Glossary term
Entropy
A thermodynamic property associated with energy dispersal, irreversibility, and the direction of spontaneous processes.
Definition
quantityA thermodynamic property associated with energy dispersal, irreversibility, and the direction of spontaneous processes.
Entropy is a state property used to evaluate reversible and irreversible processes. Entropy generation identifies lost work potential in turbines, compressors, heat exchangers, nozzles, combustion systems, refrigeration cycles, and power plants.
Entropy is a thermodynamic property that measures how energy is distributed and how much irreversibility is associated with a process. For a reversible heat transfer process,
where S is entropy, Q_{rev} is reversible heat transfer, and T is absolute temperature. Specific entropy s is entropy per unit mass.
Engineering role
Entropy is used to analyse heat engines, turbines, compressors, nozzles, heat exchangers, refrigeration cycles, combustion, mixing, throttling, and power plants. It is not just an abstract property: entropy generation shows where useful work potential is destroyed by friction, heat transfer across finite temperature differences, mixing, electrical resistance, shock waves, chemical reaction, and other irreversible effects.
Second-law meaning
For an isolated system, entropy cannot decrease. In real engineering systems, entropy may leave or enter through heat and mass flow, but total entropy generation must be non-negative. A reversible ideal process has zero entropy generation. A real process has positive entropy generation, which reduces maximum possible work output or increases required work input.
Property diagrams
Temperature-entropy diagrams are used to visualize thermodynamic cycles. For a heat engine, the area enclosed on a T-s diagram can represent net work for internally reversible cycles. Turbine and compressor performance is often compared against isentropic reference processes, where entropy remains constant in the ideal case.
Entropy and exergy
Entropy generation is directly linked to exergy destruction. The Gouy-Stodola relation states that destroyed work potential is proportional to ambient temperature and entropy generation. This makes entropy useful for locating the real sources of inefficiency, not merely calculating energy balances.
Common mistakes
Common mistakes include treating entropy as “disorder” without connecting it to thermodynamic calculations, using Celsius instead of kelvin in entropy relations, and assuming adiabatic means isentropic. An adiabatic process with friction, mixing, shock, or throttling can generate entropy even though no heat crosses the boundary.