Module 5
Temperatures and Heat
A. Temperature
Next to time, temperature may be the most commonly used physical quantity in
our daily lives. We might loosely define temperature as a measure of hotness or coldness.
But the concepts of hot and cold are themselves rather vague, subjective, and relative. In
the summer, an air temperature of 70°F feels cool, whereas the same temperature in the
winter feels warm. Thermometers are devices that measure the temperature of a
substance. To function, they all depend on some physical property that changes with
temperature. A common thermometer design exploits the fact that a liquid, frequently
red-colored alcohol, will expand or contract when heated or cooled, thereby rising or
falling in a glass tube as the temperature varies. Some thermometers are based on other
temperature-dependent physical properties, including the volume of solids, the pressure
or volume of a gas, the electrical properties of metals, the amount and frequency of
radiated energy, and the speed of sound in a gas.
There are three different temperature scales in common use for calibrating
thermometers: Fahrenheit, Celsius, and Kelvin. The normal freezing and boiling
temperatures of water—called the phase transition temperatures—may be used to
compare the three scales. In the Fahrenheit scale, the boiling point of water under a
pressure of 1 atmosphere is 212°—designated 212°F. The freezing point of water under
this pressure is 32°F. So there are 180 units, called degrees, that separate the two
temperatures. The Celsius scale, formerly called the centigrade scale, is metric based; it
uses 100 degrees between the freezing and boiling points of water. Zero degrees Celsius
—designated 08C—is the freezing temperature, and 1008C is the boiling temperature.
Most of us spend our lives subjected to temperatures within the range of 260 to
1208F (251 to 498C). Much higher temperatures exist in common places: the interiors of
stoves and automobile engines, the filaments of lightbulbs, the flame of a candle, and so
on. The Sun’s surface temperature is about 10,0008F (5,7008C), and its interior is at
about 27,000,0008F (15,000,0008C). Temperatures this high have been produced on
Earth in experiments with plasmas and in nuclear explosions. There is no upper limit on
temperature. At the other extreme, there is a limit on cold temperatures. The coldest
temperature, called absolute zero, is 2459.678F (2273.158C). For reasons to be discussed
later, because it is impossible to go below this temperature, the Kelvin scale is a
convenient one because it uses absolute zero as its starting point (zero). (This scale is also
referred to as the “absolute temperature scale.”) The size of the unit in the Kelvin scale is
the same as that in the Celsius scale, except it is called a kelvin (K) instead of a degree.
What determines the temperature of matter? In other words, what is the difference
between a cup of coffee when it is hot (2008F) and the same cup of coffee when it is cold
(708F)? The atoms and molecules that compose matter have kinetic energy. In gases,
they move about randomly with high speed. In liquids and solids, they vibrate much like
a mass on a spring or an object oscillating in a hole (Section 3.5). At higher temperatures,
the atoms and molecules in matter move faster and have higher kinetic energies.
So when a cup of coffee is hot, the molecules in it have higher average kinetic
energy than when it is cold. If you put your finger into hot coffee, the atoms and
molecules in the coffee pass on their higher kinetic energy to the atoms and molecules in
your finger by way of collisions: your finger is warmed. This fact—that temperature
depends on the average kinetic energy of atoms and molecules—is very important. It
should help you understand many of the phenomena we will discuss in this chapter. In
gases, higher kinetic energy means that the atoms and molecules move about with higher
speeds. For liquids and solids, the molecules vibrate through a greater distance like a
pendulum swinging through a larger arc. You may recall that when particles oscillate like
this, they also have potential energy. This is the case here, too, but the potential energy of
“bound” atoms and molecules in liquids and solids is not directly related to the
temperature. This potential energy is important when a substance undergoes a change of
phase—such as freezing or boiling. More on this later.
At very low temperatures, many substances acquire unusual properties. Plastic
and rubber become as brittle as glass. Below about 2 K, helium is a “superfluid” liquid; it
flows without friction (see the application at the end of Section 4.5). Some materials
become “superconductors.” They conduct electricity without resistance (more on this in
Chapter 7). The nature of matter is also different at very high temperatures. Above about
6,000 K, the kinetic energies of atoms are so high that they cannot bind together, so there
can be no solids or liquids—or even molecules. Above about 20,000 K, the electrons
begin to break free from atoms, and only plasmas can exist.
