Some processes one can imagine obey the first law of thermodynamics but still do not happen. For example, heat flows spontaneously from hot to cold but not from cold to hot even though this would conserve energy too. The second law of thermodynamics shows why some things do not happen even though they conserve energy. We have some explaining to do. The easiest definition is, a reversible process can be represented by a series of points in the PV plane, therefore, at each instant the system is near equilibrium.
Free expansion is an example of an irreversible process.
With this definition of entropy it is found that entropy is a state function, and so its change does not depend on path but only on the initial and final states of the system. Later we will give several examples on how to calculate entropy. Turning now to the second topic for today, the heat engine . Thermodynamics was developed during the 1800 s when a great deal of interest in steam engines existed.