In series RC circuit when R=XC' then the phase angle is:

Explanation

When the resistance (R) in a series RC circuit is equal to the capacitive reactance (XC'), it results in a special condition where the phase angle between the current and voltage is 45 degrees (π/4 radians). In this scenario, the circuit behaves as a half-power factor circuit, and the current and voltage are out of phase by 45 degrees. This condition occurs when the time constant of the RC circuit is equal to the period of the alternating current signal, resulting in a phase angle of 45 degrees. It's a distinctive situation where the capacitive and resistive components are balanced in such a way that the phase angle is fixed at 45 degrees.