How does a capacitor behave in AC and DC circuits?

In DC circuits, a capacitor initially allows current to flow while charging, then blocks current completely when fully charged, acting as an open circuit in steady state. In AC circuits, a capacitor continuously charges and discharges with the alternating voltage, allowing AC current to pass. Capacitive reactance Xc = 1/(2 x pi x f x C) decreases with frequency, so capacitors pass high frequencies more easily.