19.1 It’s All About Energy

Recall that energy is about systems and their interactions. In 《 SECTION 10.8 we wrote the energy principle as

Δ K + Δ U + Δ E_{th} = W_{ext}

(19.1)

This equation tells us that the energy of a system is changed when external forces do work by pushing or pulling the particles of the system through a distance. Recall that we defined the system’s mechanical energy to be $E_{mech} = K + U$ . FIGURE 19.1 reminds you that the mechanical energy is the macroscopic energy of the system as a whole, while the thermal energy $E_{th}$ is the microscopic energy of the atoms and molecules within the system. The mechanical and thermal energy together are the system energy, so we concluded Chapter 10 with the statement

Δ E_{sys} = Δ E_{mech} + Δ E_{th} = W_{ext}

(19.2)

Thus the total energy of an isolated system, for which $W_{ext} = 0$ , is constant.

The figure illustrates a rocket moving toward the upper right.

FIGURE 19.1 Full Alternative Text

The emphasis in Chapters 9 and 10 was on isolated systems. There we were interested in learning how kinetic and potential energy were transformed into each other and, where there is friction, into thermal energy. Now we want to focus on how energy is transferred between the system and its environment when $W_{ext}$ is not zero.

NOTE Strictly speaking, Equation 19.2 should use the internal energy $E_{int}$ rather than the thermal energy $E_{th}$ , where $E_{int} = E_{th} + E_{chem} + E_{nuc} + \dots$ includes all the various kinds of energies that can be stored inside a system. This textbook will focus on simple thermodynamic systems in which the internal energy is entirely thermal: $E_{int} = E_{th}$ . We’ll leave other forms of internal energy to more advanced courses.