A2 Daily A Level Physics question
A lab bench DC supply can be set either to hold a fixed output voltage or to hold a fixed output current (ideal behaviour, within limits). A purely resistive heater is connected as the load. The heater is then replaced by one with exactly double the resistance, at the same temperature. Which statement must be true about the heater’s electrical power in the two modes?
Answer
The correct answer is A.
Correct: A — In fixed-voltage mode the power halves; in fixed-current mode the power doubles. A — With V held constant, doubling R halves V^2/R, while with I held constant, doubling R doubles I^2R; in limiting cases, as R becomes very large, power → 0 in voltage mode but grows without bound in current mode for an ideal source. B — This reverses the real trends: at fixed voltage, increasing resistance cannot increase power because the current falls. C — This wrongly assumes the current halves in both cases; in current mode the supply holds I constant, so power does not halve. D — At fixed voltage, power depends on both V and I, and since I falls when R doubles, power decreases rather than staying the same; only the second clause matches current-mode behaviour.