AS Daily A Level Physics question
A travel mug warmer contains two identical resistive heating coils. A switch connects the coils either in series or in parallel across the same fixed-voltage 12 V socket. Ignoring heat losses and assuming the coils are ohmic, which statement must be true about the total rate of heat production in the mug?
Answer
The correct answer is D.
Correct: D — Parallel gives each coil the full supply voltage instead of half, so the total heating rate is about four times that in series. A Each coil in series gets about half the supply voltage, not the same as in parallel, so the total power is not the same. B In series the total resistance is larger, so the current is smaller than in parallel, not larger. C Comparing series (2R) to parallel (R/2) gives a fourfold change in total resistance, so power changes by a factor of four, not two. D With fixed supply voltage, each coil at full voltage in parallel gives twice the power of one coil, whereas halving the voltage in series quarters each coil’s power; two such coils give one-half of one-coil-at-full-voltage, hence a fourfold difference overall.