Does an inductor cause current to lead or lag voltage in an AC Circuit?

• A. It lags behind the voltage
• B. It leads the voltage
• C. It is in phase with the voltage
• D. It depends on frequency

Answer: (A)

In an inductor, the voltage is proportional to the rate of change of current (V = L di/dt). For a sinusoidal current i = I sin(ωt), the derivative is di/dt = ωI cos(ωt), which can be written as ωI sin(ωt + 90°). So the voltage across the inductor leads the current by 90 degrees. That means the current reaches its peak a quarter cycle after the voltage does, i.e., the current lags the voltage by 90 degrees.

This is why the correct choice is that the current lags behind the voltage. In real circuits with resistance, the lag is still present but may be less than 90°, depending on ωL/R; the essential behavior is that current does not lead the voltage in an inductive element.