The first law of closed-system thermodynamics was originally induced by empirically observed evidence, including calorimetric evidence. However, it is now understood to provide the definition of heat via the law of energy conservation and the definition of work in relation to changes in the external parameters of a system. The original discovery of the law occurred gradually over a period of perhaps half a century or more, and some of the first studies related to cyclical processes. [5] Sometimes the existence of internal energy is made explicit, but the work is not explicitly mentioned in the statement of the first postulate of thermodynamics. The heat supplied is then defined as the residual variation of internal energy after examination of the work in a non-adiabatic process. [32] In an open system, particles and energy can be transferred to or from the system during a process. In this case, the first law of thermodynamics always applies in the form that internal energy is a function of the state and that the change of internal energy in a process is only a function of its initial and final state, as mentioned in the next section entitled First Law of Thermodynamics for Open Systems. Classical thermodynamics initially focused on closed homogeneous systems (e.g. Planck 1897/1903[37]), which could be considered « zero-dimensional » in the sense that they have no spatial variation. However, it is desirable to also study systems with pronounced inner motion and spatial inhomogeneity.

For such systems, the principle of conservation of energy is expressed not only in relation to internal energy as defined for homogeneous systems, but also in the form of kinetic energy and potential energies of parts of the inhomogeneous system in relation to each other and in relation to external forces of great scope. [54] How the total energy of a system is divided between these three more specific types of energy depends on the objectives of the different authors; Indeed, these energetic components are, to some extent, mathematical artifacts and not physical quantities actually measured. For any closed homogeneous component of an inhomogeneous closed system, if E {displaystyle E} denotes the total energy of this system of components, it can be written that it is impossible to build a machine capable of continuously providing mechanical work without consuming energy at the same time. Such a hypothetical machine is known as a perpetual motion machine of the first type. These types of machines violate the 1st law of thermodynamics and do not exist in reality. The first law of thermodynamics is the law of conservation of energy. There is a sense in which this kind of additivity expresses a fundamental postulate that goes beyond the simplest ideas of classical thermodynamics of closed systems; the extensivity of some variables is not obvious and requires explicit expression; In fact, one author goes so far as to say that it could be recognized as a fourth law of thermodynamics, although this is not repeated by other authors. [79] [80] For the first law of thermodynamics, there is no trivial transition from the physical conception of the view of the closed system to a view of the open system. [61] [62] For closed systems, the concepts of adiabatic enclosure and adiabatic wall are fundamental. Matter and internal energy cannot penetrate or penetrate such a wall. For an open system, there is a wall that allows the penetration of matter.

In general, matter carries some internal energy with it in diffusive motion, and some microscopic potential energy changes accompany the movement. An open system is not closed adiabatic. If we look at the first law of thermodynamics, it confirms that heat is a form of energy. This means that thermodynamic processes are determined by the principle of energy conservation. The first law of thermodynamics is sometimes called the law of conservation of energy. The first law of thermodynamics indicates the relationship between the change in the total internal energy of a system, the addition of heat and the work done. This can be expressed mathematically as ΔU = Q – W. Here, ΔU is the change in internal energy, Q is the heat added to the system and W is the work done by the system. Gyarmati shows that his definition of the « heat flow vector » is strictly speaking a definition of internal energy flow, not specifically heat, and so it turns out that his use of the word heat here contradicts the strict thermodynamic definition of heat, although it is more or less compatible with historical custom. which quite often did not clearly distinguish between heat and internal energy; He writes that « this relationship must be seen as the exact definition of the concept of heat flow, which is used quite loosely in experimental physics and thermal engineering. » [96] Apparently in a different frame of thought from that of the paradoxical use mentioned above in previous sections of Prigogine`s 1947 historical work on discrete systems, this use of Gyarmati coincides with later sections of Prigogine`s 1947 work on continuous flow systems, which use the term « heat flow » in exactly this way. Glansdorff and Prigogine also follow this use in their 1971 text on continuous flow systems. You write: « Again, the internal energy flow can be divided into a convection flow and a line flow.

This line flow is by definition the heat flow W. Therefore: j[U] = ρuv + W, where u is the [internal] energy per unit mass.