Capacitors and RC Charging

Capacitors in series and parallel, and RC charging time. A capacitor's impedance decreases in response to voltage changes; in other words, a capacitor draws or supplies current to oppose voltage changes. (Therefore, the change in current through a capacitor precedes the change in voltage across the capacitor; this phase difference is 90°.) Initially, when a capacitor is fully discharged, it behaves like a short circuit (no impedance): when a constant voltage is suddenly applied, the capacitor allows current to flow freely through it, and no voltage is dropped across it – all the electrical energy goes toward charging the capacitor. Over time the capacitor's charge builds up, and its impedance increases: the current through it decreases, and the voltage across it increases. Eventually, when the capacitor is fully charged to the applied voltage, it behaves like an open circuit (infinite impedance): no current flows through it, and the entire applied voltage is dropped across it. With the capacitor fully charged, when the applied voltage is suddenly removed, the opposite occurs: the electrical energy stored in the capacitor applies the full voltage to the circuit, and supplies current in the opposite direction (assuming there is a path for current to flow). Over time, as the capacitor discharges, the voltage and current supplied by the capacitor decrease until it is fully discharged. The greater the capacitance, the greater its capacity to store electrical energy: the longer it takes to charge, and the more current it can supply for longer. (In this way, a larger capacitor, when charged, could deliver a burst of current that exceeds that of the voltage source; however, the voltage supplied by the capacitor can never exceed the voltage to which it was charged.) As described above, for DC (direct current), as the applied voltage remains constant, once the capacitor is fully charged, its impedance is effectively infinite, and it no longer allows current to flow through it. In contrast, for AC (alternating current), if the frequency and capacitance are great enough, the capacitor is constantly charging and discharging – there isn't enough time for the capacitor to fully charge before the applied voltage reverses itself. Therefore, the capacitor's impedance remains low, and it allows the AC current to flow through it. The greater the frequency and/or capacitance, the less the capacitor can charge during that time, the lower its impedance remains, and the more current it allows to flow through it. Thus, the lower the frequency and/or capacitance, the more the capacitor approaches an open circuit, whereas the greater the frequency and/or capacitance, the more the capacitor approaches a short circuit.