Question

capacitor block dc ,but allow ac to pass through a circuit .why

Answer 1

DC has zero frequency, so reactance is infinity. This is the reason DC is blocked. While AC has some frequency ,due to which capacitor lets it flow.

Answer 2

A Capacitor can store the charge as it has two electrodes with dielectric media in between. It does not allow any current to pass through it.

A capacitor stores energy as a function of voltage and resist changes in voltage by drawing current from or supplying current to the source of the voltage change, in opposition to the change.

If a constant voltage (DC) is applied across the capacitor it charges up to some value decided by the voltage and the capacitance. Q=CV

If the voltage is always changing over time (AC) the capacitor attempts to keep it constant. I=C.dV/dt. thus the current appears in the circuit.