Two bodies are in thermal equilibrium when
- A. they have the same mass
- B. they are at the same temperature, so there is no net flow of heat between them
- C. they contain the same amount of heat
- D. they are made of the same material
Explanation
Temperature is what decides the direction of heat flow, so equal temperatures mean no net transfer even if the two bodies hold very different amounts of internal energy. A cup of tea and a swimming pool at the same temperature are in equilibrium despite the pool storing far more energy. This is the content of the zeroth law, which is what makes a thermometer possible.