Vol. 9, 2024-08 Cite as: Hernandez, H. (2024). Understanding Work, Heat, and the First Law of Thermodynamics 1: Fundamentals. ForsChem Research Reports, 9, 2024-08, 1 - 40. Publication Date: 28/06/2024. Understanding Work, Heat, and the First Law of Thermodynamics 1: Fundamentals Hugo Hernandez ForsChem Research, 050030 Medellin, Colombia hugo.hernandez@forschem.org doi: 10.13140/RG.2.2.13481.20322 Abstract The First Law of Thermodynamics is the Principle of Conservation of Energy applied to the interaction between Systems. Such interaction is partially observed at a macroscopic scale, in the form of Work. The remaining interaction, taking place at the microscopic scale and not observed as macroscopic work, is denoted as Heat. Thus, the change in energy of a system can be interpreted as the sum of energies transferred in the form of (macroscopic) Work and (microscopic) Heat. However, there are different types of heat. The most common type of heat is proportional to the temperature difference between the systems, but there are other types which are independent of the systems temperatures. To avoid the incorrect use of the First Law, it is important to clearly understand the concepts of Heat and Work. In the first part of these series, these fundamental concepts are discussed in detail, and a general formulation of the First Law is presented. In the second part of the series, this general formulation is applied to a wide variety of representative interacting systems. Keywords Active Forces, Energy Conservation, Energy Exchange, Field Forces, Heat, Interaction, Internal Energy, Kinetic Energy, Local Forces, Macroscopic Energy, Mechanical Work, Open Systems, Passive Forces, Potential Energy, Reflexive, Short-range Forces, Thermodynamic Work 1. Introduction According to Enrico Fermi, “Thermodynamics is mainly concerned with the transformations of heat into mechanical work and the opposite transformations of mechanical work into heat ” [1]. Notice that two basic concepts are involved in this definition of thermodynamics: Mechanical Work and Heat. These two concepts are also the core of the First Law of Thermodynamics. In the words of Fermi this law would be expressed as follows: “The first law of thermodynamics is essentially the statement of the principle of the conservation of energy for thermodynamical