In an R-L series circuit connected to an AC source, the phase angle theta by which current lags voltage is given by:

Explanation

In an RL series circuit, inductive reactance XL = ωL opposes AC current and causes current to lag voltage by phase angle θ. Since tan(θ) = XL / R = (ωL) / R, the phase angle is θ = arctan(ωL / R).