AS Daily A Level Physics question
A bench power supply has an emf of 12.0 V and an internal resistance of 0.25 Ω. A student increases the current drawn from 2.0 A to 4.0 A by changing the external load. What happens to the terminal potential difference of the supply?
Answer
The correct answer is A.
Correct: A — It decreases by 0.50 V. Terminal pd equals emf minus the internal drop; with r = 0.25 Ω the values change from 12.0 − (2.0×0.25) = 11.5 V to 12.0 − (4.0×0.25) = 11.0 V, so it falls by 0.50 V. A uses V_terminal = emf − I r and compares the two currents correctly. B overestimates the change; the increase in internal drop is Δ(Ir) = 0.25×(4.0−2.0) = 0.50 V, not 1.0 V. C has the wrong sign: higher current increases the internal drop, so the terminal pd must decrease. D assumes an ideal source with negligible internal resistance, which contradicts the given r = 0.25 Ω.