AS Daily A Level Physics question
A technician has a 9.0 V lab power supply (assume negligible internal resistance) and two identical 6.0 Ω heating elements. In setup A the elements are wired in series; in setup B they are wired in parallel. Compared with setup A, what is the power dissipated in each element in setup B?
Answer
The correct answer is D.
Correct: D — Four times larger, because each element has the full 9.0 V rather than about 4.5 V. A … This focuses on current splitting but ignores that in series each element only had about half the supply voltage; power for a fixed resistance depends on the square of the voltage across it, so the change is larger than a factor of two. B … This overlooks that while the supply is 9.0 V, in series each element does not see 9.0 V (it sees ~4.5 V), so power per element is lower than in parallel. C … This underestimates the increase: going from ~4.5 V to 9.0 V doubles the voltage, which makes power four times larger, not twice. D … This correctly uses the fact that each element’s voltage doubles from ~4.5 V (series) to 9.0 V (parallel), so power per element increases by a factor of 2² = 4.