# why does a capacitor block dc nd allows ac????

For dc signals, tthe impedance of a capacitor (C) is very high (almost infinite). Hence, a dc signal is blocked. For an ac signal of high frequency, the impedance of Capacitor is very low. Henece ac signal is allowed to pass.

• 4

dc isnot able to cross the barrier of two plates of capacitor easily but ac due to alternatively changing frequency can overcome this barrier.

• 1

a capacitor offers infinite resistance to  flow of dc current and hence blocks it.

• 9

because in ac value of lw(omega) is small than Cw(omega) is zero so 1/Cw is infinity so resistance of ac is smaller than that of dc, that is why capacitor blocks dc and allows ac.

• 3

DC current can flow only through a continuous path, like a wire or a conducting liquid. So, DC is blocked by the gap between the capacitor plates. However, if the DC voltage across the plates of the capacitor is high enough to cause dielectric breakdown of the medium in between the plates then there will be a flow of current through the capacitor.

• 2

the capacitive reactance, Xc=1/wc=1/2(pie)fC

For d.c. f=0  therefore, Xc= infinity

i.e. capacitor offers infinite resistance to dc & hence blocks it.

For a.c. f is not = 0 therefore Xc is not = infinity, but has only finite value.

therefore a.c. can pass through the capacitor.

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