its own right. This substance was even given a name: “caloric.” However, around
1780 Benjamin Thompson, later Count Rumford, kept track of the production
of heat during the boring of cannon barrels and concluded that the amount of
heat was related to the amount of work done in the process. In the 1840s, care-
ful experiments by the English physicist James Prescott Joule (Figure 2.5) ver-
ified this. A brewer at the time, Joule used an apparatus like the one shown in
Figure 2.6 to perform the work of mixing a quantity of water using a weight
on a pulley. By making careful measurements of the temperature of the water
and of the work being performed by the falling weight (using equation 2.1),
Joule was able to support the idea that work and heat were manifestations of
the same thing. (In fact, the phrase “mechanical equivalent of heat” is still used
occasionally and emphasizes their relationship.) The SI unit of energy and
work and heat, the joule, is named in Joule’s honor.
The older unit of energy and heat and work, the calorie, is defined as the
amount of heat needed to raise the temperature of exactly 1 mL of water by
1°C from 15°C to 16°C. The relationship between the calorie and the joule is
1 calorie 4.184 joules (2.8)
Although joules are the accepted SI unit, the unit of calorie is still used often,
especially in the United States.
Heat can go into a system, so that the temperature of the system increases,
or it can come out of a system, in which case the temperature of the system
decreases. For any change where heat goes into a system,qis positive. On the30 CHAPTER 2 The First Law of ThermodynamicsFigure 2.5 James Prescott Joule (1818–1889),
English physicist. His work established the inter-
conversion of heat and work as forms of energy,
and laid the foundation for the first law of
thermodynamics.Figure 2.6 Joule used this apparatus to measure what was once
called the “mechanical equivalent of heat.”© CORBIS/Bettmann
Science & Society Picture Library, Science Museum, London