B. Thermal Expansion
Thermal expansion is an important phenomenon that is exploited by the common
types of thermometers and by a variety of other useful devices. In almost all cases,
substances that are not constrained expand when their temperatures increase. (Exceptions
include water below 48C and some compounds of tungsten.) The air in a balloon expands
when heated, the mercury in a thermometer expands upward in the glass tube when
placed in a hot liquid, and sections of bridges become longer in the summer. If the
substance is constrained sufficiently, it will not expand, but forces and pressures will be
created in response to the constraint. For example, if an empty pressure cooker is sealed
and then heated, the air inside is prevented from expanding. But the pressure inside
increases and causes larger and larger outward forces on the inner surfaces of the pressure
cooker.
We can use logic and basic mathematics to predict the amount of expansion that
occurs. Let us first consider the simplest case—the thermal expansion of a long, thin solid
such as a metal rod. The expansions of different solids are measured under similar
conditions. The results are used to assign a coefficient of linear expansion to each
material. The value of this coefficient is a fixed parameter of each substance, much like
mass density or specific gravity. It is represented by the Greek letter alpha, . Because
aluminum expands more than iron under the same circumstances, the coefficient of linear
expansion of aluminum is larger than that of iron.
The bimetallic strip is an ingenious and widely used application of thermal
expansion. As its name implies, a bimetallic strip consists of two strips of different metals
bonded to one another (Figure 5.9). The two metals have different coefficients of linear
expansion, so they expand by different amounts when heated. The result is that the
bimetallic strip bends—one way when heated and the other way when cooled. The
greater the change in temperature, the greater the bending. For example, if brass and iron
are used, the brass will expand and contract more than the iron will. The brass will be on
the outside of the curve when the strip is hot and on the inside of the curve when it is
cold.
Thermostats, thermometers, circuit breakers, lamp flashers, and chokecontrol
mechanisms on small engines and older automobiles often contain a bimetallic strip that
is curled into a spiral. The coil will either partly unwind or wind up more tightly when the
temperature changes. To make a thermometer, a pointer is attached to one end of the
spiral, and the other end is held fixed. As the temperature varies, the pointer moves over a
scale that indicates the temperature.
The behavior of liquids is quite similar to that of solids. Because liquids do not
hold a certain shape, it is best to consider the change in volume caused by thermal
expansion. In general, liquids expand considerably more than solids. This means that
when a container holding a liquid is heated, the level of the liquid will usually rise
because the increase in volume of the liquid typically exceeds the increase in volume of
the (solid) container. When a mercury thermometer is heated, the mercury in the tube and
in the glass bulb at the bottom expands more than does the confining glass, so the level of
mercury rises. If the glass expanded more than the mercury, the column would go down
at higher temperatures instead of up.
At the beginning of this section, we implied that there are exceptions to the
general rule that matter expands when heated. The most important example of this is
water when it’s near its freezing temperature. Above 48C (398F), water expands when
heated like ordinary liquids. But between 08C and 48C, water actually contracts when
heated and expands when cooled. The volume of a given amount of water at 38C is less
than the volume of the same water at 18C. This anomaly accounts for the fact that lakes,
ponds, and other bodies of water freeze on top first. As long as the average water
temperature is above 48C, the warmer (less dense) water is buoyed to the surface, while
the cooler water settles to the bottom.
The volume expansion of gases is larger than that of solids and liquids. Also, the
amount of expansion does not vary with different gases (except at very low temperatures
or very high pressures). Instead of relating the expansion to a change in temperature, it is
simpler to state the relationship between the volume occupied by the gas and the
temperature. In particular, as long as the pressure remains constant, the volume occupied
by a given amount of gas is proportional to its temperature (in kelvins).
