Nature of heat (Q)

Like work, heat is a form of energy by which the system exchanges energy with its surroundings. When the system and its surroundings are at different temperatures, heat either flows into the system or is let out of the system.

The direction of flow is decided by the temperature difference alone: heat passes from the hotter body to the colder one, and the exchange continues until the system and surroundings reach the same temperature. Heat is measured in the energy units joule (J) or kilojoule (kJ). Like work, heat is a path function (section 4.2.5) — the amount of heat exchanged depends on how the change of state is carried out, not merely on the initial and final states. The sign conven …