Q. A capacitor blocks d.c. but allows a.c. Why?
Capacitive reactance XC = 1/ωC
= 1 / 2πνC
Where ν is the frequency of the a.c. supply.
In a d.c. circuit,ν = 0 ∴ XC = ∞
Thus a capacitor tenders infinite resistance to d.c. and blocks the d.c.
For an a.c., the capacitive reactance differs inversely as the frequency of a.c. and as well inversely as the capacitance of the capacitor.