Subsubsection Heat
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 illustrate 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 had added flow.) The only kind of thermal energy that can in any sense be added to or removed from a substance is internal energy. The distinction between heat and internal energy must be clear in your own mind before you can understand the basic principles of a steam plant. Remember, steam contains internal energy but it does not “contain heat.” Heat exists when the internal energy flows from the steam to something else (another substance or system).
