JEE Main · 2021 · 17 Mar 2021 · Shift 2 · Chapter_pending

In a purely resistive circuit the current follows the applied voltage instant by instant, so the phase difference is zero.
In a pure inductor the back emf opposes any change of current, so the current builds up behind the voltage and lags it by $\frac{\pi }{2}$.
In a pure capacitor the current is largest when the plates are uncharged and the voltage is zero, so the current leads the voltage by $\frac{\pi }{2}$.
In a series LCR circuit the net reactance sets the phase, which is ${\tan }^{-1}\left(\frac{X_C-X_L}{R}\right)$ measured as the lead of the current over the voltage.
The resistive circuit pairs with zero, the inductive circuit with $\frac{\pi }{2}$ current lagging voltage, the capacitive circuit with $\frac{\pi }{2}$ current leading voltage, and the series LCR circuit with ${\tan }^{-1}\left(\frac{X_C-X_L}{R}\right)$.