- Two objects are in thermal contact when heat energy can be transferred between them.
- Two objects are in thermal equilibrium when there is no net flow of heat between two objects that are in thermal equilibrium.
- Two objects in thermal equilibrium have the same temperature.
Figure below shows 2 blocks in thermal contact with each other. Initially, the temperature of the 2 blocks are different, and there is a net flow of thermal energy from higher temperature to lower temperature.
After some time, thermal equilibrium achieved, where the temperature of the 2 blocks become the same, and there is no net flow of thermal energy between the 2 blocks.
- Initially, the temperature of block A is higher than block B.
- The rate of thermal energy transfer is higher from block A to the block B (1000J/s).
- There is also thermal energy transfer from the block B to block A, but with lower rate (only at 200J/s).
- Therefore, there is a net heat flow of thermal energy from the block A to block B.
- As a result, the temperature of block A decreases whereas the temperature of B increases.
- Thermal Equilibrium Achieved.
- The temperature of the 2 blocks become the same.
- Heat flow is still goes on between the blocks.
- However, the rate of flow of heat are equal in both direction. As a result, the net heat flow is equal to 0.