A2 Daily A Level Physics question
Two identical 12 V, 21 W automotive bulbs are first used singly on a 12 V battery. They are then connected in series and powered from the same 12 V battery. Assume the battery is ideal and each bulb behaves like a fixed resistor equal to its value at the 12 V, 21 W rating. What is the total electrical power delivered to the two-bulb series circuit?
Answer
The correct answer is B.
Correct: B — 10.5 W. Two identical resistive bulbs in series share the 12 V equally (≈6 V each), so power per bulb scales with voltage squared: (6/12)^2 × 21 W = 5.25 W; total = 2 × 5.25 W = 10.5 W. A 21 W — This assumes the total power stays the same as a single bulb, ignoring that the total resistance doubles and current falls. B — Correct because doubling the resistance at the same supply halves the total power (V^2/(2R) = 0.5 × V^2/R). C 42 W — This wrongly assumes power simply doubles with two bulbs, neglecting that in series the current drops so total power decreases. D 5.25 W — This is the power of one bulb in the series arrangement; the question asks for the total for both bulbs.