In an a-c circuit which has an inductor instead of a resistor, there will be a phase difference between voltage
and current. Shown below is a generator connected to an inductor. The sine wave represents the current
flowing through the inductor.
On any sine wave, the greatest rate of change of current occurs at 0 and 180. (Note: 360 is the same as
0.) Since the most opposition to a-c current occurs as the current is changing at its greatest rate, the
greatest amount of opposition occurs at 0 and 180.
An opposition to a changing current in an inductor is in the form of an induced voltage; therefore, the
maximum induced voltage in an inductor occurs as the current is changing at its greatest rate, the greatest
amount of opposition occurs at 0 and 180. The drawing below illustrates the relationship between the
inductor voltage and the inductor current.
From the comparison above, it can be seen that the voltage leads the current by 90 in an inductor.
(Another way to say this would be that the voltage is at its maximum value at 90 before the current at its
Q. In an inductor, the voltage ________________ the current by 90.