Heat is a more elusive concept than work. We use the word “heat” very loosely in the English language, often as synonymous with hot. We might say on a very hot day, “This heat is oppressive.” If your apartment is cold, you may say, “Turn up the heat.” These expressions date to a time long ago when it was thought that heat was a substance with fluid-like properties.

Our concept of heat changed with the work of British physicist James Joule in the 1840s. Joule was the first to carry out careful experiments to learn how it is that systems change their temperature. Using experiments like those shown in FIGURE 19.10, Joule found that you can raise the temperature of a beaker of water by two entirely different means:

1. Heating it with a flame, or

2. Doing work on it with a rapidly spinning paddle wheel.

FIGURE 19.10 Joule’s experiments to show the equivalence of heat and work.

The final state of the water is exactly the same in both cases. This implies that heat and work are essentially equivalent. In other words, heat is not a substance. Instead, heat is energy. Heat and work, which previously had been regarded as two completely different phenomena, were now seen to be simply two different ways of transferring energy to or from a system.