Overview

Alternating Currents is one of the densest numerical chapters in Grade XII Physics — the series LCR circuit dominates almost every board paper, showing up as a multi-part question that asks you to calculate impedance, current, phase angle, and often resonant frequency all in one go. The theory side mirrors this closely: expect a near-guaranteed question asking you to derive the impedance of an LCR series circuit and state the condition for resonance, alongside smaller conceptual questions on power factor, wattless current, and why 220V AC is more dangerous than 220V DC.

The numericals below follow a small number of repeating templates once you strip away the changing values: given R, L, C, and frequency, find impedance and current; given voltage readings across individual components, find the phase angle or power factor; or given a resonance condition, find the missing inductance, capacitance, or frequency. The peak-vs-rms relationship (Vrms = Vpeak/√2) also appears constantly, both as a standalone conceptual question and buried inside larger numericals.

A good chunk of marks also comes from qualitative reasoning questions — why a choke coil is preferred over a resistor to limit AC current (it doesn’t dissipate energy the way a resistor does), and how capacitive/inductive reactance change with frequency. These are quick marks if you understand the underlying reactance formulas rather than memorizing the answer as a fact.

Below is the full set of MCQs, theory questions, and numerical problems from past NEB board exams, organized by course and question type.

⚠️ Common Mistakes Students Make

• Mixing up peak value and rms value. Nepal’s 220V supply is the rms value; the peak value is Vrms × √2 ≈ 311V. Students frequently plug the rms figure directly into peak-voltage questions or vice versa.
• Getting the phase angle sign/direction wrong. When XL > XC, voltage leads current (inductive circuit); when XC > XL, current leads voltage (capacitive circuit). Forgetting which reactance dominates leads to a correct magnitude but wrong “leading/lagging” conclusion — which usually costs the justification mark.
• Confusing resonance condition with impedance minimization. At resonance, XL = XC, which makes impedance purely resistive (Z = R) and current maximum — not zero. Some students mistakenly think resonance means zero current or zero impedance.
• Forgetting that power dissipated depends on the power factor. Average power = VrmsIrmscos(φ), not just VrmsIrms. Skipping the cos(φ) term is a common error in “average power dissipated” numericals.