Earlier in this chapter we discussed molecules. You should remember that all substances are composed of very small particles called molecules. The energy associated with molecules is called thermal energy. Thermal energy, like mechanical energy, exists in two stored forms and in one transitional form. The two stored forms of thermal energy are (1) internal potential energy and (2) internal kinetic energy. Thermal energy in transition is called HEAT.

Although molecules are too small to be seen, they behave in some ways pretty much like the larger objects we considered in the discussion of mechanical energy. Molecules have energy of position (internal potential energy) because of the forces that attract molecules to each other. In this way, they are somewhat like the rock and the earth we considered before. Molecules have energy of motion (internal kinetic energy) because they are constantly in motion. Thus, the two stored forms of thermal energy-internal potential energy and internal kinetic energy-are in some ways similar to mechanical potential energy and mechanical kinetic energy, except everything is on a smaller scale.

For most purposes, we will not need to distinguish between the two stored forms of thermal energy. Therefore, instead of referring to internal potential energy and internal kinetic energy, from now on we will simply use the term internal energy. By internal energy, then, we will mean the total of all internal energy stored in the substance or system because of the motion of the molecules and because of the forces of attraction between molecules. Although the term may be unfamiliar to you, you probably know more about internal energy than you realize. Because molecules are constantly in motion, they exert a pressure on the walls of the pipe, cylinder, or other object in which they are contained. Also, the temperature of any substance arises from, and is directly proportional to, the activity of the molecules. Therefore, every time you read thermometers and pressure gauges you are finding out something about the amount of internal energy contained in the substance. High pressures and temperatures indicate that the molecules are moving rapidly and that the substance therefore has a lot of internal energy.

Heat is a more familiar term than internal energy, but may actually be more difficult to define correctly. The important thing to remember is that heat is THERMAL ENERGY IN TRANSITION-that is, it is thermal energy that is moving from one substance or system to another.

An example will help to show the difference between heat and internal energy. Suppose there are two equal lengths of pipe made of identical materials and containing steam at the same pressure and temperature. One pipe is well insulated; the other is not insulated at all. From everyday experience you know that more heat will flow from the uninsulated pipe than from the insulated pipe. When the two pipes are first filled with steam, the steam in one pipe contains exactly as much internal energy as the steam in the other pipe. We know this is true because the two pipes contain equal volumes of steam at the same pressure and at the same temperature. After a few minutes, the steam in the uninsulated pipe will contain much less internal energy than the steam in the insulated pipe, as we can tell by measuring the pressure and the temperature of the steam in each pipe. What has happened? Stored thermal energy-internal energy-has moved from one system to another, first from the steam to the pipe, then from the uninsulated pipe to the air. This MOVEMENT or FLOW of thermal energy from one system to another is called heat.

A good deal of confusion exists concerning the use of the word heat. For example, you will hear people say that a hot object contains a lot of heat when they really mean that it contains a lot of internal energy. Or you will hear that heat is added to or removed from a substance. Since heat is the FLOW of thermal energy, it can no more be added to a substance than the flow of water could be added to a river. (You might add water, and this addition might increase the flow, but you could hardly say that you added flow. ) The only thermal energy that can in any sense be added to or removed from a substance is INTERNAL ENERGY.