AS Daily A Level Physics question
A night-light uses an LDR in a potential divider to produce a control voltage V_out for a microcontroller. A 6.0 V supply feeds a fixed 2.0 kΩ resistor in series with the LDR to ground. V_out is taken across the LDR (from the junction to ground). In bright light the LDR is 1.0 kΩ; in darkness it rises to 4.0 kΩ. What happens to V_out when the hallway goes dark?
Answer
The correct answer is C.
Correct: C — Increases from 2.0 V to 4.0 V. With V_out across the LDR, V_out = 6.0 × R_LDR / (2.0 kΩ + R_LDR); in bright light: 6.0 × 1.0/(2.0+1.0) = 2.0 V; in darkness: 6.0 × 4.0/(2.0+4.0) = 4.0 V. A confuses the direction (or which component V_out is across); increasing the LDR’s resistance gives it a larger share of the supply, not a smaller one. B assumes the fixed supply fixes V_out, ignoring that the divider ratio changes with resistance. C matches the calculated divider outputs for 1.0 kΩ and 4.0 kΩ. D underestimates the change, treating it as a small shift rather than using the correct ratio R_LDR/(R_fixed + R_LDR).