Cangel & Boles Translate
Cangel & Boles Translate
Cangel & Boles Translate
per unit area and discuss absolute and gage pressures, the
Objectives
• Review the metric SI and the English unit systems that will
and cycle.
• Review concepts of temperature, temperature scales,
technique.
The name thermodynamics stems from the Greek words therme (heat) and
heat into power. Today the same name is broadly interpreted to include all
principle. It simply states that during an interaction, energy can change from
one form to another but the total amount of energy remains constant. That is,
energy cannot be created or destroyed. A rock falling off a cliff, for example,
energy (Fig. 1–1). The conservation of energy principle also forms the backbone
of the diet industry: A person who has a greater energy input (food)
than energy output (exercise) will gain weight (store energy in the form of
fat), and a person who has a smaller energy input than output will lose
weight (Fig. 1–2). The change in the energy content of a body or any other
system is equal to the difference between the energy input and the energy
property. The second law of thermodynamics asserts that energy has quality
decreasing quality of energy. For example, a cup of hot coffee left on a table
eventually cools, but a cup of cool coffee in the same room never gets hot
engines were very slow and inefficient, but they opened the way for the
the 1850s, primarily out of the works of William Rankine, Rudolph Clausius,
is the result of momentum transfer between the molecules and the walls of
the container. However, one does not need to know the behavior of the gas
direct and easy way to the solution of engineering problems. A more elaborate
is rather involved and is used in this text only in the supporting role.
Application Areas of Thermodynamics
All activities in nature involve some interaction between energy and matter;
and other aspects of life, and one does not need to go very far to see some
application areas of it. In fact, one does not need to go anywhere. The heart
is constantly pumping blood to all parts of the human body, various energy
conversions occur in trillions of body cells, and the body heat generated is
the rate of this metabolic heat rejection. We try to control this heat transfer
Some examples include the electric or gas range, the heating and
cooker, the water heater, the shower, the iron, and even the computer and
the TV. On a larger scale, thermodynamics plays a major part in the design
nuclear power plants, solar collectors, and the design of vehicles from ordinary
cars to airplanes (Fig. 1–5). The energy-efficient home that you may be
living in, for example, is designed on the basis of minimizing heat loss in
winter and heat gain in summer. The size, location, and the power input of
the fan of your computer is also selected after an analysis that involves
thermodynamics.
assigned to the dimensions are called units. Some basic dimensions such as
A number of unit systems have been developed over the years. Despite
world with a single unit system, two sets of units are still in common use
today: the English system, which is also known as the United States Customary
units, and it is being used for scientific and engineering work in most of the
no apparent systematic numerical base, and various units in this system are
related to each other rather arbitrarily (12 in 1 ft, 1 mile 5280 ft, 4 qt
gal, etc.), which makes it confusing and difficult to learn. The United
States is the only industrialized country that has not yet fully converted to
units dates back to 1790 when the French National Assembly charged the
early version of the metric system was soon developed in France, but it did not find universal
acceptance until 1875 when The Metric Convention
States. In this international treaty, meter and gram were established as the
metric units for length and mass, respectively, and a General Conference
of Weights and Measures (CGPM) was established that was to meet every
six years. In 1960, the CGPM produced the SI, which was based on six
fundamental quantities, and their units were adopted in 1954 at the Tenth
General Conference of Weights and Measures: meter (m) for length, kilogram
(kg) for mass, second (s) for time, ampere (A) for electric current,
degree Kelvin (°K) for temperature, and candela (cd) for luminous intensity
was officially dropped from the absolute temperature unit, and all unit
from proper names (Table 1–1). However, the abbreviation of a unit was to
be capitalized if the unit was derived from a proper name. For example, the
unit may be pluralized, but its abbreviation cannot. For example, the length
The recent move toward the metric system in the United States seems to
the rest of the world, passed a Metric Study Act. Congress continued to
September 1992 deadline for all federal agencies to convert to the metric
system. However, the deadlines were relaxed later with no clear plans for
the future.
The industries that are heavily involved in international trade (such as the
automotive, soft drink, and liquor industries) have been quick in converting to
the metric system for economic reasons (having a single worldwide design,
fewer sizes, smaller inventories, etc.). Today, nearly all the cars manufactured
in the United States are metric. Most car owners probably do not realize this
until they try an English socket wrench on a metric bolt. Most industries,
however, resisted the change, thus slowing down the conversion process.
