Editing Electrical impedance (section)

==== Capacitive reactance ====
<!--[[File:Photo-SMDcapacitors.jpg|thumb|right|200px|Capacitors: [[Surface-mount technology|SMD]] ceramic at top left; SMD tantalum at bottom left; [[through-hole]] tantalum at top right; through-hole electrolytic at bottom right. Major scale divisions are cm. [[Media:Resistors.jpg|Actual size]]]]-->
{{Main|Capacitance}}

A capacitor has a purely reactive impedance that is [[Inversely proportional#Inverse proportionality|inversely proportional]] to the signal [[frequency]]. A capacitor consists of two [[Electrical conduction|conductor]]s separated by an [[Electrical insulation|insulator]], also known as a [[dielectric]].

:<math>X_\mathsf{C} = \frac{-1\ ~}{\ \omega\ C\ } = \frac{-1\ ~}{\ 2\pi\ f\ C\ } ~.</math>

The minus sign indicates that the imaginary part of the impedance is negative.

At low frequencies, a capacitor approaches an open circuit so no current flows through it.

A DC voltage applied across a capacitor causes [[Electrical charge|charge]] to accumulate on one side; the [[electric field]] due to the accumulated charge is the source of the opposition to the current. When the [[potential]] associated with the charge exactly balances the applied voltage, the current goes to zero.

Driven by an AC supply, a capacitor accumulates only a limited charge before the potential difference changes sign and the charge dissipates. The higher the frequency, the less charge accumulates and the smaller the opposition to the current.