A2 Daily A Level Physics question
In a lab temperature probe, a 12 V supply is applied across a series pair: a fixed 4.0 kΩ resistor on top and an NTC thermistor on the bottom. The output V_out is measured across the thermistor. The thermistor is 4.0 kΩ at 20 °C and 2.0 kΩ at 40 °C. As the room warms from 20 °C to 40 °C, what happens to V_out?
Answer
The correct answer is D.
Correct: D — Decreases to 4.0 V. With V_out taken across the thermistor, V_out = 12 × R_therm / (4.0 + R_therm in kΩ). At 20 °C: 12 × 4.0/(4.0+4.0) = 6.0 V; at 40 °C: 12 × 2.0/(4.0+2.0) = 4.0 V, so it decreases by one-third. A Increases to 8.0 V — This is the voltage across the fixed 4.0 kΩ (the top resistor), not across the thermistor. B Remains at 6.0 V — Only true when the two resistances are equal; at 40 °C they are not, so V_out changes. C Decreases to 3.0 V — Halving the thermistor’s resistance does not halve V_out because the total series resistance in the ratio also changes.