The Micro/Macro Connection
Recommended class days:
The kinetic theory of gases is the centerpiece of the micro/macro connection. There are two
important aspects to emphasize to students. First, the microscopic view of gases helps us understand
of how pressure is established, energy is transferred, systems reach equilibrium, and
so on. Second, the macroscopic predictions of kinetic theory, such as the values of specific heats,
are strong evidence for the atomic theory of matter.
Much of this chapter is fairly traditional—kinetic theory is kinetic theory, and there’s not much
leeway for alternative presentations. Note that the root-mean-square speed, which plays an impor-
tant role in kinetic theory, will be a new concept for most or all students. For most students, it
seems an unnecessary and confusing idea. It’s important to spend some time introducing
main point is that
, for practical purposes, is just about the same as the average speed, but it is
easier to calculate and it arises naturally in the analysis of molecular motions. Students who had
difficulty with kinematics, and who have forgotten that
is a signed quantity, may need some help
recognizing that the average velocity (as distinct from the average speed) is zero.
Student Learning Objectives
• To recognize the connection between temperature, thermal energy, and the average translational
kinetic energy of the molecules in the system.
• To understand the molecular basis for pressure and the ideal-gas law.
• To use the micro/macro connection to predict the molar specific heats of gases and solids.
• To understand how heat is transferred via molecular collisions and how thermally interacting
systems reach equilibrium.
• To obtain a qualitative understanding of entropy, the second law of thermodynamics, and some
of the implications of the second law.
The kinetic theory introduction of mean free path, pressure, and the root-mean-square speed are all
fairly standard. A main point to emphasize is the underlying reasoning: each molecular collision
with a wall exerts a small force on it, and pressure is due to the vast number of collisions that occur
each second in a macroscopic system. The derivations are straightforward once students understand
the reasoning. It’s more important to emphasize the results and their consequences than the details
of the derivations.