Presently the United States is a dual-system society, and it will stay that
way until the transition to the metric system is completed. This puts an extra
working in terms of the SI. Given the position of the engineers in the transition
period, both unit systems are used in this text, with particular emphasis
on SI units.
The prefixes used to express the multiples of the various units are listed in
Table 1–2. They are standard for all units, and the student is encouraged to
memorize them because of their widespread use (Fig. 1–6). Some SI and English Units
In SI, the units of mass, length, and time are the kilogram (kg), meter (m),
and second (s), respectively. The respective units in the English system are
the pound-mass (lbm), foot (ft), and second (s). The pound symbol lb is
actually the abbreviation of libra, which was the ancient Roman unit of
weight. The English retained this symbol even after the end of the Roman
occupation of Britain in 410. The mass and length units in the two systems
dimension whose unit is derived from Newton’s second law, that is,
or
(1–1)
In SI, the force unit is the newton (N), and it is defined as the force required
force unit is the pound-force (lbf) and is defined as the force required to
accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s2 (Fig. 1–7). That
is,
102 g), whereas a force of 1 lbf is roughly equivalent to the weight of four
medium apples (mtotal 454 g), as shown in Fig. 1–8. Another force unit in
second law W = mg (N) where m is the mass of the body, and g is the local gravitational acceleration
(g is 9.807 m/s2 or 32.174 ft/s2 at sea level and 45° latitude). An ordinary
The mass of a body remains the same regardless of its location in the universe.
mass of 1 lbm, however, weighs 1 lbf, which misleads people to believe that
It should be noted that the gravity force acting on a mass is due to the
depends on the local density of the earth’s crust, the distance to the center
of the earth, and to a lesser extent, the positions of the moon and the sun.
The value of g varies with location from 9.8295 m/s2 at 4500 m below sea
to 30,000 m, the variation of g from the sea-level value of 9.807 m/s2 is less
note that at locations below sea level, the value of g increases with distance
from the sea level, reaches a maximum at about 4500 m, and then starts
decreasing. (What do you think the value of g is at the center of the earth?)
The primary cause of confusion between mass and weight is that mass is
approach also assumes that the forces exerted by other effects such as air
buoyancy and fluid motion are negligible. This is like measuring the distance
a known mass. This is cumbersome, however, and it is mostly used for calibration
Work, which is a form of energy, can simply be defined as force times distance;
A more common unit for energy in SI is the kilojoule (1 kJ 103 J). In the
English system, the energy unit is the Btu (British thermal unit), which is
68°F by 1°F. In the metric system, the amount of energy needed to raise the
Dimensional Homogeneity
We all know from grade school that apples and oranges do not add. But we
must have the same unit (Fig. 1–11). If, at some stage of an analysis,
some stage:
E = 25 kJ + 7 kJ>kg
where E is the total energy and has the unit of kilojoules. Determine how to
correct the error and discuss what may have caused it.
determined.
Analysis The two terms on the right-hand side do not have the same units,
and therefore they cannot be added to obtain the total energy. Multiplying
the last term by mass will eliminate the kilograms in the denominator, and
the whole equation will become dimensionally homogeneous; that is, every
Discussion Obviously this error was caused by forgetting to multiply the last
We all know from experience that units can give terrible headaches if they
are not used carefully in solving a problem. However, with some attention
and skill, units can be used to our advantage. They can be used to check formulas;
they can even be used to derive formulas, as explained in the following
example.
A tank is filled with oil whose density is Ro / p = 850 kg/m3 . If the volume of the
Solution The volume of an oil tank is given. The mass of oil is to be determined.
Analysis A sketch of the system just described is given in Fig. 1–12. Suppose
we forgot the formula that relates mass to density and volume. However,
we know that mass has the unit of kilograms. That is, whatever calculations
we do, we should end up with the unit of kilograms. Putting the given information
these two quantities. Therefore, the formula we are looking for should be
Thus,
Discussion Note that this approach may not work for more complicated
formulas.
You should keep in mind that a formula that is not dimensionally homogeneous
expressed as N = kg
N / kg m s2 = 1 and
Lbf / 32.174 lbm ft s2 = 1
Unity conversion ratios are identically equal to 1 and are unitless, and thus
such ratios (or their inverses) can be inserted conveniently into any calculation
conversion ratios such as those given here when converting units. Some
= 32.174
conversion ratios.
