# Energy conservation law (1st law of thermodynamics)

Energy cannot disappear into thin air, not appear out of the blue. With a total energy amount $E_1$ at any moment, energy can be added/removed as work $W$ or heat $Q$, giving the new total energy amount $E_2$ at a later moment.

$$E_1+W+Q=E_2$$

Sign convention: $W$ is work done on the system and $Q$ heat added to the system.