AS Daily A Level Physics question
In a lab investigation of a light sensor, a student makes a potential divider with a 10 kΩ fixed resistor in series with an LDR across a 6.0 V supply. The output V_out is measured across the LDR using a digital voltmeter with 100 kΩ input resistance. The light level is varied from very low light (LDR very large) to bright light (LDR a few kΩ). Which statement must be true about how the voltmeter affects the measured V_out compared with the value that would exist at the LDR terminals with an ideal (infinite-resistance) meter?
Answer
The correct answer is A.
Correct: A — The measured V_out is too low in very low light and close to correct in bright light, because the meter in parallel with a large LDR significantly reduces the effective resistance. B is wrong because the voltmeter is connected in parallel, not in series, so it does not add series resistance to raise V_out. C is wrong because the loading depends on the LDR’s resistance (the parallel combination changes with light), so the error is not the same at all levels. D is wrong because real voltmeters have high but finite input resistance, so they draw some current and can load the divider when the LDR resistance is large.