Circuit analysis (every paper)

Series: same I, voltages add. Parallel: same V, currents add. Exam-style 12 V battery, 4 Ω and 8 Ω in series Current, and V across each resistor. R_total = 12 Ω → I = 12/12 = 1.0 A (same everywhere) V₄ = 4.0 V; V₈ = 8.0 V (sum = 12 ✓) P in the 8 Ω: P = I²R = 8.0 W ✓ 1.0 A; 4 V + 8 V; 8 W. ℹ

Parallel shortcut

Two in parallel: R₁R₂/(R₁+R₂). Ammeter in series, voltmeter in parallel (ideal 0 Ω / ∞ Ω). ✕

A parallel branch failing does not touch the OTHER branch's power

When a lamp or element sits on its own PARALLEL branch of a fixed-voltage supply, a different branch failing changes the TOTAL current and resistance, but that branch still gets the same voltage as before — so its own current and power are unchanged. This exact trap has appeared repeatedly: a lamp failing does not dim or brighten an element wired in parallel with it, because V = IR for that branch has not changed. Only a component in SERIES with the failure is affected.

Practice for this standard covers the whole topic — flashcards and quiz.