AS Daily A Level Physics question
A student makes a mid-supply reference using two identical 3.0 kΩ resistors in series across a 9.0 V battery; V_out is taken across the lower resistor relative to ground. They then connect a 3.0 kΩ load in parallel with the lower resistor. Which statement must be true about the new V_out?
Answer
The correct answer is B.
Correct: B — It decreases to 3.0 V because the lower branch is 1.5 kΩ in parallel. With R_top = 3.0 kΩ and the loaded lower branch R_bottom' = (3.0 kΩ || 3.0 kΩ) = 1.5 kΩ, V_out = 9.0 × 1.5 / (3.0 + 1.5) = 3.0 V. A The lower branch is no longer equal to the top; adding a 3.0 kΩ in parallel halves the lower resistance, so V_out must drop below 4.5 V. B This uses the correct parallel reduction and divider ratio to give 3.0 V. C More current through the network does not increase V_out here; the smaller lower resistance takes a smaller share of the supply voltage, so V_out falls, not rises to 6.0 V. D Adding a parallel path decreases the total series resistance, not increases it, and the correct calculation gives 3.0 V rather than about 4.0 V.