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CONCEPTS Applied A B
Weight
Why Is It Called a “Pop” Can?
Piston
Obtain two empty, clean pop cans, a container of ice water
with ice cubes, and a container of boiling water. You might
want to “dry run” this experiment to make sure of the
procedure before actually doing it. Weight
Place about 2 cm of water in a pop can and heat
it on a stove until the water boils and you see evidence Piston
of steam coming from the opening. Using tongs, quickly Air
invert the can halfway into a container of ice water. Note
how much water runs from the can as you remove it from
the ice water. Air
Repeat this procedure, this time inverting the can
halfway into a container of boiling water. Note how much
water runs from the can as you remove it from the boiling
water.
Explain your observations in terms of the kinetic
molecular theory, evaporation, and condensation. It is also
important to explain any differences observed between
what happened to the two pop cans.
FIGURE 4.23 A very simple heat engine. The air in (B)
has been heated, increasing the molecular motion and thus
the pressure. Some of the heat is transferred to the increased
gravitational potential energy of the weight as it is converted to
4.5 THERMODYNAMICS mechanical energy.

The branch of physical science called thermodynamics is con-
cerned with the study of heat and its relationship to mechani-

cal energy, including the science of heat pumps, heat engines, identified, everything else is called the surroundings. A system
and the transformation of energy in all its forms. Th e laws of can exist in a number of states since the variables that char-
thermo dynamics describe the relationships concerning what acterize a state can have any number of values and combina-
happens as energy is transformed to work and the reverse, tions of values. Any two systems that have the same values of
also serving as useful intellectual tools in meteorology, chem- variables that characterize internal energy are said to be in
istry, and biology. the same state.
Mechanical energy is easily converted to heat through
friction, but a special device is needed to convert heat to THE FIRST LAW OF THERMODYNAMICS
mechanical energy. A heat engine is a device that converts
Any thermodynamic system has a unique set of properties that will
heat into mechanical energy. The operation of a heat engine

identify the internal energy of the system. This state can be changed
can be explained by the kinetic molecular theory, as shown in

in two ways, (1) by heat flowing into (Q in ) or out (Q out ) of the sys-

Figure 4.23. This illustration shows a cylinder, much like a big
tem, or (2) by the system doing work (W out ) or by work being done

can, with a closely fitting piston that traps a sample of air. Th e
on the system (W in ). Thus, work (W) and heat (Q) can change the

piston is like a slightly smaller cylinder and has a weight rest-
internal energy of a thermodynamic system according to
ing on it, supported by the trapped air. If the air in the large
cylinder is now heated, the gas molecules will acquire more JQ – W = U 2 – U 1
kinetic energy. This results in more gas molecule impacts with

equation 4.7
the enclosing surfaces, which results in an increased pressure.
Increased pressure results in a net force, and the piston and where J is the mechanical equivalence of heat (J = 4.184
weight move upward as shown in Figure 4.23B. Thus, some of joules/calorie), Q is the quantity of heat, W is work, and

the heat has now been transformed to the increased gravita- (U 2 – U 1 ) is the internal energy difference between two states.

tional potential energy of the weight. This equation represents the first law of thermodynamics,

Thermodynamics is concerned with the internal energy which states that the energy supplied to a thermodynamic
(U), the total internal potential and kinetic energies of mol- system in the form of heat, minus the work done by the sys-
ecules making up a substance, such as the gases in the simple tem, is equal to the change in internal energy. Th e fi rst law
heat engine. The variables of temperature, gas pressure, vol- of thermodynamics is an application of the law of conser-

ume, heat, and so forth characterize the total internal energy, vation of energy, which applies to all energy matters. Th e
which is called the state of the system. Once the system is first law of thermodynamics is concerned specifi cally with

104 CHAPTER 4 Heat and Temperature 4-20