Explain why, when a hot metal spoon is placed into a cup of cold water and the system is left for a long time, the final temperature of the spoon and the water is the same.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A metal spoon at 80 °C is placed into a cup of water at 20 °C. The cup is thermally insulated from its surroundings.
Model answer (4 marks)
The particles in the hot spoon have a higher average kinetic energy than those in the cold water. Heat is transferred from the spoon to the water by conduction – collisions between particles at the interface transfer energy. As a result the spoon loses internal energy (cools) while the water gains internal energy (warms). This process continues until the average kinetic energies of the particles in both bodies are equal. At that point the system is at thermal equilibrium, so there is no net transfer of energy and the temperature of the spoon and the water remains constant and equal.
Examiner tips
- Use the word ‘conduction’ to show the mechanism of heat transfer.
- Explain the change in internal energy of each body.
- State that equilibrium is reached when temperatures are equal.
- Show the logical sequence from higher to lower kinetic energy to equalisation.
Common mistakes
- Not mentioning conduction or particle collisions as the transfer mechanism.
- Failing to state that the spoon cools and the water warms.
- Assuming the final temperature is the initial temperature of one body instead of the equilibrium temperature.
Mark scheme (4 marks)
- The particles of the hot spoon have greater average kinetic energy than the particles of the cold water.
- Thermal energy is transferred from the spoon to the water (by conduction through collisions between particles at the interface).
- The spoon loses internal energy (cools) and the water gains internal energy (warms) until their average particle kinetic energies are equal.
- At thermal equilibrium, there is no net transfer of energy between the spoon and the water, so the temperature remains constant and equal for both.
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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