Heat

Two types of experiments suggest that heating is a form of energy transfer. First of all, on the macroscopic or everyday scale of things, there are forces which are apparently nonconservative. This is in marked contrast to the microscopic world, where forces are either conservative (gravity, electrostatics), or don't change a particle's energy (magnetic force), or convert energy from one known form to another (non-static electric forces). With these fundamental forces all energy is accounted for -- it is neither created or destroyed.

In contrast, macroscopic energy routinely disappears in the everyday world. Cars once set in motion don't continue in motion forever on a level road once the engine is stopped; a soccer ball once kicked eventually comes to rest; electrical energy powering a light bulb appears to be lost. Careful measurements show that whenever this type of energy loss is found, heating occurs. Since we believe that macroscopic forces are really just large scale manifestations of fundamental microscopic forces, we do not believe that energy really disappears as a result of these forces -- it must simply be converted from a form visible to us into an invisible form. We now know that such forces convert macroscopic energy to internal energy, a form of energy which is just the kinetic and potential energy of atomic and molecular motions. Thus, the apparent disappearance of macroscopic energy is just a consequence of the conversion of this energy into microscopic form.

Figure 22.2: Conversion of internal energy of gas in the cylinder to macroscopic energy. The work done by the force of the gas on the piston as it moves outward results in a decrease in temperature of the gas.

The second type of experiment which suggests that heating converts macroscopic energy to internal energy is one in which this energy is converted back to macroscopic form. An example of this process is illustrated in figure 22.2. As the piston moves out of the cylinder under the force exerted on it by the gas, work is done which can be stored or used by, say, compressing a spring or running an electric generator. As the piston moves out, the gas in the cylinder decreases in temperature, which indicates that the gas is loosing microscopic energy.