Explain why the rate of thermal energy transfer by conduction between two objects in contact decreases over time until it eventually reaches zero.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two solid blocks, one initially at 80 °C and one initially at 20 °C, are placed in contact with each other and thermally insulated from their surroundings.
Model answer (4 marks)
The hotter block transfers thermal energy to the cooler block by conduction because a temperature gradient exists between them.
As conduction proceeds the hotter block cools and the cooler block warms, so the temperature difference ΔT between the two blocks decreases.
The rate of conduction is proportional to ΔT (Fourier’s law), therefore the rate falls as ΔT falls.
When the two blocks reach the same temperature (thermal equilibrium) ΔT = 0, so no net energy is transferred and the rate becomes zero.
As conduction proceeds the hotter block cools and the cooler block warms, so the temperature difference ΔT between the two blocks decreases.
The rate of conduction is proportional to ΔT (Fourier’s law), therefore the rate falls as ΔT falls.
When the two blocks reach the same temperature (thermal equilibrium) ΔT = 0, so no net energy is transferred and the rate becomes zero.
Examiner tips
- State that conduction depends on the temperature difference
- Explain that ΔT decreases as the blocks exchange energy
- Mention Fourier’s law or the proportionality to ΔT
- Show the final state is equilibrium with zero ΔT
Common mistakes
- Confusing heat transfer with heat content; writing that heat itself decreases
- Forgetting to mention that the rate is proportional to the temperature difference
- Stating that the blocks stop exchanging energy before equilibrium is reached
Mark scheme (4 marks)
- Thermal energy is transferred by conduction from the hotter block to the cooler block due to the temperature difference between them.
- As conduction proceeds, the temperature of the hotter block decreases and the temperature of the cooler block increases.
- The rate of conduction decreases because the temperature difference between the two blocks decreases over time.
- When both blocks reach the same (equilibrium) temperature, the temperature difference is zero and there is no net transfer of thermal energy, so the rate becomes zero.
Key terms in this question
conduction · rate of thermal energy transfer
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Thermal energy transfers questions
- Explain why, when a hot metal spoon is placed into a cup of cold water and the s…
- Explain why thermal energy transfer by convection cannot occur in a solid.
- Explain why a gas in a thermally insulated container rises in temperature when i…
- Explain why evaporation from the surface of a liquid causes the temperature of t…