Every physical system has "locked up" within it a store of energy which can be decreased or increased by interaction with other systems. The internal energy is related to the internal structure and internal processes of the system. For convenience, we consider the internal energy as consisting of two parts. One part consists of some kind of mechanical potential energy-for example, magnetic energy, or the chemical energy related to the molecular structure, or even the zero-point-energy that every material system has in order to exist at all. The other part consists of thermal energy. What are the important differences between these two parts of the internal energy? Energy in transit produces changes. So a store of energy can produce external results by diminishing, i.e. by losing energy. A store of mechanical energy can be used to shift a load from one place to another - by work. A compressed spring can exert a force and push the load. An electric battery can produce an electromagnetic field that can propel the load on a magnetic carriage. A raised weight in falling can produce the necessary traction. And so on. But how can the store of energy in a furnace, say, be used to shift a load? Heat cannot move anything. In this Study Unit, we are to study a concept that can help us to understand the internal characteristics of systems associated with their thermal energy, and how work and heat differ This concept is entropy.