The First Law of Thermodynamics

Empirical Law: The First Law of Thermodynamics

The infinitesimal change $\mathrm{d}{E}_{\text{int}}$ in the Internal Energy of a Physical System is the difference of the infinitesimal Heat $\mathrm{d}Q\ge 0$ which flows into the system and the infinitesimal Work $\mathrm{d}W\ge 0$ which the system does on its environment.

$$\mathrm{d}{E}_{\text{int}}=\mathrm{d}Q-\mathrm{d}W$$