ABSTRACT
Let us consider a simple system that is closed and fixed in space. As for most practical applications, the interconversion between mass and energy occurring in atomic or nuclear processes can be safely ignored; we consider here that the principles of conservation of mass and energy can be separately stated. In this case, a change in the internal energy, U, of this simple system is due only to an exchange of energy between the system and its surroundings, be this exchange in the form of heat, Q, or work, W. According to the International Union of Pure and Applied Chemistry (IUPAC) [1] convention of signs, energy is positive when it enters the system, thus increasing its internal energy. Using the symbol Δ to indicate the difference of the final value of a function minus its initial value, we write https://www.w3.org/1998/Math/MathML"> Δ U = Q + W https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315399065/5df30985-80c1-4e11-9533-10c20ed2604c/content/equ02_01a.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
