# Why does a capacitor block DC current, but allow AC current to pass through?

william1941 | Student

The way capacitors and resistors behave is totally different. While resistors allow a current to flow through them which is proportional to the voltage drop across the resistor, capacitors oppose a change in voltage across them by either drawing in or supplying current as they charge or discharge resp. The flow of current through a capacitor is thus directly proportional to the rate of change of voltage across it.

This is given by the relation, i = C* (de/dt) where de/dt is the instantaneous change in voltage.

As the voltage does not change in the case of DC, de/dt = 0 and the current that is allowed to pass through by the capacitor is 0. For AC voltage the voltage changes in a regular manner. Hence here de/dt is not 0 and a current is allowed to flow through by the capacitor.

crosswind5 | Student

Dont be so panic, u dnt have to learn huge theory. U cn just learn it with the formula ------- xc=1/(2 pi f c). Where xc=capacitive reactance, f=frequency, c=capacitance and pi=3.14.
So in case of an dc frequency,i.e, f is zero. So reactance i.e., impedance is infinite. ( According to formula,put 0 in place of f, u get xc=~infinite). Thats why capacitor gives infinite impedance or resistance to dc signal... Got it?

astrosonuthird | Student

The capacitors block DC current because there is an insulating layer between one part and the other part of the circuit.

We know that direct current cannot pass though a open circuit.

Am I right!!!????