temperature increased; and when caloric was removed from a body, its tem-
perature decreased. When a body could not contain any more caloric, much
the same way as when a glass of water could not dissolve any more salt or
sugar, the body was said to be saturated with caloric. This interpretation gave
rise to the terms saturated liquidand saturated vaporthat are still in use
today.
The caloric theory came under attack soon after its introduction. It main-
tained that heat is a substance that could not be created or destroyed. Yet it
was known that heat can be generated indefinitely by rubbing one’s hands
together or rubbing two pieces of wood together. In 1798, the American
Benjamin Thompson (Count Rumford) (1754–1814) showed in his papers
that heat can be generated continuously through friction. The validity of the
caloric theory was also challenged by several others. But it was the careful
experiments of the Englishman James P. Joule (1818–1889) published in
1843 that finally convinced the skeptics that heat was not a substance after
all, and thus put the caloric theory to rest. Although the caloric theory was
totally abandoned in the middle of the nineteenth century, it contributed
greatly to the development of thermodynamics and heat transfer.
Heat is transferred by three mechanisms: conduction, convection, and
radiation. Conductionis the transfer of energy from the more energetic par-
ticles of a substance to the adjacent less energetic ones as a result of interac-
tion between particles. Convectionis the transfer of energy between a solid
surface and the adjacent fluid that is in motion, and it involves the combined
effects of conduction and fluid motion. Radiationis the transfer of energy
due to the emission of electromagnetic waves (or photons). An overview of
the three mechanisms of heat transfer is given at the end of this chapter as a
Topic of Special Interest.2–4 ■ ENERGY TRANSFER BY WORK
Work, like heat, is an energy interaction between a system and its surround-
ings. As mentioned earlier, energy can cross the boundary of a closed sys-
tem in the form of heat or work. Therefore,if the energy crossing the
boundary of a closed system is not heat, it must be work. Heat is easy to
recognize: Its driving force is a temperature difference between the system
and its surroundings. Then we can simply say that an energy interaction that
is not caused by a temperature difference between a system and its sur-
roundings is work. More specifically,work is the energy transfer associated
with a force acting through a distance. A rising piston, a rotating shaft, and
an electric wire crossing the system boundaries are all associated with work
interactions.
Work is also a form of energy transferred like heat and, therefore, has
energy units such as kJ. The work done during a process between states 1
and 2 is denoted by W 12 , or simply W. The work done per unit massof a
system is denoted by wand is expressed as(2–17)The work done per unit timeis called powerand is denoted W.
(Fig. 2–17).
The unit of power is kJ/s, or kW.wW
m¬¬ 1 kJ>kg 2
62 | Thermodynamics
Hot
bodyCold
bodyContact
surfaceCaloricFIGURE 2–16
In the early nineteenth century, heat
was thought to be an invisible fluid
called the caloricthat flowed from
warmer bodies to the cooler ones.
W = 30 k = 30 kJ
m = 2 kg = 2 kg
t = 5 s = 5 sW = 6 kW = 6 kW
w = 15 k = 15 kJ/kJ/kg30 kJ30 kJ
workwork
∆FIGURE 2–17
The relationships among w,W, and .W
#SEE TUTORIAL CH. 2, SEC. 4 ON THE DVD.INTERACTIVE
TUTORIAL