Although the symbolic expression of the First Law is very simple and is almost obvious, its interpretation in practice is not so simple. The main reason for difficulty is that "internal energy" is a single concept covering a very complicated state of affairs. We have seen how it can be treated as made up of three parts--thermal, potential and chemical-and how each of these three parts can interchange energy with the others. This "inner complication" exactly parallels the "outer-complication" in how energy is transferred to and from a system. The separate concepts of heat and work take us only so far; in certain instances they have to be taken simultaneously or even combined together (as in electrical heating).
In a very abstract way, the composition of the internal energy is shown in fig. 2.9. The three main arrows represent the three main kinds of change in the internal energy - heat, work, and heat of reaction. There can of course be interchange between the potential and the thermal components. What faces us next is the investigation of what it is that differentiates heat transfer from work transfer. Many of our problems will recur again, though in a different form. Remember, that exactitude is gained at the expense of rejecting something of the complexity of the actual world.