Unfortunately, the numbering system for the three laws of thermodynamics is a bit confusing. We begin with the zeroth law. The zeroth law of thermodynamics begins with a simple definition of thermodynamic equilibrium . It is observed that some property of an object, like the pressure in a volume of gas, the length of a metal rod, or the electrical conductivity of a wire, can change when the object is heated or cooled. If two of these objects are brought into physical contact there is initially a change in the property of both objects.
Thermodynamic equilibrium leads to the large scale definition of temperature.
When two objects are in thermal equilibrium they are said to have the same temperature. During the process of reaching thermal equilibrium, heat , which is a form of energy, flows between the objects. The details of the process of reaching thermal equilibrium are described in the first , and second, laws of thermodynamics. The zeroth law of thermodynamics is an observation which is given in black on the slide. As an illustration, we show three objects on the slide.