Capacitance

A capacitor effectively conducts alternating current even though electrons do not cross from one plate to other plate. Alternating current that appears to pass through a capacitor is actually, the charge and discharge current resulting from the constantly-changing voltage across the capacitor.

An uncharged capacitor always appears as a short circuit because its voltage must equal zero when its stored charge is zero. A capacitor carrying an alternating current continually charges and discharges, spending much of the time in a near-zero charge state. The resulting low voltage across its terminals means that it is often less significant in limiting circuit than other components in the circuit.

A capacitor's opposition to alternating current is called reactance. Higher capacitance introduces less reactance and higher frequencies result in lower reactance.