Current lags voltageInductance, as you recall, stores electrical energy as a magnetic field. When a voltage isplaced across a coil, it takes awhile for the current to build up to full value.When ac is placed across a coil, the current lags the voltage in phase.Pure inductanceSuppose that you place an ac voltage across a low-loss coil, with a frequency highenough so that the inductive reactance, XL, is much larger than the resistance, R. In thissituation, the current is one-quarter of a cycle behind the voltage. That is, the currentlags the voltage by 90 degrees (Fig. 13-8).At very low frequencies, large inductances are normally needed in order for thiscurrent lag to be a full 1⁄4 cycle. This is because any coil has some resistance; no wire isa perfect conductor. If some wire were found that had a mathematically zero resistance,and if a coil of any size were wound from this wire, then the current would lag the volt-age by 90 degrees in this inductor, no matter what the ac frequency.When the value of XL is very large compared with the value of R in a circuit—thatis, when there is an essentially pure inductance—the vector in the RL plane pointsstraight up along the XL axis. Its angle is 90 degrees from the R axis, which is consid-ered the zero line in the RL plane.Current lags voltage23713-7Four vectors in the RL impedance plane.