Thermodynamics

When an object of interest (usually called the system) is left alone for a sufficiently long time, and is subject to no outside influences from the surroundings, measurements of the properties of the object do not change

with time; it is in a state of thermal equilibrium. It is found experimentally that there are certain measurable quantities that give complete information about the state of the system in thermal equilibrium (this is similar to the idea that measurements of the velocity and acceleration of an object give complete information about the mechanical state of a system). For each such state relationships can be found which hold true over a wide range of values of the quantities. These relationships are known as equations of state.