Current leads voltageCapacitance stores energy in the form of an electric field. When a current is driventhrough a capacitor, it takes a little time before the plates can fully charge to the full po-tential difference of the source voltage.When an ac voltage source is placed across a capacitor, the voltage in the capacitorlags the current in phase. Another way of saying this is that the current leads the volt-age. The phase difference can range from zero, to a small part of a cycle, to a quarter ofa cycle (90 degrees).Pure capacitanceImagine placing an ac voltage source across a capacitor. Suppose that the frequency ishigh enough, and/or the capacitance large enough, so that the capacitive reactance, XC,is extremely small compared with the resistance, R. Then the current leads the voltageby a full 90 degrees (Fig. 14-8).At very high frequencies, it doesn’t take very much capacitance for this to happen.Small capacitors usually have less leakage resistance than large ones. At lower frequen-cies, the capacitance must be larger, although high-quality, low-loss capacitors are not toodifficult to manufacture except at audio frequencies and at the 60-Hz utility frequency.The situation depicted in Fig. 14-8 represents an essentially pure capacitive reac-tance. The vector in the RC plane points just about straight down. Its angle is 90 de-grees from the R axis or zero line.254 Capacitive reactance14-7Four vectors in the RC impedance plane.