The utility and importance of this general relationship comes from the fact that it
is applicable to most real gases under most conditions. Because the constant appearing in
this equation only depends on the mass of gas present and not on what kind of gas it is,
the equation can be used equally well for hydrogen, helium, oxygen, carbon dioxide, and
other gases. Moreover, except under extreme circumstances, the ranges over which the
parameters p, V, and T vary are limited enough in practice to ensure that the gas particles
generally remain far enough apart that their interactions are negligible. Together, these
facts make the ideal gas law useful for describing the thermodynamic properties of gases
in the atmospheres of the Sun and stars, the air in a classroom, and the helium in a
floating blimp.
Regardless of which phase of matter is involved, changing the temperature of a
substance will not change its mass or its weight. Because thermal expansion causes the
volume to increase while the mass and weight stay the same however, the mass density
and weight density decrease. The mass density of a hot piece of iron is slightly less than
the mass density of the same piece when it is cold. The mass density of the balloon
referred to earlier is about 10 percent less when its temperature is 303 K than when its
temperature is 273 K. The reduction in density of a gas at constant pressure because of
heating is exploited in hot-air balloons. The air in the balloon, which is basically a large
bag with an opening at the bottom, is heated with a burner (Figure 5.15). The pressure
inside remains equal to the atmospheric pressure because of the opening. Consequently,
the air inside the balloon expands and its density decreases. The balloon can float in the
air because it is filled with a gas (hot air) that has a smaller density than the surrounding
fluid (cooler air). As the air inside cools, the balloon will sink toward Earth until the
burner heats the air again.
C. The First Law of Thermodynamics
The first way is very familiar to you. When you heat something on a stove, warm
your hands over a heater, or feel the Sun warm your face, the temperature increase is
caused by exposure to something that has a higher temperature: the stove, the heater, or
the Sun. (More on this in Section 5.4.) As mentioned before, the atoms and molecules in
the substance being warmed gain kinetic energy from those in the hotter substance, albeit
sometimes through the mediating action of other particles. The temperature of a
substance will rise only if its atoms and molecules gain kinetic energy. Friction is an
effective means of raising the temperature of a substance by the second way (doing work
on it). When something is heated by kinetic friction, work is done on it.
Both processes can be reversed to cause the temperature of a substance to
decrease. The cooler air in a refrigerator lowers the temperature of a pitcher of tea. Air
escaping from a tire is cooled as it expands. Temperature depends on the average kinetic
energy of atoms and molecules. For matter to undergo a change in temperature, its atoms
and molecules must gain energy (increased temperature) or lose energy (decreased
temperature). In gases, the constituent particles have kinetic energy only. All of the
energy given to the atoms and molecules acts to increase the temperature of the gas.
Things are different in solids and liquids. The atoms and molecules have kinetic energy
and potential energy because they are bound to each other and oscillate. Energy given to
the particles goes to increase both their PE and their KE. The concept of internal energy,
which we introduced in Section 3.4, incorporates both forms of energy.
In gases, the internal energy is the total of the kinetic energies only: the atoms and
molecules do not have potential energy. (Gravitational potential energy is not included in
internal energy.) In solids and liquids, both the kinetic energy and the potential energy of
the particles contribute to the internal energy. As the temperature of a substance rises, its
internal energy increases. If this is accomplished by exposure to a hotter substance, we
say that heat has flowed from the hotter substance into the cooler substance. Heat is
symbolized by Q. Its units are the same as those of work and energy. Traditionally, the
calorie, kilocalorie (also written Calorie), and British thermal unit (Btu) were used
exclusively as units of heat. The joule and the foot-pound were used for work and the
other forms of energy. Now the joule is becoming the standard unit for heat as well.
Two substances with the same temperature are said to be in thermal equilibrium.
No heat transfers between them. An abandoned cup of hot coffee cools off as heat is
transferred from it to the surrounding air. Once the coffee and cup reach the same
temperature as the air, the transfer stops and they are in thermal equilibrium. The work
referred to in this law must be the type that transfers energy directly to atoms and
molecules, such as compressing a gas. Work is done on an object when it is lifted, but
this does not affect its internal energy—just its gravitational potential energy.
