In an AC circuit, under what condition are current and voltage in phase?

In an alternating current (AC) circuit, current and voltage are in phase when the circuit is purely resistive, meaning that the total reactive impedance is zero. This occurs when the inductive reactance (XL) and capacitive reactance (XC) balance each other out perfectly, resulting in their algebraic sum being zero.

When the condition XL + XC = 0 is met, it implies that the reactances are equal in magnitude but opposite in phase. Therefore, the inductive reactance, which causes the current to lag behind the voltage, is canceled out by the capacitive reactance, which causes the current to lead the voltage. This balance ensures that the voltage and current waveforms reach their maximum and minimum points simultaneously, indicating they are in phase with one another.

In contrast, the other options suggest conditions in which either the resistive elements or reactance do not equate properly, thereby resulting in phase shifts between the current and voltage. This would involve either the resistive impedance not accounting for the entire reactance or excess reactance remaining unbalanced, leading to a phase relationship where current either lags or leads the voltage. Thus, only the condition where XL + XC = 0 allows for perfect alignment between the voltage