How capacitor behave in alternating current?

Capacitors have an equivalent reactance of 1/jwC (ohms) where w is the angular frequency of the AC signal and C is the capacitance. As the frequency of the signal across the capacitor increases, the capacitor reactance approaches 0 (capacitor acts like a short circuit). As the frequency of the signal across the capacitor decreases, the capacitor reactance approaches infinity (capacitor acts like an open circuit). So, if you have a high frequency signal (like a step input) the capacitor will momentarily act like a short.