1.21.2: Thermodynamic Energy
The thermodynamic energy \(\mathrm{U}\) of a closed system increases when work \(\mathrm{w}\) is done by the surroundings on the system and heat \(\mathrm{q}\) flows from the surroundings into the system.
\[\Delta \mathrm{U}=\mathrm{q}+\mathrm{w} \nonumber \]
Equation (a) uses the acquisitive convention. In effect we record all changes from the point of view of the system.