AQA A-Level Physics Paper 1, November 2020: Question 27

1 mark · Medium difficulty · Multiple Choice

Determine which circuit modification restores a bridge circuit containing a thermistor to balance when the temperature is increased.

Practise this question

Question

A multiple-choice question featuring a bridge circuit diagram with four components labelled P, Q, R, and a thermistor T, connected to a cell and a voltmeter V across the middle junctions. The text asks which change will restore the voltmeter reading to zero when the temperature of T increases, with four options A to D provided.
Question text

27 In the circuit below, the voltmeter reading is zero.

When the temperature of the thermistor T is increased, the voltmeter reading changes.

Which change to the circuit will restore the voltmeter to zero?

[1 mark]

A a reduction in the emf of the cell

B a reduction in the resistance of P

C an increase in the resistance of Q

D a reduction in the resistance of R

Mark scheme

Show the mark scheme The mark scheme table showing the correct answer for question 27 is D.

27 D

How to answer it

Wheatstone Bridge Balancing and Thermistors

What this question tests

This question assesses your understanding of potential divider circuits arranged as a Wheatstone bridge, the behavior of Negative Temperature Coefficient (NTC) thermistors, and the condition required for a zero potential difference across a bridge circuit.

Question 2.7

Analysis and Full Solution

✅ Correct Answer

Option D: a reduction in the resistance of R

Marks available: 1 mark

💡 Key Knowledge

  • Balanced Bridge Condition: The voltmeter reads zero when the potential at the junction between P and R equals the potential at the junction between Q and T.
  • Mathematically, the bridge is balanced when: P / R = Q / T (or equivalent ratio formulations).
  • NTC Thermistor: As temperature increases, the resistance of thermistor T decreases ( R_T ↓ ).

🧠 Exam Technique

Trace the bridge arms systematically. Treat the parallel branches as two potential dividers connected across the same emf source. Write out the ratio condition before and after the disturbance to predict how to restore equilibrium.

❌ Common Errors

  • Assuming changing the emf of the cell affects the balance point (it shifts absolute potentials equally across symmetric branches, but doesn't fix a disrupted ratio).
  • Confusing NTC behavior (thinking resistance increases with temperature).

📐 Step-by-Step Breakdown

  1. Initial State: The circuit is initially balanced, meaning P / R = Q / T .
  2. Disturbance: Temperature of T increases, causing its resistance to decrease ( T ↓ ). This lowers the fraction of voltage dropped across the right-hand lower branch, breaking the ratio.
  3. Restoring Balance: To restore the balance condition P / R = Q / T when the denominator of the right side ( T ) has decreased, we must decrease the denominator of the left side ( R ) by the same proportional factor. Thus, reducing the resistance of R restores the balance.

Topics

Physics · 3.5 Electricity

Question and mark scheme from the AQA A-Level Physics examination, Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.