Using unity conversion ratios, show that 1.00 lbm weighs 1.00 lbf on earth
(Fig. 1–13).
Analysis We apply Newton’s second law to calculate the weight (force) that
corresponds to the known mass and acceleration. The weight of any object is
equal to its mass times the local value of gravitational acceleration. Thus,
When you buy a box of breakfast cereal, the printing may say “Net
weight: One pound (454 grams).” (See Fig. 1–14.) Technically, this means
that the cereal inside the box weighs 1.00 lbf on earth and has a mass of
453.6 g (0.4536 kg). Using Newton’s second law, the actual weight of the
study. The mass or region outside the system is called the surroundings.
The real or imaginary surface that separates the system from its surroundings
is called the boundary. These terms are illustrated in Fig. 1–15. The
boundary of a system can be fixed or movable. Note that the boundary is the
contact surface shared by both the system and the surroundings. Mathematically
speaking, the boundary has zero thickness, and thus it can neither contain
fixed mass or a fixed volume in space is chosen for study. A closed system
mass can cross its boundary. That is, no mass can enter or leave a closed
system, as shown in Fig. 1–16. But energy, in the form of heat or work, can
cross the boundary; and the volume of a closed system does not have to be
fixed. If, as a special case, even energy is not allowed to cross the boundary,
Consider the piston-cylinder device shown in Fig. 1–17. Let us say that
we would like to find out what happens to the enclosed gas when it is
heated. Since we are focusing our attention on the gas, it is our system. The
inner surfaces of the piston and the cylinder form the boundary, and since
may cross the boundary, and part of the boundary (the inner surface of the
piston, in this case) may move. Everything outside the gas, including the
devices is best studied by selecting the region within the device as the
control volume. Both mass and energy can cross the boundary of a control
volume.
car radiator, a turbine, and a compressor all involve mass flow and should
as a control volume. There are no concrete rules for the selection of control
volumes, but the proper choice certainly makes the analysis much easier. If
we were to analyze the flow of air through a nozzle, for example, a good
choice for the control volume would be the region within the nozzle.
The boundaries of a control volume are called a control surface, and they
can be real or imaginary. In the case of a nozzle, the inner surface of the nozzle
forms the real part of the boundary, and the entrance and exit areas form
the imaginary part, since there are no physical surfaces there (Fig. 1–18a).
A control volume can be fixed in size and shape, as in the case of a nozzle,
control volumes, however, have fixed boundaries and thus do not involve
any moving boundaries. A control volume can also involve heat and work
Fig. 1–19. Let us say that we would like to determine how much heat we
must transfer to the water in the tank in order to supply a steady stream of
hot water. Since hot water will leave the tank and be replaced by cold
water, it is not convenient to choose a fixed mass as our system for the
analysis. Instead, we can concentrate our attention on the volume formed
by the interior surfaces of the tank and consider the hot and cold water
streams as mass leaving and entering the control volume. The interior surfaces
of the tank form the control surface for this case, and mass is crossing
and defining the system may seem like a tedious and unnecessary task. In
other cases, however, the system under study may be rather involved, and a
properties are those that are independent of the mass of a system, such as
values depend on the size—or extent—of the system. Total mass, total volume,
divide the system into two equal parts with an imaginary partition, as shown
in Fig. 1–20. Each part will have the same value of intensive properties as
the original system, but half the value of the extensive properties.
mass m being a major exception), and lowercase letters are used for intensive
Extensive properties per unit mass are called specific properties. Some
examples of specific properties are specific volume (v = V/m) and specific
Continuum
Matter is made up of atoms that are widely spaced in the gas phase. Yet it is
This idealization is valid as long as the size of the system we deal with
is large relative to the space between the molecules. This is the case in practically
oxygen molecule is about 3 x 10-10 m and its mass is 5.3 x 10-26 kg. Also,
the mean free path of oxygen at 1 atm pressure and 20°C is 6.3 x 10-8 m.
(about 200 times of its diameter) before it collides with another molecule.
Also, there are about 3 x 1016 molecules of oxygen in the tiny volume of
1 mm3 at 1 atm pressure and 20°C (Fig. 1–21). The continuum model is
applicable as long as the characteristic length of the system (such as its diameter) is much larger than
the mean free path of the molecules. At very
high vacuums or very high elevations, the mean free path may become large
km). For such cases the rarefied gas flow theory should be used, and the