First Law of Thermodynamics

The first law of thermodynamics is the total energy balance for closed systems. The variation in total energy \(d E^{tot}\) of a closed system is due to the work \(\delta L^{ext}\) performed on the system by external macroscopic actions and the heat \(\delta Q^{ext}\) transferred to the system from the outside,

\[d E^{tot} = \delta L^{ext} + \delta Q^{ext} \ .\]

Classical thermodynamics provides an average macroscopic description of the microscopic dynamics of a large number of elementary components (todo atomic theory). The total energy of the system can thus be interpreted as the sum of a macroscopic kinetic contribution and a microscopic content, both kinetic and potential; heat can be interpreted as the work performed on the system by microscopic actions.