
\vec can be replaced — courtesy of Stokes's
theorem — with the integral of \vec \cdot \mathrm
\vec over a closed contour, thus demonstrating the interconnection
with Ampère's formulation.)
Coefficients of Potential
The discussion above is limited to the case of two conducting plates, although of arbitrary size and shape. The definition C=Q/V still holds if only one plate is given a charge, provided that we recognize that the field lines produced by that charge terminate as if the plate were at the center of an oppositely charged sphere at infinity.C=Q/V does not apply when there are more than two
charged plates, or when the net charge on the two plates is
nonzero. To handle this case, Maxwell introduced his "coefficients
of potential". If three plates are given charges Q_1, Q_2, Q_3,
then the voltage of plate 1 is given by
 V_1 = p_ Q_1 + p_ Q_2 + p_ Q_3 ,
and similarly for the other voltages. Maxwell
showed that the coefficients of potential are symmetric, so that
p_=p_, etc.
Capacitance/inductance duality
In mathematical terms, the ideal capacitance can be considered as an inverse of the ideal inductance, because the voltagecurrent equations of the two phenomena can be transformed into one another by exchanging the voltage and current terms.Selfcapacitance
In electrical circuits, the term capacitance is usually a shorthand for the mutual capacitance between two adjacent conductors, such as the two plates of a capacitor. There also exists a property called selfcapacitance, which is the amount of electrical charge that must be added to an isolated conductor to raise its electrical potential by one volt. The reference point for this potential is a theoretical hollow conducting sphere, of infinite radius, centred on the conductor. Using this method, the selfcapacitance of a conducting sphere of radius R is given by:Typical values of selfcapacitance are:
 for the top "plate" of a van de Graaf generator, typically a sphere 20 cm in radius: 20 pF
 the planet Earth: about 710 µF
Elastance
The inverse of capacitance is called elastance, and its unit is the reciprocal farad.Stray capacitance
Any two adjacent conductors can be considered as a capacitor, although the capacitance will be small unless the conductors are close together or long. This (unwanted) effect is termed "stray capacitance". Stray capacitance can allow signals to leak between otherwise isolated circuits (an effect called crosstalk), and it can be a limiting factor for proper functioning of circuits at high frequency.Stray capacitance is often encountered in
amplifier circuits in the form of "feedthrough" capacitance that
interconnects the input and output nodes (both defined relative to
a common ground). It is often convenient for analytical purposes to
replace this capacitance with a combination of one inputtoground
capacitance and one outputtoground capacitance. (The original
configuration — including the inputtooutput capacitance
— is often referred to as a piconfiguration.) Miller's
theorem can be used to effect this replacement. Miller's
theorem states that, if the gain ratio of two nodes is 1:K,
then an impedance
of Z connecting the two nodes can be replaced with a Z/(1k)
impedance between the first node and ground and a KZ/(K1)
impedance between the second node and ground. (Since impedance
varies inversely with capacitance, the internode capacitance, C,
will be seen to have been replaced by a capacitance of KC from
input to ground and a capacitance of (K1)C/K from output to
ground.) When the inputtooutput gain is very large, the
equivalent inputtoground impedance is very small while the
outputtoground impedance is essentially equal to the original
(inputtooutput) impedance.
Capacitors
The capacitance of the majority of capacitors used in electronic circuits is several orders of magnitude smaller than the farad. The most common subunits of capacitance in use today are the millifarad (mF), microfarad (µF), the nanofarad (nF) and the picofarad (pF)The capacitance can be calculated if the geometry
of the conductors and the dielectric properties of the insulator
between the conductors are known. For example, the capacitance of a
parallelplate capacitor constructed of two parallel plates of area
A separated by a distance d is approximately equal to the
following:
 C = \epsilon_\epsilon_ \frac (in SI units)
 C is the capacitance in farads, F
 A is the area of each plate, measured in square metres
 εr is the relative static permittivity (sometimes called the dielectric constant) of the material between the plates, (vacuum =1)
 ε0 is the permittivity of free space where ε0 = 8.854x1012 F/m
 d is the separation between the plates, measured in metres
 C = \epsilon_ \frac
The dielectric constant for a number of very
useful dielectrics changes as a function of the applied electrical
field, e.g. ferroelectric materials,
so the capacitance for these devices is no longer purely a function
of device geometry. If a capacitor is driven with a sinusoidal
voltage, the dielectric constant, or more accurately referred to as
the relative static permittivity, is a function of frequency. A
changing dielectric constant with frequency is referred to as a
dielectric
dispersion, and is governed by dielectric
relaxation processes, such as Debye
relaxation.
References
 Tipler, Paul (1998). Physics for Scientists and Engineers: Vol. 2: Electricity and Magnetism, Light (4th ed.). W. H. Freeman. ISBN 1572594926
 Serway, Raymond; Jewett, John (2003). Physics for Scientists and Engineers (6 ed.). Brooks Cole. ISBN 0534408427
 Saslow, Wayne M.(2002). Electricity, Magnetism, and Light. Thomson Learning. ISBN 0126194556. See Chapter 8, and especially pp.255259 for coefficients of potential.
External links
capacitance in Bosnian: Električni
kapacitet
capacitance in Bulgarian: Електрически
капацитет
capacitance in Catalan: Capacitància
capacitance in Czech: Elektrická kapacita
capacitance in Danish: Kapacitans
capacitance in German: Elektrische
Kapazität
capacitance in Spanish: Capacidad
eléctrica
capacitance in Esperanto: Kapacitanco
capacitance in French: Capacité électrique
capacitance in Korean: 전기용량
capacitance in Croatian: Električni
kapacitet
capacitance in Icelandic: Rafrýmd
capacitance in Italian: Capacità elettrica
capacitance in Latvian: Elektriskā
kapacitāte
capacitance in Lithuanian: Elektrinė talpa
capacitance in Malay (macrolanguage):
Kapasitans
capacitance in Mongolian: Багтаамж
capacitance in Dutch: Elektrische
capaciteit
capacitance in Japanese: 静電容量
capacitance in Norwegian: Kapasitans
capacitance in Polish: Pojemność
elektryczna
capacitance in Portuguese: Capacitância
capacitance in Russian: Электрическая
ёмкость
capacitance in Slovak: Elektrická kapacita
capacitance in Slovenian: Kapacitivnost
capacitance in Serbian: Капацитивност
capacitance in Swedish: Kapacitans
capacitance in Vietnamese: Điện dung
capacitance in Turkish: Kapasite
(elektrik)
capacitance in Ukrainian: Ємність
(електрика)
capacitance in Chinese: 電容