What is internal energy, as defined for a physical system?

1. Internal energy is defined as the sum of the kinetic and potential energies of a system's atoms and molecules. 2. It depends only on the current state of the system, not on the history of heat or work that produced that state. 3. So describing it as the total heat or total work ever exchanged is incorrect. 4. Surface temperature alone does not capture the combined kinetic and potential energy of every particle. 5. So the sum of kinetic and potential energies is correct. _Source: OpenStax Physics (CC BY 4.0), Ch 12 "Thermodynamics", section 12.2 First law of Thermodynamics: Thermal Energy and Work_