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

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alexb2 | eNotes Employee

Posted October 26, 2010 at 2:40 AM via web

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

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william1941 | College Teacher | (Level 3) Valedictorian

Posted October 26, 2010 at 3:11 AM (Answer #1)

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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.

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astrosonuthird | Student | (Level 1) Valedictorian

Posted October 22, 2012 at 7:39 AM (Answer #2)

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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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crosswind5 | Student | eNotes Newbie

Posted May 14, 2013 at 5:11 AM (Answer #3)

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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?

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