irreversibilities

Irreversibilities are processes that cannot be reversed to restore a system and its surroundings to their exact initial states without leaving net changes. They contrast with idealized reversible processes, which could, in principle, be reversed by infinitesimal changes and do not generate net entropy.

Most real processes exhibit irreversibilities. Common sources include friction and viscous dissipation, electrical resistance, heat transfer across finite temperature differences, inelastic deformation, mixing of different substances, chemical reactions, phase changes, turbulent flows, and radiation losses. These factors cause irreversible changes in the microstate of the system and increase disorder.

Thermodynamically, irreversibilities are associated with entropy production. The second law states that the total entropy of the universe does not decrease; irreversible processes generate positive entropy. This entropy production reduces the maximum possible efficiency of devices such as heat engines; no real engine can reach the Carnot efficiency because of irreversibilities. Exergy destruction or exergy loss is a common way to quantify the work potential lost due to these irreversible effects.

Irreversibilities can be categorized as internal or external. Internal irreversibilities occur within the working substance or device (for example, viscous heating in a fluid). External irreversibilities arise from interactions with the surroundings, such as heat transfer across a finite temperature difference. In some theoretical frameworks, irreversible thermodynamics studies systems near equilibrium, using entropy production rate as a central measure of dissipation.

Applications include the analysis and optimization of engines, refrigerators, heat exchangers, and many natural processes like diffusion, chemical reactions, and shock waves, where accounting for irreversibilities is essential to understand real performance.