why does a capacitor block dc nd allows ac????
DC current can flow only through a continuous path, like a wire or a conducting liquid. So, DC is blocked by the gap between the capacitor plates. However, if the DC voltage across the plates of the capacitor is high enough to cause dielectric breakdown of the medium in between the plates then there will be a flow of current through the capacitor.
the capacitive reactance, Xc=1/wc=1/2(pie)fC
For d.c. f=0 therefore, Xc= infinity
i.e. capacitor offers infinite resistance to dc & hence blocks it.
For a.c. f is not = 0 therefore Xc is not = infinity, but has only finite value.
therefore a.c. can pass through the capacitor.