AS Daily A Level Physics question
A porch light controller uses a 6.0 V supply with a fixed 1.0 kΩ resistor in series with an LDR. The output voltage V_out is measured across the LDR to ground. In dim light the LDR is 2.0 kΩ and V_out = 4.0 V. In bright light the LDR’s resistance halves to 1.0 kΩ. Which statement about V_out is correct?
Answer
The correct answer is B.
Correct: B — It decreases from 4.0 V to 3.0 V. With V_out taken across the LDR, V_out = 6.0 × R_LDR/(1.0 kΩ + R_LDR): halving R_LDR from 2.0 kΩ to 1.0 kΩ changes V_out from 6 × 2/(1+2) = 4.0 V to 6 × 1/(1+1) = 3.0 V, so it decreases. A It cannot increase: a smaller share of the series resistance means a smaller share of the supply voltage, not a larger one. B Matches the divider calculation for R_LDR halving. C The supply is unchanged but the resistance ratio changes, so the divided output does not stay the same. D The output is across only part of the divider and cannot rise to the full 6.0 V in this arrangement.