Energy

Energy is a feature of matter. Nothing in this physical world exists independent of matter, and energy is no exception. Energy, entropy, electric charge, momentum, chemical potential, temperature, pressure, volume, all of these are features of matter whose values exist only in relation to the physical features of some material. There is no free-floating energy off by itself. Energy is always the energy of something.

Stated as a feature, energy takes its place on the core list of features a phase has: temperature, pressure, amount of substance, volume, mass, energy, entropy, chemical potential, momentum, electric charge. Each of these is a measurable quantity that a particular material has at a particular time, and each has a value. Energy belongs on that list. When we say a material has energy, we are naming one of its features; when we attach a value to that feature, we are stating a property of the material.

Why put energy on the same list as volume and mass? Because, treated macroscopically, that is the honest description of what energy is. A material has some. We can measure how much. We can watch the value change as the material interacts with its surroundings. We do not need to reach beneath the description to find energy; the description is where energy lives. Treated this way, energy loses some of its mystique and becomes tractable, a number attached to a material.

Energy also does work in the story of change. From the third assumption comes the concept of transformation caused by the interactions of materials with each other and with energy and entropy. The driving force of those changes is chemical potential. Energy and entropy are paired in that sentence on purpose; they travel together. Where a material is changing, the values of its energy and its entropy are in motion, and the difference in chemical potential is what aims that motion. Energy is one of the features that makes change describable. It does not sit off to the side; it is inside the process.