Work, Heat, and Irreversibility
A reversible transformation is one which is carried out in such a way that the substance is always nearly in equilibrium—for example, when a gas expands against a movable piston which is itself backed by nearly the same pressure, or when heat flows from a warm body into another one which is at nearly the same temperature. A cyclic process is a process comprised of a set of transformations which eventually return the substance back to its initial state (i.e. the same temperature, the same pressure, and most importantly, the same internal energy). Consequently, during a cyclic processes any mechanical work done on the substance must be either (i) expended as work done by the substance, or (ii) expelled from the substance in the form of heat. This follows form the first law of thermodynamics. For a non-cyclic process, on the other hand, the internal energy of a substance may change. This is the topic to which Clausius now turns in Art. 5 of his sixth memoir. Then, in Art. 11, he broadens the discussion to examine irreversible transformations, such as the conduction of heat between bodies at significantly different temperatures.