Energy Transfer by Heat

Energy can enter and leave a system by heat transfer, $Q$, which is caused by a temperature difference between the system and surroundings.

• If $T_A>T_B$, heat is transferred out of system $Q(-)$
• If $T_B>T_A$, heat is transferred into system $Q(+)$

$Q$ is in units of $[\text{kJ}]$, and $q=\frac{Q}{m}\left[\frac{\text{kJ}}{\text{kg}}\right]$. This is a process, not a property; it’s only quantified across a boundary.