Fundamentals of Renewable Energy Processes, 3rd Edition

Chapter 2

A Minimum of Thermodynamics and of the Kinetic Theory of Gases

2.1 The Motion of Molecules

A gas is a collection of particles (molecules) that, to a first approximation, interact with one another solely through elastic collisions—in other words, through collisions that conserve both energy and momentum. If molecules were dimensionless, point-like objects, their thermal energy would be only that of linear motion in three dimensions—they would have only three degrees of freedom. In reality, molecules are more complicated. Even simple monatomic ones, such as helium, may be able to spin (because it has a finite dimension) and may, therefore, have more than three degrees of freedom. Multi-atomic molecules can also vibrate and this confers to ...