For both SL and HL, the direction of net thermal energy transfer is determined by the temperature difference between the bodies. Thermal energy transfers spontaneously from higher temperature to lower temperature until thermal equilibrium is reached.
The Reasoning
Kelvin temperature is a measure of the average random kinetic energy of particles. When bodies at different temperatures interact, microscopic energy exchanges occur in both directions through conduction, convection, or thermal radiation.
If , particles in A have greater average kinetic energy. Statistically, more energy is transferred from A to B than from B to A, so the net thermal energy transfer is from A to B.
For example, if a metal block at is placed in water at , energy transfers from the block to the water. The block cools and the water warms until both reach the same temperature. There is then no net transfer, although microscopic exchanges continue.
| Quantity or condition | Effect on direction |
|---|---|
| Temperature difference | Higher temperature to lower temperature |
| Mass or total internal energy | Does not by itself determine direction |
| Specific heat capacity | Affects temperature change, not direction |
| Conductivity and surface area | Affect rate, not direction |
| Equal temperatures | No net transfer |
A common misconception is that energy must transfer from the body with greater internal energy. This is incorrect. A large cold body may have more internal energy than a small hot body, but net transfer still occurs from hotter to colder.
Exam Technique
This is syllabus topic B.1 Thermal energy transfers for SL and HL. For an Explain question, state the temperature difference, identify the direction from higher to lower temperature, and link the transfer to thermal equilibrium. Avoid saying “heat rises” or that temperature flows: it is thermal energy that is transferred.
