Heat flow and thermal equilibrium
What happens when a hot piece of iron is dropped into water at room temperature?
Activity 9.1
You will need: a vessel of water at room temperature, a hot piece of iron, a thermometer, a stirrer
- Put the hot iron into the cold water.
- Watch the thermometer reading.
Figure to be added
The observation
The water's temperature rises: heat flows from the hotter iron to the cooler water. The vessel warms too. The iron cools, and before long the water and the iron are at the same temperature; after that no heat flows either way.
Thermal equilibrium
When two objects in contact reach the same temperature and no more heat flows between them, they are in thermal equilibrium.
As water flows from a higher level to a lower one, heat flows from a hotter object to a cooler one: the cooler object's temperature rises and the hotter one's falls.
Heat is energy, so it is measured in joules (J), the international unit. The calorie is also widely used.