In a direct current (DC) circuit, or in an alternating current (AC) circuit whose impedance is a pure resistance, the voltage and current are in phase, and the following formula holds: P = E_rmsI_rms where P is the power in watts, E_rms is the root-mean-square (rms) voltage in volts, and I_rms is the rms current in amperes. But in an AC circuit whose impedance consists of reactance as well as resistance, the voltage and current are not in phase. This complicates the determination of power. In the absence of reactance, the product E_rmsI_rms represents true power because it is manifested in tangible form (radiation, dissipation, and/or mechanical motion). But when there is reactance in an AC circuit, the product E_rmsI_rms is greater than the true power. The excess is called reactive power, and represents energy alternately stored and released by inductors and/or capacitors. The vector sum of the true and reactive power is known as apparent power