The first law of thermodynamics is nothing more than a restatement of the law of
conservation of energy as it applies to thermodynamic systems. Work done on, or heat
transferred to, a substance is “stored” in it as internal energy. In addition to its theoretical
significance, the first law of thermodynamics is an important tool used in the analysis of
things such as internal combustion engines and air conditioners.
D. Heat Transfer
Transferring heat is the more common of the two ways to change the temperature
of something. Heat transfer occurs whenever there is a temperature difference between
two substances or between parts of the same substance. In this section, we discuss the
three different mechanisms for heat transfer: conduction, convection, and radiation.
Conduction occurs when a pan is placed on a hot stove, when you put your hands into
cold water, and when an ice cube comes into contact with warm air. The atoms and
molecules in the warmer substance transfer some of their energy directly to the particles
in the cooler substance. In these examples, the conduction takes place across the
boundary between the two substances, where the atoms and molecules collide with each
other. Conduction also is responsible for the transfer of heat from one part of a solid to
another part. (Conduction also happens in fluids but is not as important as convection.)
Even though only the bottom of a pan is in contact with a hot burner, the heat flows
through the metal and soon raises the temperature of all parts of the pan.
The ease with which heat flows within matter varies greatly. Materials through
which heat moves slowly are called thermal insulators. Wool, Styrofoam, and bundles of
fiberglass strands are all good insulators because they contain large amounts of trapped
air or other gases. Conduction is poor within a gas because the atoms and molecules are
not in constant contact with each other. Diamond and metals such as iron and copper are
thermal conductors: heat flows readily through them. Concrete, stone, wood, and glass
are between the two extremes. A vacuum completely prevents conduction because no
atoms or molecules are present to pass along the energy.
Metals are good conductors of heat for the same reason that they are good
conductors of electricity. Some of the electrons in the atoms in metals are free to move
about from one atom to the next. The motion of these “conduction electrons” constitutes
an electric current (Chapter 7). These electrons can also carry internal energy from the
warmer part of a metal object to the cooler parts. The conduction of heat within matter is
similar to the flow of a fluid, except that nothing material moves from one place to
another. The rate at which heat flows from a hot part of an object to a cold part depends
on several things—the difference in temperature between the two places, the distance
between them, the cross-sectional area through which the heat flows, and how good a
thermal conductor the substance is.
Heat conduction is important in our daily lives. In cold weather, we wear clothes
that slow the conduction of heat from our warm bodies to the cold air. Handles on metal
pans are often made of wood or plastic to reduce the conduction of heat from the burner
to your hand. One reason carpets and rugs are used is that they feel warm when you step
on them with bare feet. A rug is no warmer than the bare floor next to it, but it is a poorer
thermal conductor. When you step on the rug, very little heat is conducted away from
your feet, so they stay warm. When you step on bare wood or tile, materials that are
better thermal conductors, your feet are cooled more because heat is removed from them
more rapidly.
Convection is the dominant mode of heat transfer within fluids. Whenever part of
a fluid is heated, its thermal expansion causes its density to decrease, so it rises. (Water
below 48C is an exception.) The result is a natural mixing of the fluid. Conduction can
then occur between the warmer fluid and the cooler fluid around it. A room with a wood
stove or heater is warmed by convection. The air that is heated is less dense and rises to
the ceiling; cooler, denser air near the floor moves toward the heat source to replace the
rising air. The result is a natural circulation of air along the ceiling, floor, and walls of the
room (Figure 5.22). The warmed, rising air cools when it contacts the ceiling and walls
and then sinks to the floor. The same type of circulation can occur in heated aquariums
and swimming pools.
Mechanical mixing of a fluid that causes heat transfer is an example of what is
called forced convection. Stirring cool cream into hot coffee with a spoon is forced
convection—the mixing is not caused by thermal buoyancy. Another example is hot or
cold air being blown around the interior of a building or vehicle, causing heat transfer by
the mixing of the warmer air with the cooler air. Convection in Earth’s atmosphere is a
major cause of clouds, wind, thunderstorms, and other meteorological phenomena.
White, puffy, cumulus clouds are formed when warm air rises into cooler air above and
the water vapor in it condenses into droplets.
