Capacitors behave differently with direct current (DC) compared to alternating current (AC) due to their inherent characteristics. In a DC circuit, when a capacitor is connected, initially, a surge of current flows through it as the capacitor charges or discharges to match the voltage of the DC source. However, once the capacitor is fully charged, no steady-state current flows through it. This is because capacitors block the steady flow of DC after charging up to the source voltage. In essence, while DC can momentarily flow through a capacitor during charging or discharging, once equilibrium is reached, no continuous current passes through it.

When a capacitor is connected to a direct current (DC) source, it charges up until it reaches the same voltage as the DC source. During this charging process, an initial surge of current flows through the capacitor as it accumulates charge on its plates. This charging current gradually decreases as the capacitor approaches full charge, following an exponential decay pattern governed by the RC time constant (where R is the resistance and C is the capacitance). Once fully charged, the capacitor blocks any further DC current flow because it acts like an open circuit to DC, maintaining the voltage across its terminals but not allowing a steady current to pass through.

Capacitors are designed to block direct current (DC) while allowing alternating current (AC) to pass through them. This behavior arises because capacitors store energy in an electric field between their plates. In an AC circuit, the capacitor alternately charges and discharges as the AC signal alternates direction, allowing current to flow back and forth through the capacitor. The impedance (resistance to current flow) of a capacitor in an AC circuit decreases as the frequency of the AC signal increases, allowing higher-frequency AC signals to pass through more easily. This property makes capacitors useful in AC circuits for applications such as coupling, filtering, and tuning, where they can block DC components while allowing AC signals to pass through based on their capacitance and the frequency of the AC signal.