A particularly important concept is thermodynamic equilibrium. In general, when two objects are brought into thermal contact, heat will flow between them until they come into equilibrium with each other. When a temperature difference does exist, heat flows spontaneously from the warmer system to the colder system. Heat transfer occurs by conduction or by thermal radiation. When the flow of heat stops, they are said to be at the same temperature. They are then said to be in thermal equilibrium.
See also: What is Temperature.
For example, you leave a thermometer in a cup of coffee. As the two objects interact, the thermometer becomes hotter, and the coffee cools off a little until they come into thermal equilibrium. Two objects are defined to be in thermal equilibrium if, when placed in thermal contact, no net energy flows from one to the other, and their temperatures don’t change. We may postulate:
When the two objects are in thermal equilibrium, their temperatures are equal.
This is a subject of a law that is called the “zeroth law of thermodynamics”.
We can discover an important property of thermal equilibrium by considering three systems. A, B, and C, which initially are not in thermal equilibrium. We separate systems A and B with an adiabatic wall (ideal insulating material), but we let system C interact with systems A and B. We wait until thermal equilibrium is reached; then, A and B are in thermal equilibrium with C. But are they in thermal equilibrium with each other?
According to many experiments, there will be no net energy flow between A and B. This is experimental evidence of the following statement:
If two systems are both in thermal equilibrium with a third, then they are in thermal equilibrium with each other.
This statement is known as the zeroth law of thermodynamics. It has this unusual name because it was not until after the great first and second laws of thermodynamics were worked out that scientists realized that this obvious postulate needed to be stated first.
This law provides a definition and method of defining temperatures, perhaps the most important intensive property of a system when dealing with thermal energy conversion problems. Temperature is a system property that determines whether the system will be in thermal equilibrium with other systems. When two systems are in thermal equilibrium, their temperatures are, by definition, equal, and no net thermal energy will be exchanged between them. Thus the importance of the zeroth law is that it allows a useful definition of temperature.
For example, most materials expand when their temperature is increased. This property is very important in all of science and engineering, even in nuclear engineering. The thermodynamic efficiency of power plants changes with inlet steam temperature or even with the outside temperature. At higher temperatures, solids such as steel glow orange or even white, depending on temperature. The white light from an ordinary incandescent lightbulb comes from an extremely hot tungsten wire. It can be seen temperature is one of the fundamental characteristics that describe matter and influences matter behavior.