• id: JTHERM-01-N04
• title: First Law — Energy Conservation
• tags: first-law, conservation, heat-work

$\Delta Q = \Delta U + \Delta W$: heat added to a system equals the increase in internal energy plus the work done by the system. Sign convention: Q positive when absorbed, W positive when done by system (expansion). Equivalently: $\Delta U = \Delta Q - \Delta W$. The First Law is a statement of energy conservation — energy can neither be created nor destroyed, only converted. It places no restrictions on the direction of processes (that's the Second Law's role).