AS Daily A Level Physics question
A 12 V bench power supply powers a single resistor R; the supply reads current I and the resistor dissipates power P. The setup is changed so that two identical resistors, each of resistance R, are connected in parallel across the same 12 V. Assume the supply maintains 12 V and the leads are negligible. Which statement must be true about the total power drawn from the supply compared with P?
Answer
The correct answer is B.
Correct: B — It doubles, because the total resistance halves so the supply current doubles while the voltage stays the same. A incorrectly assumes parallel branches must each take half the original current; in fact, the total resistance falls, not rises, so total power does not halve. B correctly recognises that at fixed voltage, halving the total resistance doubles the current and thus doubles the power drawn. C ignores that adding a second identical branch doubles the total current at the same voltage, so the power cannot stay the same. D double-counts the effect: the current in each branch equals the original I (not twice I), so the total current is 2I, giving double the power, not four times.