A2 Daily A Level Physics question
In a lab dimming circuit for an LED, a charged capacitor is allowed to discharge through a resistor when the power is switched off. Two versions are tested, both starting from the same initial voltage: Setup 1 uses a 200 kΩ resistor with a 10 μF capacitor; Setup 2 uses a 100 kΩ resistor with a 20 μF capacitor. Which statement must be true about the time taken for the voltage to fall to half its initial value?
Answer
The correct answer is A.
Correct: A — It is the same in both setups. A is correct because halving the resistance while doubling the capacitance leaves the product R×C unchanged, so any fixed fraction of the initial voltage (like half) is reached in the same time. B is wrong because it considers only the smaller resistance making discharge faster and ignores the simultaneously larger capacitance that slows it by the same factor. C is wrong because it considers only the larger capacitance making discharge slower and ignores the smaller resistance that speeds it by the same factor. D is wrong because the relative timing follows directly from the R×C product; a full plot is unnecessary here.