Question

A capacitor acts as an infinite resistance for
A. DC
B. AC
C. DC as well as AC
D. neither AC nor DC

Answer 1

A capacitor is not closed circuit. There is an insulator in between the metallic conductor. When there is potential difference across the capacitor in a DC circuit, charges deposit on the capacitor and there is a current. When the capacitor is fully charged, there is no more current. In an AC circuit there is continuous charging and discharging of the capacitor.

Complete step-by-step answer :
Capacitor is a device that stores electric charges. Hence, it stores electrical energy. A capacitor consists of two metallic conductors separated by some distance. When a potential difference is created across these metallic conductors or when the capacitor is connected in a circuit, there is current and charges from the negative terminal of the capacitor get deposited on the positive terminal of the capacitor. And we say that the capacitor is getting charged.
The medium between the metallic conductors of the capacitor is air or any other insulator. When the capacitor connected to a DC circuit, is fully charged or the potential difference across it has become maximum (equal to the voltage of the source), it acts as an open circuit and no charges deposit on the positive terminal of the capacitor. Hence, there is zero current in the circuit.
Therefore, we can say that a fully charged capacitor acts as an infinite resistance in a DC circuit.
In an AC circuit, the deposition of charges on the capacitor is an oscillating (sinusoidal) function of time. Therefore, the potential difference across the capacitor is continuously changed. Due to this, the capacitor is continuously charged and discharged. As a result, it always acts as a closed circuit.
Therefore, A capacitor acts as an infinite resistance only for a DC circuit.
Hence, the correct option is A.

Note : Points to remember.
(i) In a DC circuit, a capacitor acts as a closed circuit and there is a current until the capacitor is not fully charged.
(ii) Once the capacitor is fully charged, it acts as an open circuit or an infinite resistance that allows no current to flow in the circuit.