A capacitor and a coil in an AC circuit

Direct current does not pass through a capacitor (there is a dielectric between the plates), but alternating current does: the capacitor is charged and discharged in turn and charges keep moving in the wires. If the voltage u = Uₘcosωt is applied to a capacitor, the charge is q = C·u = CUₘcosωt and the current is the rate of change of charge: i = Iₘcos(ωt + π/2), Iₘ = ωC·Uₘ. So the current leads the voltage by π/2 (a quarter period). Ohm's law reads Iₘ = Uₘ/X_C, where the capacitive reactance X_C = 1/(ωC) = 1/(2πνC) is measured in ohms (Ω). The greater the frequency and capacitance, the smaller X_C and the larger the current; for direct current (ν = 0) X_C is infinite. X_C is a reactive resistance: energy is not turned into heat but is stored as electric field energy during part of the period and returned to the source, so the mean power is zero.