# Explain why the rate of thermal energy transfer by conduction between two objects in contact decreases over time until it eventually reaches zero.

> IB DP Physics Standard Level (2023 syllabus) — B.1 Thermal energy transfers · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Thermal energy is transferred by conduction from the hotter block to the cooler block due to the temperature difference between them.
2. As conduction proceeds, the temperature of the hotter block decreases and the temperature of the cooler block increases.
3. The rate of conduction decreases because the temperature difference between the two blocks decreases over time.
4. 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

- [conduction](https://www.gradenine.co.uk/glossary/conduction)
- [rate of thermal energy transfer](https://www.gradenine.co.uk/glossary/rate-of-thermal-energy-transfer)

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-rate-of-thermal-0ff935d3) · Published by Druglandscape Ltd.