Why capacitor block direct current?

Answer 1

A capacitor opposes a change in voltage, but it will help to look at both the device and at a circuit up close to see what's going on.

Any capacitor is two "plates" separated by a dielectric or insulator. Connect a wire to each plate and you've got the device. In a direct current circuit, the voltage source will cause current flow in only one direction. A common battery is a good example. Let's look further.

When a capacitor is connected in a DC circuit and the circuit is energized, the voltage source will want to cause current to flow in only the one direction. In the initial moment when the power is switched on, electrons will flow in the circuit. Electrons will leave the negative terminal of the source and enter the positive terminal. The current flow will travel through the wire, and electrons will "pile up" on one of the plates of the capacitor. As electrons are "piling up" on one plate, their presence there will create an electric field across the dielectric to the other plate. This electric field will cause electrons on that other plate to leave. The capacitor is charging, and the voltage source will, for the first instant of time, think that things are "fine" and current will flow. But as the capacitor charges, current flow drops off, and it eventually stops when the voltage across the plates equals the source voltage.

In review, as the DC power is switched on in a circuit with a capacitor in it, current will flow "normally" for the first instant. But as the first electrons arrive on one plate and force them off the other plate, current in the circuit will begin dropping off. The voltage developing across the plates of the capacitor opposes the battery voltage. Eventually the capacitor is charged and all current flow has stopped. There is some math that says something slightly different, but for all practical purposes, the capacitor is considered fully charged in a very short period of time. This will depend on circuit resistance and the ability of the source to deliver current, of course. But that capacitor will, when charged, not "pass" any more current. The voltage across the plates is equal to (an opposing) the source voltage, and no more electrons can get onto the negative plate to force more off the positive plate.

Answer 2

In a sense, yes. You can use a capacitor to transfer an AC signal from one side to the other, while the DC voltage on either side of the capacitor is different. So you can use the capacitor to "block" DC while "passing" AC.

In another sense, no. If you applied an ideal direct current source to an ideal capacitor, current would continue to flow through it indefinitely. The voltage would just rise to infinity.

It does not actually "block" current; it just charges up to a certain voltage and then stops. In reality, the capacitor will explode before this happens.

Answer 3

capacitors block dc because the capacitive reactant, Xc=1/(2*pi)fc

here 'f' is the frequency of the signal and 'c' is the value of the capacitor.

for dc supply frequency is zero,(dc is independent of frequency) .

so the reactant similar to resistance becomes infinity.

So it blocks the dc current

Answer 4

It changes current. It's job is to rebuild it in another sequence.