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CHAPTER 1

MOLECULAR-KINETIC THEORY OF IDEAL GASES

1.1 What is Molecular Physics?

The dependence of the structure and physical properties of bodies on the

type of motion and interaction between the particles bodies consist of is
studied in the branch of physics called molecular physics.
Molecular physics is based on the molecular-kinetic theory of matter
building. In accordance with this theory all bodies are composed of small

particles (atoms, molecules and ions), which are in uninterrupted chaotic
motion called the heat motion. The molecular-kinetic theory is
experimentally confirmed by the Brownian movement, transfer

phenomena in different aggregate states etc. The intensity of chaotic
motion depends on the temperature. The smallest unit into which a
substance can be divided without decomposition is known as a molecule.

A molecule can consist of one or several atoms of one or different
chemical elements. A molecule is electrically neutral.
Particles of all matter are in constant motion. Atoms in solids vibrate

back and forth in complex motions about their equilibrium positions.
Molecules in a liquid wander around among other molecules, having
frequent collisions with them and, thus, exchanging energy. In gases the
molecules travel at higher speeds and have frequent elastic collisions with

their neighbors. Possible transitions from one aggregate state to another
are shown in Fig. 9.1.

1.2. Methods of Molecular Physics

There are such two methods for theoretical investigation of matter as
statistical and thermodynamic.

Macroscopic properties of systems consisting of a huge number of
particles are studied by the statistical method. This method is based on the
theory of probabilities and certain models of building of systems. In the

systems with a big number of particles there exists a special kind of
objective laws — statistical objective laws. Properties of such systems are
described by the average meanings of physical values.

The average meanings for accidental value A are defined as the limit
approached by the arithmetic average in respect to the number of values N:

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