Series RL circuit — phase behavior with changing inductance: Consider a series RL circuit driven by a sinusoidal source. If the inductance L is increased (R fixed and frequency fixed), the phase angle between source voltage and current will ______.

Introduction / Context:
In AC circuits, inductors cause current to lag voltage. The phase angle between source voltage and current depends on the ratio of reactance to resistance. Understanding how this angle changes with L helps in power factor correction and filter design.

Given Data / Assumptions:

• Series RL circuit at fixed frequency f.
• Resistance R is constant.
• Inductance L increases.

Concept / Approach:
The impedance is Z = R + jX_L with X_L = 2πfL. The phase angle φ (current lag) satisfies tan φ = X_L / R. As L increases, X_L increases linearly, so tan φ increases and thus φ increases (i.e., the current lags by a larger angle). Therefore, the angle does not decrease; it grows toward 90 degrees as the inductive reactance dominates.

Step-by-Step Solution:

Verification / Alternative check:
Phasor diagrams show the reactive voltage drop vector lengthening with larger X_L, rotating the current vector further behind the voltage vector.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing series RL (current lags) with RC (current leads); forgetting that φ depends on X_L/R, not on absolute voltage levels.

Final Answer:
increase