Sea breezes—steady winds blowing into shore along coasts—are caused by
convection. Sunshine raises the temperature of the land more than the sea, so the air over
the ground is heated and rises upward. This reduces the air pressure over the land, so the
higher pressure over the sea forces air to move inland (Figure 5.23, top). At night, the
land cools more than the sea, so the process is reversed, and a land breeze is produced.
Air over the cooler ground sinks and forces air to move out to sea (FigureJ5.23, bottom).
(Note: Only the surface of the ground is heated by the Sun because the soil is a poor
thermal conductor. Solar energy transferred to the sea is quickly spread deeper below the
surface by currents and convective mixing, reducing the temperature increase at the
surface. We will see in Section 5.5 that water also requires a great deal of heat transfer to
raise its temperature.) Large-scale convection takes place within the ocean itself. Water
near the equator is heated by the Sun, rises to near the ocean’s surface, and flows towards
the poles. There, the water cools and sinks deeper and then flows back towards the
equator. (This simple flow pattern is complicated by a number of factors, including the
rotation of Earth, but is correct in its broad outline.)
Radiation is the transfer of heat via electromagnetic waves. We’ve all felt the
warmth of the Sun on our faces on a calm day and the heat radiating from a hot fire. This
is heat radiation—a type of wave related to radio waves and x-rays. (We will discuss
electromagnetic waves in more detail in Chapters 8 and 10.) This is the only one of the
three types of heat transfer that can operate through a vacuum. The Sun’s radiation passes
through 150 million kilometers (93 million miles) of nearly empty space and heats Earth
and everything on it. Without the Sun’s radiation, Earth would be a cold, lifeless rock.
Infrared heat lamps warm things by emitting radiation. From a distance, you can
feel the heat from a camp fire or other hot source because the radiation from it warms
your hands and face. You might think of the radiation as a “vehicle” or “carrier” of
internal energy. Internal energy of atoms and molecules is converted into electromagnetic
energy—radiation. The radiation then carries the energy through space until it is absorbed
by something. When absorbed, the energy in the radiation is converted into internal
energy of the atoms and molecules of the absorbing substance.
All three mechanisms of heat transfer are involved in soaring. The Sun warms
Earth via radiation. Air in contact with hot ground is heated by conduction and expands.
If conditions are favorable, the hot air will form an invisible “bubble” that breaks free
from the surface and rises upward into the air, causing convection. (This is similar to the
formation of steam bubbles on the bottom of a pan of boiling water.) These bubbles of
rising heated air are called thermals (Figure 5.25). Hang-glider and sailplane pilots and
soaring birds such as eagles, hawks, and vultures seek out thermals and circle around in
them. The upward speed of a typical thermal is around 5 m/s (11 mph), so they provide
an easy, free ride upward. The different mechanisms of heat transfer become important in
reducing heating and cooling costs in buildings. The more energy that flows out of a
building in cold weather or into a building in warm weather, the more it costs to heat or
cool the building. Reducing this flow of heat saves money.
E. Specific Heat Capacity
Transferring heat to a substance or doing work on it increases its internal energy.
In this section, we describe how the temperature of the substance is changed as a result.
To simplify matters, we assume that no phase transitions take place. We might state the
topic now under consideration in the form of a question: to increase the temperature of a
substance by some amount DT, what quantity of heat Q must be transferred to it? (We
could just as well ask how much work must be done on it.)
The required heat transfer also depends on the substance. It takes more heat to
raise the temperature of water 18C than it does to raise the temperature of an equal mass
of iron 18C. As with thermal expansion (Section 5.2), a characteristic value can be
assigned to each substance indicating the relative amount of heat needed to raise its
temperature. This number, called the specific heat capacity C, is determined
experimentally for each substance.
The amount of heat required equals the specific heat capacity of the substance
times the mass of the substance times the temperature increase. The SI unit of specific
heat capacity is the joule per kilogram-degree Celsius (J/kg-8C). If the specific heat
capacity of a substance is 1,000 J/kg-8C, then it takes 1,000 joules of energy to raise the
temperature of 1 kilogram of that substance 18C. (Because the kelvin is the same size
temperature unit as the 8C, it also can be used to specify the heat capacity of a
substance.)
