# Why capacitor conducts a.c. but not d.c.(except before charging time)? Answer in microscopic view(How electron flows) not mathematically.

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Sudhindra Sukumar

• Member Since Dec-2007 | Mar 1st, 2008

The capacitor is actually a pair of plates (electrodes) between which a dielectric medium is placed which helps conduction of charges.

Proof 1:
The capacitive reactance (otherwise known as resistance) is described by a formula Xc = 1/(2*pi*f), where pi = 3.14 and fc is the frequency. From basic electrical principles it's clear that for DC the frequency is zero. Therefore the capacitor offers infinite resistance to DC and allows ac to pass freely because the ac has a fixed or defined frequency.

Proof 2:
A microscopic view of it would be like this : the capacitor plates are separated by a little distance and hence when a DC supply is increased across it the plate connected across the +ve terminal of the battery and that connected across the -ve terminal of the battery is charged to the potential of the battery and hence it starts blocking the charge entering it form a DC source. Whereas from an ac source, the voltage changes continuously in magnitude and direction and hence the capacitor plate does not get a fixed charge to block any of the voltage or current that is alternating in nature. hence capacitor blocks only DC and acts as a normal conducting element for ac supply.

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#### Sudhindra Sukumar Profile Answers by Sudhindra Sukumar

• Mar 1st, 2008

The capacitor is actually a pair of plates (electrodes) between which a dielectric medium is placed which helps conduction of charges.

Proof 1:
The capacitive reactance (otherwise known as resistance) is described by a formula Xc = 1/(2*pi*f), where pi = 3.14 and fc is the frequency. From basic electrical principles it's clear that for DC the frequency is zero. Therefore the capacitor offers infinite resistance to DC and allows ac to pass freely because the ac has a fixed or defined frequency.

Proof 2:
A microscopic view of it would be like this : the capacitor plates are separated by a little distance and hence when a DC supply is increased across it the plate connected across the +ve terminal of the battery and that connected across the -ve terminal of the battery is charged to the potential of the battery and hence it starts blocking the charge entering it form a DC source. Whereas from an ac source, the voltage changes continuously in magnitude and direction and hence the capacitor plate does not get a fixed charge to block any of the voltage or current that is alternating in nature. hence capacitor blocks only DC and acts as a normal conducting element for ac supply.