Why capacitor allows ac and not dc?

Answer 1

CAPACITOR BLOCKS DC SUPPLY .THERE ARE MAINLY TWO ANSWERS

1.CAPACITIVE REACTANCE Xc=1/( 2*3.1416*F*C) , HERE THE FREQUENCY OF DC SUPPLY IS ZERO .HENCE THE REACTANCE VALUE IS INFINITY .SO IT BLOCKS THE DC SUPPLY.

2.CURRENT THROUGH A CAPACITOR IS PROPORTIONAL TO THE RATE OF CHANGE OF CURRENT . BUT IN THE CASE OF DC SUPPLY, THE RATE OF CHANGE OF CURRENT IS ZERO. HENCE THE CAPACITOR CURRENT IS ALSO ZERO.

H.L.kiran

Answer 2

A capacitor blocks DC, while allowing AC to pass through it, because of its nature in resisting a change in voltage. The equation of a capacitor is ...

dv/dt = i/c

... which means that the rate of change of the voltage is proportional to current and inversely proportional to capacitance.

If you place DC across a capacitor, it will initially allow current to flow. As it charges, if the current is coming through a resistor, the current will exponentially decrease. (If the current source is a constant current source, the capacitor will charge linearly, until the underlying voltage source is exhausted.) At some point, the voltage on the capacitor will match the voltage of the source, and current will stop. No current means infinite resistance, by Ohm's Law. Thus, the DC is blocked.

If, however, the input voltage is changing, either because its AC, or because it represents some kind of signal, then the capacitor will come into equilibrium with an average voltage equal to the DC component of the signal. If you place the capacitor in series between two nodes, the output node will have only the varying signal present, with the DC bias removed.

In this configuration, the capacitor is functioning as a high pass filter. This is often done in AC coupled amplifiers so as to remove the DC component of a signal so as to not affect the DC bias of the following stage.

You can make a low pass filter just as easily. Just place the capacitor in parallel with the signal, and DC will go around the capacitor, while AC/Signal will be shunted through the capacitor.

Often, there are other components, such as resistors and inductors, in a low pass or high pass filter. They simply go into the design of the filter and what we call its roll-off frequency.

Answer 3

A: Consider a capacitor a container when it if full it has reached status quo With AC the container is filled then emptied then fill again and so on.