The first law of thermodynamics insists that energy should be conserved in any process, including the exchange of heat and work between a system and its surroundings. A machine that violated the first law would undoubtedly be called a perpetual motion machine of the first kind because it would undoubtedly manufacture its very own energy out of absolutely nothing and also, therefore, run forever. Such a machine would certainly be impossible, even theoretically. However, this unfeasibility would certainly not prevent the construction of a machine that could extract unlimited amounts of heat from its surroundings (earth, air, and sea) and transform it entirely into work. Although such a hypothetical machine would indeed not violate the conservation of energy, the total failure of inventors to build such a machine, referred to as a perpetual motion machine of the second kind, led to the discovery of the second law of thermodynamics. The second law of thermodynamics can be precisely stated in the following two types, as originally formulated in the 19th century by the Scottish physicist William Thomson (Lord Kelvin) and also the German physicist Rudolf Clausius, respectively:
A cyclic transformation whose only final result is to transform heat extracted from a source at the same temperature level throughout right into work is impossible.
A cyclic transformation whose only final result is to transfer heat from a body at a given temperature level to a body at a higher temperature level.
Both statements are, in fact, equivalent because, if the first were feasible, then the work obtained can be used, as an example, to generate electrical energy that can then discharge with an electrical heating unit installed in a body at a higher temperature level. The net impact would undoubtedly be a flow of heat from a lower temperature level to a higher temperature level, therefore violating the second (Clausius) form of the second law. Conversely, if the second form were feasible, they could use the heat transferred to the higher temperature level to run a heat engine that would certainly transform components of the heat right into work. The result would undoubtedly be a conversion of heat right into work at a constant temperature level- a violation of the first (Kelvin) form of the second law.
Central to the following discussion of entropy is the concept of a heat reservoir that can provide essentially limitless amounts of heat at a fixed temperature level. It is an idealization; however, the temperature level of a large body of water such as the Atlantic Ocean does not materially change if a small amount of heat is taken out to run a heat engine. The essential factor is that the heat reservoir is assumed to have a well-defined temperature level that does not change as an outcome of the process being considered.