Thermal Equilibrium
Thermal Equilibrium
- Before any formulas, Paper 4 wants one idea stated exactly: what temperature does, and when energy transfer stops.
What temperature does
- Put a hot object and a cold object in contact. Thermal energy flows from the higher-temperature object to the lower-temperature object — always that direction, never the reverse on its own.
- So temperature is the property that decides the direction of energy transfer. It is not the same thing as the amount of energy: a bathtub of warm water holds more thermal energy than a red-hot nail, but energy still flows from the nail into the water.
- What temperature measures on the particle scale — the average kinetic energy of the molecules — is made exact in the Ideal Gases chapter. This chapter works at the object scale.
Thermal equilibrium: the two-mark sentence
- The transfer does not continue forever. The hot object cools, the cold object warms, and the flow stops when they reach .
- The definition is worth two marks, one per phrase: the objects are at the same temperature, so there is no net transfer of thermal energy between them (9702/42/O/N/25 Q3(a)) (9702/42/F/M/25 Q3(a)(i)).
- The word net carries a mark. Molecules at the boundary still exchange energy in both directions all the time — equilibrium means the two flows cancel, not that they stop. Writing “no transfer of energy” without net can lose the mark.
Thermal equilibrium = same temperature = no net transfer of thermal energy. Keep the word “net”.
Your turn— tap to reveal the worked answer (quick check)
A metal spoon at 20 °C is placed in a large tank of water, also at 20 °C. Molecules of water keep hitting the spoon. Is energy transferred between water and spoon? Is there net transfer?
Yes and no. Individual collisions pass energy both ways all the time. But because spoon and water are at the same temperature, the two flows are equal in size — the net transfer is zero. That is exactly what thermal equilibrium means.