Editing Capacitance (section)

==Capacitors==
{{Main|Capacitor}}
The capacitance of the majority of capacitors used in electronic circuits is generally several orders of magnitude smaller than the [[farad]]. The most common units of capacitance are the [[micro-|micro]]farad (μF), [[nano-|nano]]farad (nF), [[pico-|pico]]farad (pF), and, in microcircuits, [[femto-|femto]]farad (fF). Some applications also use [[supercapacitors]] that can be much larger, as much as hundreds of farads, and parasitic capacitive elements can be less than a femtofarad.  Historical texts use other, obsolete submultiples of the farad, such as "mf" and "mfd" for microfarad (μF); "mmf", "mmfd", "pfd", "μμF" for picofarad (pF).<ref>{{cite web |url=http://www.justradios.com/MFMMFD.html |title=Capacitor MF-MMFD Conversion Chart |website=Just Radios}}</ref><ref>{{cite book |url=https://archive.org/details/FundamentalsOfElectronics93400A1b |title=Fundamentals of Electronics |volume=1b – Basic Electricity – Alternating Current |publisher=Bureau of Naval Personnel |year=1965 |page=[https://archive.org/details/FundamentalsOfElectronics93400A1b/page/n58 197]}}</ref>

The capacitance can be calculated if the geometry of the conductors and the dielectric properties of the insulator between the conductors are known. Capacitance is proportional to the area of overlap and inversely proportional to the separation between conducting sheets. The closer the sheets are to each other, the greater the capacitance.

An example is the capacitance of a capacitor constructed of two parallel plates both of area  <math display="inline">A</math> separated by a distance <math display="inline">d</math>. If <math display="inline">d</math> is sufficiently small with respect to the smallest chord of <math display="inline">A</math>, there holds, to a high level of accuracy:
<math display="block">\ C=\varepsilon\frac{A}{d};</math>

<math display="block">\varepsilon=\varepsilon_0 \varepsilon_r,</math>

where
*<math display="inline">C</math> is the capacitance, in farads;
*<math display="inline">A</math> is the area of overlap of the two plates, in square meters;
*<math display="inline">\varepsilon_0</math> is the [[vacuum permittivity|electric constant]] {{nowrap|(<math display="inline">\varepsilon_0 \approx 8.854\times 10^{-12} ~ \mathrm{F{\cdot}m^{-1}}</math>);}} 
*<math display="inline">\varepsilon_r</math> is the [[relative permittivity]] (also dielectric constant) of the material in between the plates {{nowrap|(<math display="inline">\varepsilon_r \approx 1</math>}} for air); and
*<math display="inline">d</math> is the separation between the plates, in meters.

The equation is a good approximation if ''d'' is small compared to the other dimensions of the plates so that the electric field in the capacitor area is uniform, and the so-called ''fringing field'' around the periphery provides only a small contribution to the capacitance.

Combining the equation for capacitance with the above equation for the energy stored in a capacitor, for a flat-plate capacitor the energy stored is:
<math display="block"> W_\text{stored} = \frac{1}{2} C V^2 = \frac{1}{2} \varepsilon \frac{A}{d} V^2.</math>
where <math display="inline">W</math> is the energy, in joules; <math display="inline">C</math> is the capacitance, in farads; and <math display="inline">V</math> is the voltage, in volts.