Thermal energy moves by conduction, convection and radiation, and the explanation for each one depends on what the particles are doing. Strong answers name the process, describe the particles, and then link both to the observation.
This lesson belongs to integrated physical reasoning because the same particle model supports physics here and returns in chemistry topics such as states of matter and reaction rates.
What are the particles doing in each process?
In every case, the hotter region has particles with more energy. What differs is how that energy gets to the cooler region.
- Conduction (solids): particles vibrate harder and pass energy to neighbours by collisions. In metals, free electrons also carry energy, which is why metals conduct well.
- Convection (liquids and gases): a heated fluid expands, becomes less dense and rises. Cooler, denser fluid sinks to replace it, forming a circulation.
- Radiation (no particles needed): hot objects emit infrared waves, which can cross a vacuum. Dark matt surfaces absorb and emit more than light shiny ones.
- Evaporation: the most energetic particles escape the liquid surface, so the remaining liquid cools.
Worked example (invented data)
A student pours hot tea into a cup in a room at 25 °C and records the temperature every 2 minutes.
| Time (min) | 0 | 2 | 4 | 6 |
|---|---|---|---|---|
| Temperature (°C) | 80 | 68 | 59 | 52 |
Describe the pattern, find the average rate of cooling over 6 minutes, and explain the pattern using particles.
Pattern. The temperature falls each interval, but by less each time: 12 °C, then 9 °C, then 7 °C.
Average rate. The fall is 80 − 52 = 28 °C over 6 minutes, so the rate is 28 ÷ 6 = 4.7 °C/min (to 2 significant figures).
Explanation. The tea is hotter than the air, so its particles pass energy to the cup and to the surrounding air by conduction, convection and radiation, and by evaporation at the surface. When the tea is much hotter than the room, the particles gain and lose energy at very different rates, so energy leaves quickly. As the tea cools, the difference shrinks, so the net transfer slows and the temperature falls by less in each interval.
Re-check: 12 + 9 + 7 = 28 °C, which matches 80 − 52.
The mistake to watch for
A student writes that “cold travels into the tea” or that “the particles in the tea get smaller as it cools.”
Mistaken answer: “The tea cools because the cold air moves into it and the particles shrink.”
Thermal energy moves from hot to cold. Cold is not a substance that travels, and the particles do not change size; their average kinetic energy decreases and they move more slowly.
The correction is to describe the direction of energy transfer: from the hotter tea to the cooler surroundings. Then describe the particles: slower on average, so the temperature is lower.
Check yourself
All data are invented.
1. Explain why a metal spoon feels colder than a plastic spoon, both at room temperature.
Show answer
The metal conducts thermal energy away from your hand much faster, because free electrons and vibrating particles pass energy on quickly. The plastic is a poor conductor, so less energy leaves your skin each second. Both are at the same temperature; the difference is the rate of transfer.
2. A beaker of water is heated gently from underneath. Explain why the water circulates.
Show answer
Water near the heat source gains energy, its particles move further apart, and it becomes less dense. The warmer water rises, and cooler, denser water sinks to take its place. This forms a convection current.
3. Another cup of tea cools from 90 °C to 66 °C in 8 minutes. What is the average rate of cooling?
Show answer
Fall = 90 − 66 = 24 °C. Rate = 24 ÷ 8 = 3.0 °C/min.
Where this leads next
Thermal ideas also appear in circuits, where energy transferred as heat matters, so continue to explaining a circuit change using more than one quantity. The scientific investigation critic is useful when you plan a cooling experiment and need to judge the control variables, and the integrated practice set mixes these ideas.
If your answers are accurate but thin on particle detail, our teachers can help you build sentences that carry the reasoning. That is part of online one-to-one Co-ordinated Sciences tuition.