thermodynamics. The second basic principle, which deals with the inevitable
increase of a quantity called entropy, is the subject of Lesson 21. These two
abstract principles, plus a few other concepts and laws and the vocabulary needed
for literacy in the field, are the entire content of thermodynamics. The energy
and entropy principles form the framework that governs all energy conversions
involving heat; they are the touchstones we must rely on as we attempt to create
new energy devices, such as solar converters or fusion reactors, to limit the
wasteful exploitation of the earth's resources.The approach of this module is
macroscopic; that is, we shall deal with systems that are approximately of human
scale in size and mass (thermometers, blocks of ice, heat engines), and we shall
choose observable quantities such as pressure, volume, and temperature to
describe the behavior of these systems. The macroscopic approach should be seen
as supplementary to the microscopic approach, which regards the behavior of the
atoms and molecules as fundamental. This latter framework chooses the
molecular velocities, energies, and momenta as the starting point, and values for
macroscopic observables are derived from the microscopic picture. The
microscopic approach is treated later where the behavior of gases is interpreted
kinetically, that is, in terms of molecular energies and collisions.
PREREQUISITE: Recognizing methods and units used for measuring pressure

Week of April 20, 1998
Lesson 14Kinetic Theory of Matter
Keywords: heat and thermodynamics, kinetic theory, macroscopic description of
ideal gases, ideal gas law, pressure in gases, volume;
OBJECTIVE:
* Define an ideal gas from a macroscopic point of view using the equation of state,
pV = nRT, and use this equation to solve problems involving the relations among
the macroscopic variables p, V, n, and T.
Comments: As you read this sentence you will experimentally demonstrate
the general gas law at least once by breathing in and out. As you expand the volume
in your lungs, the pressure drops and air comes in; as you decrease the lung volume
the pressure rises and air goes out. Pressure (p) and volume (V) are related; at
constant temperature, pV = const.This relation, called Boyle's law, was well
established before the atomic theory of matter was accepted. In this lesson, you will
learn how to apply much of your knowledge of Newton's laws, kinetic energy,
momentum, and elastic collisions to molecular motion. The fact that you can derive
the macroscopic general gas law from Newtonian mechanics applied to molecules
should give you further insight into the remarkable simplification Newton
wrought in our understanding of matter. You will see that Newton's laws hold
(with some limitations) for masses 1027 times smaller and dimensions 10 10 times
smaller than the blocks and inclined planes we have discussed in previous lessons.
Newton's laws are useful in microphysics as well as in planetary physics, an
astronomical range.To be sure, we have overstated the case in order to make a
point. The behavior of an ideal gas is just that, idealized, and real gases only
approximately behave this way. At the molecular level the fundamental