Phase

Author: J.B. Hoag

If an a.c. generator is connected to a pure resistance, the voltage and current through the resistance will be "in phase" with each other. This means that they both reach their positive maximum values at the same instant. Similarly they have their zero value and their negative maximum value at the same instant. See a in Fig. 4 E. If an a.c. generator is connected to a pure inductance,¹ the electrical "inertia" of the inductance causes the current to lag behind the impressed e.m.f. by 90°, or one-quarter of a period. This is shown in b of Fig. 4 E.

Fig. 4 E. The current i is "in-phase" with the impressed voltage e in a resistance; it lags 90° in an inductance and it leads 90° in a capacitance

In case a condenser is connected to an a.c. generator, the voltage across the condenser terminals is zero when there is no charge in it. Hence, then, even a small e.m.f. can send electricity at a great rate into the condenser. The current decreases as the voltage of the condenser builds up and opposes that of the generator. Hence the current in a capacitative circuit leads the impressed voltage by 90° or one-quarter of a cycle, as shown in c of Fig. 4 E.

In a series circuit of inductance and capacitance, the current will lag behind the impressed e.m.f. somewhere between 0° and 90° if the inductive effect is greater, and will lead somewhere between 0° and 90° if the capacitative effect is predominant.

1	A coil is not a "pure" inductance since there is always some resistance in its wires. In addition, there is capacitance between the turns of the coil. A coil is to be treated as a resistance and an inductance in series with each other, shunted by the "distributed" capacitance.