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For an AC circuit the applied voltage is V = 100 sin(100 t) volt and the resulting current is I = 100 sin(100 t + pi/3) mA. What is the phase difference between the voltage and the current, and is the circuit predominantly inductive or capacitive?

1. pi/3, current leads voltage -> capacitive
2. pi/3, current lags voltage -> inductive
3. pi/6, current leads voltage -> capacitive
4. pi/2, current in phase -> purely resistive

Correct answer: pi/3, current leads voltage -> capacitive

Solution

V = 100 sin(100 t) has phase 0; I = 100 sin(100 t + pi/3) has phase +pi/3. The current's phase angle exceeds the voltage's by pi/3, so the current leads the voltage by pi/3 radians (60 deg). A leading current means the circuit is capacitive (or RC). The power factor is cos(pi/3) = 0.5.

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