We all have heard that a capacitor blocks DC and passes AC. But what is the reason behind this behavior of a capacitor? A capacitor blocks DC in a steady state only. When a capacitor gets charged fully and the voltage across it becomes equal and opposite to the DC input voltage, no more current can flow through it.
If you apply a direct current source to a capacitor, it will pass DC just fine. (The voltage will increase until the cap explodes, of course...) If you apply DC voltage to a capacitor it is not at all blocked at first. Eventually, the capacitor gets charged and puts out its ow n DC. At that point no current flows through it.
DC current does not pass through because there is no conducting path from one side of the capacitor to the other. AC current "passes through" in a different sense than conduction through the device. As much charge enters one side as leaves the other. But this cannot be maintained forever; eventually one side will run out of electrons.
Note: Only capacitor with no polarity passes AC. For example, Tantalum and electrolytic are polarized i.e., cannot be used in reverse polarity. Hence, they can’t pass AC. For a more detailed and mathematical answer keep reading. How does a capacitor work in DC? How does a capacitor work in DC?
A capacitor does not block DC immediately! Even when it is directly connected to a DC source, it takes some time for it to charge. And in order for it to charge current must flow through the circuit. So current flows through the circuit and when it gets charged to the source voltage level, no more current passes through it.
A capacitor blocks DC in a steady state only. When a capacitor gets charged fully and the voltage across it becomes equal and opposite to the DC input voltage, no more current can flow through it. This is when we say the capacitor is blocking DC. Whereas in the case of input AC supply, the voltage drops, becomes zero and reverses.