Heat, Internal Energy and Work

So far we have only discussed thermal equilibrium — objects settling to a common temperature. But this alone does not explain everyday observations like why rubbing your palms together makes them warm, or why a running engine gets hot even though nothing outside is heating it. To explain these, we need a deeper concept: the internal energy of a system, and the two distinct ways that internal energy can change — by heat exchange, and by work being done on or by the system. The next four subsections build up this picture piece by piece: what internal energy actually is at the molecular level, what we mean formally by a 'system' and a …