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

Asked on by alexb2

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william1941's profile pic

william1941 | College Teacher | (Level 3) Valedictorian

Posted on

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's profile pic

crosswind5 | Student | eNotes Newbie

Posted on

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's profile pic

astrosonuthird | Student, Kindergarten | (Level 1) Valedictorian

Posted on

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!!!????

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