Introduction

Note:

This chapter will deal with a new form of energy, heat. It is therefore beneficial to review quickly what we have learned so far about energy and its different manifestations. We will discuss these concepts in words, omitting equations. However, the following text on this page is heavily decorated with hyperlinks so that you can easily look up the corresponding equations in order to refresh your memory.

In our discussion of mechanical energy, we introduced the concepts of kinetic and potential energy. We mentioned that work done to a system resulted in a change in the system's energy. Then we introduced the law of total energy conservation for an isolated system.

Later, we extended the concept of kinetic energy when we introduced the rotational kinetic energy in addition to the translational kinetic energy mentioned above. But this did not change the validity of the conservation law of energy when one used the augmented definition of the total kinetic energy = rotational kinetic energy + translational kinetic energy in the law of energy conservation.

In the context of our discussion of the law of energy conservation, we emphasized that the total mechanical energy was only conserved for so-called conservative forces. Of the non-conservative forces, we prominently mentioned friction. In this context we said that "When one deals with non-conservative forces, energy is "lost'". Well, where does this energy go?

Here is an observation that we have all made before: when you touch the tires of a car right after driving it they feel warm. Another one: when you rub your hand really fast back and forth across a flat surface, like a table, the hand starts to feel pretty . What warms up the tire and also the hand is the effect of friction. The effect of friction is to convert ordered mechanical energy in random thermal motion,thus producing heat.

This leads us to the conclusion that heat is a form of energy. We will later give another extended form of the conservation law that now also includes heat as an energy form. We will find out, that in an isolated system the total energy, mechanical + heat, is always conserved. What is needed, of course, is a way to calculate the equivalent amount of energy contained in heat. We will be introduced to the mechanical equivalent of heat, which accomplishes exactly this goal.

Now, clearly, when we heat something up, we increase the temperature. So we expect that the concepts of temperature and heat are intimately linked. This is correct, and we will explore this connection in some detail in this chapter. Since temperature and average kinetic energy of molecules in a gas are proportional to each other, as we have observed in our introduction to kinetic theory (applet), this has to play a role in our extension of the energy concept to include heat.

One final introductory thought: we had seen that matter exists in different phases, and that these phases can change (applet) as we change the temperature. But an important question remained unanswered so far: how much energy in the form of heat does one have to put into a substance to raise its temperature by a certain amount. We will see that there exists a linear relationship, with a proportionality constant called specific heat capacity. When the material makes a phase transition, it takes some additional heat to convert from one phase to another, and we will learn the meaning of the term "latent heat'.