Playbook
AC Circuits
129 q - 3.00/paper - 12% HARD. Reactance and power are the cheap half; one impedance triangle does most of the rest.
- Questions in the bank
- 129
- q/paper (2024–25 papers)
- 3.00
- Tagged HARD
- 12%
- Subtopics
- 6
Strand: Quick-Win
When you’ll see it
An alternating voltage with an R, L or C, a series LCR circuit and its impedance, resonance, the power factor, or a transformer.
How this chapter is tested
129 q at 3.00 a paper, 12% HARD. The cheap half is single elements: Reactance (23 q, 0% HARD) is X_L = ωL and X_C = 1/ωC, and Power in AC Circuit (25 q, 4%) is P = V_rms I_rms cos φ with cos φ = R/Z.
Series LCR — Impedance, Phase and Phasors (33 q) and Resonance (32 q) are each about 15% HARD, and one right-angled triangle does both: Z = √(R² + (X_L − X_C)²), tan φ = (X_L − X_C)/R. At resonance X_L = X_C, so Z = R, the current is largest, and ω₀ = 1/√(LC).
LC Oscillations, Transformer, and AC Generator is small (9 q) and 33% HARD. Transformer questions are the ratio V_s/V_p = N_s/N_p = I_p/I_s.
The sub-skills
The distinct skills inside the chapter, in the order to learn them.
RMS and peak
V_rms = V₀/√2 for a sine wave. Meters read RMS.
Reactance
X_L = ωL rises with frequency; X_C = 1/ωC falls with it. Current leads in a capacitor, lags in an inductor.
Impedance triangle
Z = √(R² + (X_L − X_C)²), cos φ = R/Z. Voltages across L and C are 180° apart.
Resonance and quality factor
ω₀ = 1/√(LC), Z = R at resonance, Q = ω₀L/R.
Power
P = V_rms I_rms cos φ. A pure L or C draws wattless current: cos φ = 0.
Traps to expect
Distractor shapes this chapter reuses. The Traps page covers the patterns that cut across chapters.
Adding voltages across R, L and C directly
In a series LCR circuit the voltages are out of phase: V = √(V_R² + (V_L − V_C)²). Adding them as numbers gives a voltage larger than the source.
Using peak values in the power formula
P = V_rms I_rms cos φ. With peak values it is ½V₀I₀ cos φ — missing the half doubles the power.
Learn it before you drill it
This chapter has full teaching notes — foundations, worked examples, self-checks and a per-subtopic mastery checkpoint. Read the notes once, then drill subtopic by subtopic below.
AC Circuits notesDrill every ac circuits question
129 questions from the bank, scoped to 6 bundled subtopics.
Drill one subtopic at a time
The 6 subtopics this playbook covers, in catalog order.
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