The primary focus of this chapter is learning the Energy-Interaction Model and how to use it to construct particular models of various kinds of phenomena. We will encounter two other modes in these two chapters as well, the Three-Phase Model of Pure Substances and the Intro Spring-Mass Oscillator Model (that will be substantially augmented and extended later in the course).
We will see that by applying the Energy-Interaction Model to these seemingly very strange thermal phenomena, we can make sense of them, we can explain what is going on, and we can answer all kinds of questions about the phenomena (including some seemingly hard questions on exams). To see the universal applicability of the Energy-Interaction Model, we will also apply this model to several chemical reactions.
You likely use the term "energy" in a reasonably accurate sense -- that is, a physicist would likely not cringe when hearing the way the word is used in daily contexts. Phrases such as "I burned the energy in the ice cream by jogging", or "the car is out of energy" are motivated largely by the origins of the word in physics. However, the story of energy in science is, like most things, long and complicated.
Temperature, heat, thermal energy, and entropy are all what we call macroscopic measurements. That is, they can be used to describe large systems with many particles, but it is not necessary to know that the objects are made of particles to take these measurements. This makes them very useful for describing overall changes in a system. Measuring any of these is typically a simple process, since no knowledge of the system's microscopic configuration is needed.
The goal of this section is simply to extend the naive discussion of energy and temperature we have running so far to include phase changes. I use the word naive in the sense that, at this point in the course, we still don't understand what the underlying mechanisms in thermal phenomena really are. There were a lot of questions and only a few half-answers on the previous page. Keep in mind that the take away from these first sections are not fundamental, but are still of value.
The scientific meaning of energy is rather tricky to convey in a sentence or two. There is a good reason for this: energy is an abstract concept that took scientists a long time to figure out. Although the concept of energy is truly universal in the sense that energy changes are associated with nearly all phenomena and processes, energy is not related to a single property of matter. For example, we all have an intuitive sense of “hotness” and we associate the concept of temperature.
