Fundamental Relations in Thermodynamics: The First and Second Laws (Post #1)
First Law of Thermodynamics for a closed system, starting from the universal principle of conservation of energy . 1. Total Energy Conservation: Because the universe is isolated, the sum of all energy changes must equal zero: energy cannot be created or destroyed: $$\Delta(E_{kin} + E_{pot} + U)_{Sys} + \Delta(E_{kin} + E_{pot} + U)_{Surr} = 0\tag{1}$$ The universe is divided into two parts: the System ( Sys , the region of interest) and the Surroundings ( Surr , everything else) . The total energy of any body or region consists of: \(E_{kin}\): macroscopic kinetic energy (movement of the center of mass) \(E_{pot}\): macroscopic potential energy (position in external fields, like gravity) . \(U\): microscopic internal energy (molecular vibrations, rotations, electronic states, intermolecular forces) 2. Practical Approximations & Boundary Transfers $$\Delta(U)_{Sys} = -\Delta(E_k + E_p + U)_{Surr} = Q + W\tag{2}$$ Neglecting macroscopic motion of the system: In ...