I
n Chapter 19 we discussed the properties of an ideal gas, using such macroscopic vari-
ables as pressure, volume, and temperature. We shall now show that such large-scale
properties can be related to a description on a microscopic scale, where matter is
treated as a collection of molecules. Newton’s laws of motion applied in a statistical
manner to a collection of particles provide a reasonable description of thermodynamic
processes. To keep the mathematics relatively simple, we shall consider primarily the
behavior of gases, because in gases the interactions between molecules are much
weaker than they are in liquids or solids. In our model of gas behavior, called
kinetic
theory, gas molecules move about in a random fashion, colliding with the walls of
their container and with each other. Kinetic theory provides us with a physical basis for
our understanding of the concept of temperature.
21.1 Molecular Model of an Ideal Gas
We begin this chapter by developing a microscopic model of an ideal gas. The model
shows that the pressure that a gas exerts on the walls of its container is a consequence
of the collisions of the gas molecules with the walls and is consistent with the macro-
scopic description of Chapter 19. In developing this model, we make the following as-
sumptions:
1.
The number of molecules in the gas is large, and the average separation
between them is large compared with their dimensions. This means that the
molecules occupy a negligible volume in the container. This is consistent with the
ideal gas model, in which we imagine the molecules to be point-like.
2.
The molecules obey Newton’s laws of motion, but as a whole they move ran-
domly. By “randomly” we mean that any molecule can move in any direction with
any speed. At any given moment, a certain percentage of molecules move at high
speeds, and a certain percentage move at low speeds.
3.
The molecules interact only by short-range forces during elastic collisions.
This is consistent with the ideal gas model, in which the molecules exert no long-
range forces on each other.
4.
The molecules make elastic collisions with the walls.
5.
The gas under consideration is a pure substance; that is, all molecules are
identical.
Although we often picture an ideal gas as consisting of single atoms, we can assume
that the behavior of molecular gases approximates that of ideal gases rather well at low
pressures. Molecular rotations or vibrations have no effect, on the average, on the
motions that we consider here.
For our first application of kinetic theory, let us derive an expression for the pres-
sure of N molecules of an ideal gas in a container of volume V in terms of microscopic
quantities. The container is a cube with edges of length d (Fig. 21.1). We shall first
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Figure 21.1 A cubical box with
sides of length d containing an
ideal gas. The molecule shown
moves with velocity v
i
.
d
d
d
z
x
y
m
v
xi
v
i
Assumptions of the molecular
model of an ideal gas