AQA AS Level Physics Paper 2, June 2023: Question 18

1 mark · Medium difficulty · Multiple Choice

Determine the changes to the voltmeter and ammeter readings when the temperature of a thermistor in a parallel circuit with negligible internal resistance is increased.

Practise this question

Question

A multiple-choice question showing a circuit diagram with a cell of negligible internal resistance connected in parallel to two branches: one containing an ammeter and a resistor, and the other containing a thermistor in series with a voltmeter across it. Below the diagram is a table of four options (A to D) indicating possible changes to the voltmeter and ammeter readings when the thermistor's temperature is increased.
Question text

18 A circuit contains a thermistor and a resistor in parallel. The internal resistance of the cell

is negligible.

The temperature of the thermistor is increased.

The temperature of the resistor is kept constant.

What is observed on the voltmeter and the ammeter?

[1 mark]

Voltmeter reading Ammeter reading

A decreases increases

B increases increases

C no change increases

D no change no change

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 18 is option D, where both the voltmeter reading and ammeter reading show no change.

18 D no change no change

How to answer it

Thermistor and Resistor in Parallel Circuit

What this question tests

This question assesses your understanding of parallel DC circuits, the electrical characteristics of NTC (Negative Temperature Coefficient) thermistors, and the impact of internal resistance (or lack thereof) on terminal potential difference across parallel branches.

Question 18 • Multiple Choice [1 mark]

Analyzing the Circuit Behavior

✅ Correct Answer: D (no change, no change)

Because the internal resistance of the cell is negligible, the terminal potential difference across the parallel combination is equal to the electromotive force (emf) of the cell, which remains constant.

💡 Key Knowledge

  • Thermistors (NTC): As temperature increases, resistance decreases ( R ↓ as T ↑ ).
  • Parallel Circuits: Components in parallel across an ideal voltage source experience the full emf.
  • Ammeter position: The ammeter is in series with the fixed resistor, measuring only the current through that specific branch.

🧠 Exam Technique

Don't fall into the trap of analyzing the total circuit resistance first! Look closely at what the meters are connected to and whether the power supply has internal resistance.

❌ Common Errors

  • Assuming that because the thermistor's resistance decreases, the total circuit current changes the voltmeter reading (forgetting the cell has negligible internal resistance).
  • Assuming a change in temperature of one branch affects the fixed resistor branch's current or potential difference.

Topics

Physics · 3.5 Electricity

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.