We’ll complete our energy overview—a roadmap of the next two chapters—with the basic energy model.

MODEL 9.1 Basic energy model

Energy is a property of the system.

• Energy is transformed within the system without loss.

• Energy is transferred to and from the system by forces from the environment.

  • The forces do work on the system.

  • $W>0$ for energy added.

  • $W<0$ for energy removed.

• The energy of an isolated system—one that doesn’t interact with its environment—does not change. We say it is conserved.

• The energy principle is $\text{Δ}{E}_{\text{sys}}=W_{\text{ext}}$.

• Limitations: Model fails if there is energy transfer via thermal processes (heat).

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Workbook Exercise 1

We call this model basic because, for now, the only forms of energy we’ll consider are kinetic energy, potential energy, and thermal energy, and the only energy-transfer process we’ll consider is work. This is an excellent model for a mechanical process, but it’s not complete. We’ll expand the model when we get to thermodynamics by adding chemical energy, another form of energy, and heat, another energy-transfer process. And this model, although basic, still has many complexities, so we’ll be developing it piece by piece in this chapter and the next.