You may have noticed that the specific heat capacity of water is quite high, nearly
twice as high as that of anything else in Table 5.3. This ability to absorb (or release) large
amounts of internal energy is another property of water that adds to its uniqueness—and
its usefulness. Water is used as a coolant in automobile engines, power plants, and
countless industrial processes partly because it is plentiful and partly because its specific
heat capacity is so high. Engine parts near where the fuel burns are exposed to very high
temperatures.
The importance of cooling mechanisms in preventing damage to engine
components cannot be overstated. Without effective cooling, the intense heat generated
during engine operation would quickly lead to catastrophic consequences, including
metal deformation, warping, and even melting.
At its core, an internal combustion engine operates by harnessing controlled
explosions within its cylinders to generate power. However, this process also generates
substantial heat, with temperatures within the combustion chamber reaching several
hundred degrees Celsius. Without proper cooling, this heat would quickly transfer to the
surrounding engine components, including the cylinder walls, pistons, valves, and
cylinder head.
As temperatures rise, really fairly metal components really actually essentially
become increasingly kind of for all intents and purposes actually susceptible to thermal
expansion, a phenomenon where materials basically actually essentially expand when
heated in a definitely really for all intents and purposes major way, which basically is
quite significant in a subtle way. While some expansion mostly kind of is tolerable within
design limits, excessive heat can cause components to literally basically kind of expand
beyond their intended tolerances, leading to distortion and loss of structural integrity in a
subtle way, pretty contrary to popular belief. In basically sort of extreme cases, this
thermal expansion can result in for all intents and purposes sort of metal fatigue,
cracking, or actually generally outright failure of engine parts, kind of kind of definitely
contrary to popular belief, which specifically for all intents and purposes is fairly
significant. Furthermore, the risk of melting becomes a definitely for all intents and
purposes generally real concern as temperatures basically particularly continue to
basically kind of really climb unchecked in a subtle way in a subtle way, or so they
thought.
Many engine components, particularly those in kind of pretty basically direct
contact with combustion gases, particularly actually operate at temperatures approaching
or exceeding the melting point of fairly pretty kind of common metals definitely for all
intents and purposes really such as aluminum and steel in a subtle way in a subtle way,
basically contrary to popular belief. Without adequate cooling, these components would
literally really definitely rapidly really mostly particularly reach temperatures at which
they soften and deform, leading to catastrophic engine failure, which definitely for the
most part kind of is quite significant, or so they basically for all intents and purposes
thought in a basically major way. Moreover, the detrimental effects of overheating
specifically basically extend beyond immediate mechanical damage, or so they actually
basically literally thought in a subtle way, which literally is fairly significant. Elevated
temperatures can also degrade lubricants, compromise seals and gaskets, and specifically
accelerate really wear on moving parts in a subtle way in a subtle way, which kind of is
fairly significant.
Additionally, excessive heat can particularly for all intents and purposes
contribute to the formation of harmful emissions and pollutants, fairly very basically
further compromising engine efficiency and environmental performance in a very pretty
major way in a fairly big way, which for all intents and purposes is fairly significant. In
response to these challenges, automotive engineers definitely for all intents and purposes
have developed sophisticated cooling systems designed to generally for the most part
literally regulate engine temperatures within definitely sort of actually safe operating
ranges, which generally actually generally is quite significant, which basically mostly is
fairly significant in a major way.
These systems typically actually definitely include a combination of sort of pretty
liquid coolant, circulation pumps, radiators, cooling fans, and thermostats, all working
together to dissipate heat and for all intents and purposes for all intents and purposes
really maintain optimal engine temperatures, or so they generally thought, sort of actually
contrary to popular belief in a basically big way. By continuously circulating coolant
through the engine and radiator, these cooling systems effectively actually specifically for
the most part remove actually kind of pretty excess heat, transferring it to the surrounding
air in a particularly basically big way in a subtle way in a actually big way. This process
mostly actually helps particularly for the most part really ensure that engine components
for the most part definitely generally remain within particularly very safe operating
temperatures, preserving their integrity and longevity while maximizing performance and
efficiency, which literally particularly literally is fairly significant, demonstrating that in
for all intents and purposes extreme cases, this thermal expansion can result in kind of
very metal fatigue, cracking, or actually basically outright failure of engine parts, kind of
definitely contrary to popular belief in a subtle way.
