Questions & Answers

In a purely resistive AC circuit
A. voltage leads current
B. voltage lags current
C. voltage and current are in same phase
D. nothing can be said

Answer Verified Verified
Hint: We know that an AC circuit has input voltage and current, in the sinusoidal form. Thus there are 3 possible relationships between voltage and current in the circuit. 1. Voltage lags current or current leads voltage, 2. Voltage leads current or current lags voltage and 3. Voltage and current are in the same phase.

Complete answer:
A purely resistive circuit or a non-inductive circuit is a circuit where inductance is very small or negligible as compared to the resistance. Here the voltage applied is used to overcome the ohmic resistance of the circuit.
We know that the source of an AC circuit is sinusoidal. Then there is a phase difference between the voltage and current. If the phase difference between the current and voltage is zero, then both are said to be in phase, and if the phase difference is not equal to zero, then both are said to be out of phase.

If the load of the circuit is purely resistive, then there is no lag between the voltage and the current in the circuit. This means that both the current and voltage reverse their polarity at the same time. Then at any given instant of time, the product of voltage and current i.e. the power is either positive or zero. In AC circuits, there are two types of power namely, the active power which is the average of the one complete wave form, and the other is the reactive power, which is due to the stored energy.

Thus clearly, in purely resistive AC circuits, current and voltage are in the same phase or have no lag.

So, the correct answer is “Option C”.

The phase difference between voltage and current is zero in a purely resistive AC circuit. i.e. that both the current and voltage reverse their polarity at the same time. Then, the active power of the circuit is always positive or zero and never negative.