A student leaves a glass of cold water on a table in a warm room. After some time, the water reaches the same temperature as the room. Explain how energy is transferred from the warm air in the room to the cold water, and describe what happens to the particles of water as the water warms up.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Energy is transferred from the warm air to the water by conduction through the glass and by convection of the air around the glass.
Energy transfer occurs because there is a temperature difference between the warm air and the cold water.
As the water absorbs energy, the water molecules gain kinetic (thermal) energy.
The molecules move faster and vibrate more rapidly as the water warms.
When the water reaches the same temperature as the room, the temperature difference disappears, so there is no further net transfer of energy.
Energy transfer occurs because there is a temperature difference between the warm air and the cold water.
As the water absorbs energy, the water molecules gain kinetic (thermal) energy.
The molecules move faster and vibrate more rapidly as the water warms.
When the water reaches the same temperature as the room, the temperature difference disappears, so there is no further net transfer of energy.
Examiner tips
- Use the words ‘conduction’ and ‘convection’ to show understanding of transfer mechanisms.
- Explain the role of a temperature difference as the driving force.
- Describe the increase in kinetic energy and faster motion of the molecules.
- Mention that the process stops when temperatures equalise.
Common mistakes
- Forgetting to mention both conduction and convection.
- Saying energy is transferred without explaining the temperature difference.
- Describing the particles as ‘moving faster’ but not linking it to kinetic energy.
Mark scheme (5 marks)
- Energy is transferred from the warm air to the water by conduction and/or convection
- Energy is transferred because there is a temperature difference between the warm air and the cold water
- As the water warms up, the particles gain (kinetic/thermal) energy
- The particles move faster / vibrate faster as the water warms up
- When the water reaches the same temperature as the room, there is no longer a temperature difference, so there is no net transfer of energy
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