In an inductive circuit, which quantity leads?

In an inductive circuit, which quantity leads?

• A. Current leads voltage
• B. Voltage lags current
• C. Voltage leads current
• D. Voltage and current are in phase

Answer: (C)

In an inductive circuit, the inductor resists changes in current, so the voltage across it is tied to how quickly the current is changing (V = L di/dt). Because the current is what’s changing in response to the applied voltage, the voltage must reach its peak before the current does. In sinusoidal steady state, that means the current lags the voltage by 90 degrees for a pure inductor, so the voltage leads the current. In real circuits with some resistance, the lead is less than 90 degrees but the voltage still leads the current. That’s why the statement “voltage leads current” is the best description. The other ideas describe a capacitor (current leads voltage) or a resistive case (they’re in phase), which don’t apply to a purely inductive relationship.

In an inductive circuit, the inductor resists changes in current, so the voltage across it is tied to how quickly the current is changing (V = L di/dt). Because the current is what’s changing in response to the applied voltage, the voltage must reach its peak before the current does. In sinusoidal steady state, that means the current lags the voltage by 90 degrees for a pure inductor, so the voltage leads the current. In real circuits with some resistance, the lead is less than 90 degrees but the voltage still leads the current. That’s why the statement “voltage leads current” is the best description. The other ideas describe a capacitor (current leads voltage) or a resistive case (they’re in phase), which don’t apply to a purely inductive relationship.