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AS Daily A Level Physics question

2025-08-13 OCR A A Level Physics 4.2.4(c) 4.3.3(c)(i)

A potential divider circuit consists of a fixed resistor R and an NTC thermistor T connected in series to a power supply with negligible internal resistance. A high-resistance voltmeter is connected in parallel with the fixed resistor R. The temperature of the thermistor is significantly increased. Which statement correctly describes and explains the change in the voltmeter reading?

  1. A A. The reading decreases, because the thermistor's resistance increases with temperature.
  2. B B. The reading decreases, because the ratio of the resistance of R to the total circuit resistance decreases.
  3. C C. The reading increases, because the total resistance of the circuit falls, leading to a larger current through R. (correct)
  4. D D. The reading stays the same, because the resistance of R and the e.m.f. of the supply are both constant.

Answer

The correct answer is C.

The key idea is to analyse how the change in the thermistor's resistance affects the current in the series circuit. For an NTC thermistor, increasing the temperature causes its resistance (R_T) to decrease. This reduces the total circuit resistance (R_total = R + R_T). According to Ohm's law for the whole circuit (I = V_supply / R_total), a decrease in total resistance causes the current (I) to increase. The voltmeter measures the potential difference across the fixed resistor R, given by V_R = I * R. Since I increases and R is constant, the voltmeter reading V_R increases. Therefore, C is correct. Option A is incorrect because an NTC thermistor's resistance decreases with temperature. Option B is incorrect because as R_T decreases, the ratio R / (R + R_T) actually increases. Option D incorrectly assumes the current in the circuit remains constant, ignoring the effect of the thermistor's change in resistance on the whole series circuit.

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