AS Daily A Level Physics question
In a lab, a 9.0 V d.c. supply runs a small motor. Initially the current is 0.20 A. After a light mechanical load is added, the current rises by 50% while the supply voltage remains at 9.0 V. By what percentage does the electrical power delivered by the supply change?
Answer
The correct answer is D.
Correct: D — increases by 50%. With the supply voltage fixed, the power delivered is proportional to the current, so a 50% rise in current gives a 50% rise in power. A no change — wrongly assumes power depends only on voltage; at fixed voltage, changing current changes power. B increases by 25% — a common miscalculation; the current rises by a factor of 1.5, so power also scales by 1.5, not 1.25. C increases by 125% — misapplies a square-law (as if resistance were fixed so P ∝ I^2), but here voltage is fixed, not resistance.