Intelligent assessment and revision tools for OCR A Physics

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?

• 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 The measured V_out is too high in very low light and close to correct in bright light, because the meter adds extra resistance in series with the divider.
• C The measured V_out is reduced by the same amount at all light levels, because the meter draws a constant current independent of the circuit.
• D The measured V_out is unaffected at all light levels, because voltmeters do not draw current.