In a series RL circuit, the phase difference $\phi$ between the applied voltage and the current is determined by the tangent of the ratio of the inductive reactance ($X_L$) to the resistance ($R$). The relationship is given by $\tan \phi = \frac{X_L}{R}$ .

Given: $R = 3 \, \Omega$ $X_L = 3 \, \Omega$

Substituting these values: $\tan \phi = \frac{3}{3} = 1$

Therefore: $\phi = \tan^{-1}(1) = 45^\circ = \frac{\pi}{4}$ rad.