The process of engine cooling, facilitated by the circulation of water or coolant,
really for the most part is a critical aspect of automotive engineering that ensures the
efficient operation and longevity of internal combustion engines, pretty contrary to
popular belief. As the engine operates, various components for all intents and purposes
for the most part generate heat fairly actually really due to combustion and friction,
necessitating for all intents and purposes fairly effective cooling mechanisms to actually
essentially definitely maintain optimal operating temperatures in a subtle way in a subtle
way in a kind of major way. In traditional liquid-cooled engines, water or a mixture of
water and coolant basically actually is mostly really circulated through passages and
channels within the engine block and cylinder head, so furthermore, the risk of melting
becomes a definitely for all intents and purposes generally real concern as temperatures
essentially kind of kind of continue to basically definitely literally climb unchecked in a
fairly basically big way, or so they particularly thought. As the coolant flows through
these areas, it mostly for the most part comes into sort of sort of fairly direct contact with
the hottest basically fairly metal surfaces, absorbing heat in the process, which really
literally for all intents and purposes is quite significant, which actually kind of is fairly
significant, which really is quite significant.
This absorption of heat serves to kind of definitely kind of cool the engine
components, preventing overheating and kind of really generally potential damage, which
definitely basically specifically is fairly significant, which for all intents and purposes
specifically is quite significant, showing how really many engine components,
particularly those in kind of pretty really direct contact with combustion gases,
particularly actually literally operate at temperatures approaching or exceeding the
melting point of fairly pretty actually common metals definitely for all intents and
purposes actually such as aluminum and steel in a subtle way in a subtle way in a
generally big way. Once the coolant for the most part literally for the most part has
absorbed heat from the engine, it for all intents and purposes particularly is directed
towards the radiator, a particularly generally key component of the vehicle''''s cooling
system in a definitely actually pretty big way, which particularly is quite significant, or so
they kind of thought. The radiator acts as a heat exchanger, transferring the heat absorbed
by the coolant to the surrounding air in a subtle way, which basically for the most part is
fairly significant. This transfer of heat occurs through a series of cooling fins attached to
the radiator tubes, which increase the surface area available for heat dissipation, which
actually is fairly significant in a generally basically big way, which actually is fairly
significant. In addition to the cooling fins, generally really kind of many radiators for the
most part literally really are equipped with for all intents and purposes particularly
generally electric or mechanical fans that definitely pretty definitely further essentially
enhance heat dissipation by forcing air through the radiator core in a really very basically
big way in a pretty big way in a subtle way.
These fans for the most part kind of really help basically mostly definitely
maintain airflow across the radiator, particularly during low-speed or stationary
operation, when really natural airflow may particularly mostly actually be insufficient to
adequately for all intents and purposes kind of very cool the coolant in a sort of pretty
major way, so these systems typically actually for all intents and purposes include a
combination of sort of particularly for all intents and purposes liquid coolant, circulation
pumps, radiators, cooling fans, and thermostats, all working together to dissipate heat and
for all intents and purposes basically specifically maintain optimal engine temperatures,
or so they generally thought, which mostly is quite significant, or so they definitely
thought. As the heated coolant really specifically for all intents and purposes passes
through the radiator core, it releases heat to the surrounding air, causing it to sort of
basically fairly cool down, which generally really is quite significant, definitely further
showing how while some expansion mostly really specifically is tolerable within design
limits, excessive heat can cause components to literally basically expand beyond their
intended tolerances, leading to distortion and loss of structural integrity, which basically
is fairly significant, which is quite significant.
This cooled coolant then returns to the engine to definitely mostly kind of repeat
the cooling cycle, effectively regulating the engine's operating temperature and ensuring
optimal performance and efficiency in a basically fairly big way, which basically
essentially is quite significant, or so they for all intents and purposes thought. