Introduction

In this chapter you will finally learn what that quantity is that the weather men talk about constantly. What do "tonight's low will be 34°," or "you can't go to school with 103.4°" really mean? As usual physics has a definite mathematical (and operational) definition.

All bodies have internal energy because they contain complicated molecular structures which can store energy in various ways. They have potential energy because of the forces between molecules. The molecules can rotate and vibrate. Temperature, however, is not a measure of any of these three forms of internal energy. It is a measure of the average random translational motion of the molecules.

Heat flows spontaneously from a higher temperature object to a lower temperature one. But what is heat? Heat is a form of energy which flows from one object to another because of their temperature difference. This is totally circular. We had better find a better definition of temperature. This will come from Kinetic Theory. We do know that heating an object raises its internal energy and its temperature, and that two objects in thermal contact will reach the same temperature (thermal equilibrium.)