Why does a capacitor block DC but pass AC?
The DC voltage value of DC remains constant all the time. However, AC has a variable voltage with respect to time. So, that for some amount of time it will have a certain polarity but in the next amount of time, it will have another polarity.
The DC voltage is constant at all times. But in AC, this is a time-varying voltage. This means that there will be a certain polarity for some time (T1), but it will be another polarity after some time (T2).
When a capacitor is connected to a voltage source, the positive charge from the positive terminal and the negative charge from the negative terminal travel and get on the plates of the capacitor. Once the capacitor is fully charged, it will work as an insulator and will not permit current to pass through it.
D.C Supply
The capacitor gets charged to its total limit when the voltage source charges it. Once fully charged, positive charge is adjusted on a plate and negative charge are adjusted on the second plate. Hence the capacitor gets charged to its total limit and thus does not allow any current to flow. So it blocked DC.
A.C Supply
The polarity time of AC source varies after T1 and time changes again after T2. For this reason, for T1, the capacitor is charged in such a way that the positive charge is put up on the left plate. After time T1, the polarity changes and now at the start of T2, the voltage source V will draw a negative and positive charge from the respective plates to its terminals and the current will flow in the other direction. And because the capacitor is losing its charges - the capacitors are being discharged.
After this, once the polarity will switch (after T2), the capacitor will be charged again. In this way, the capacitor gets charged and repeatedly discharges and the AC current flows.
So ac can flow through a capacitor because of the alternate charging and discharging of the capacitor. Dc will be blocked because the charging of the capacitor will take place, but discharging won’t take place due to its constant nature.
Another Way-
The capacitive reactance is
Xc = 1 / (2*Pi*f*C)
In DC frequency f = 0.
So, Xc = ∞ and DC is blocked.
In AC, f is not Zero and hence AC current can flow.
