|Modified diagram of the original 1824 Carnot heat engine showing the hot body, working body (system), and cold body, the letters labeled according to the stopping points in the various steps of the seven-step Carnot cycle.|
“A thermodynamic system may contain no substance at all, in the ordinary sense, and consist of radiant energy, or an electric or magnetic field … [but] usually comprises a substance, which may be homogeneous or heterogeneous.”
|Left: Modern generic version of the thermodynamics system, showing that both radiation, heat, and work can pass the boundary, often defined as an "closed system", and that also mass can pass across the boundary, often defined as an "open system". Mass can pass the boundary as either inert transport mass, in which a certain amount of energy is assigned to the work of moving the mass; or in the case of chemically reactive mass, the effect is quantified by the chemical potential terms, taking into affect the nature of the change of the internal energy in a chemical reaction sense. Right: a close-up of the original Carnot engine (1824).|
|Watt engine (c. 1780)||Papin engine (1690)||Papin digester (1679)||Vacuum pump (1850)|
See main: Earth-bound thermodynamic systemsIn subjects such as life thermodynamics, evolutionary thermodynamics, biological thermodynamics, ecological thermodynamics, economic thermodynamics, sociological thermodynamics, or human thermodynamics in general, the diagramming of the boundary of the thermodynamic system becomes a paramount issue. The basic model delineates a cylinder type imaginary volumetric region on the surface (substrate) of the earth, viewed such, while remaining in contact with the surface of the earth, it rotates in a 24-hour, two-part heat cycle, being put in contact with a hot body (the sun) for approximately 12-hours, the expansion phase, and then put in contact with a cold body (the night sky) for approximately 12-hours, the contraction phase, whereby after the body, if it is considered to be reversible, returns to its original condition.