A2 Daily A Level Physics question
A metal wire used as a small heater behaves as an ohmic conductor and its resistance can be taken as constant. It is connected to a variable DC supply. If the supply voltage is increased by 50%, what happens to the power dissipated in the wire?
Answer
The correct answer is D.
Correct: D — It increases by 125% (becomes 2.25 times the original). A 50% rise in voltage is a factor of 1.5; with constant resistance, power scales with the square of voltage, so the factor is 1.5^2 = 2.25, i.e. a 125% increase. A assumes power is proportional to voltage, but with fixed resistance power depends on the square of voltage, not linearly. B treats a 50% voltage increase as causing only a doubling of power; squaring 1.5 gives 2.25, not 2.0. C overestimates the effect, implying power would triple; there is no basis for a factor of 3 from a 1.5× voltage change with constant resistance. D is correct because with fixed resistance current scales with voltage, so power (voltage × current) scales as voltage squared, giving 2.25×.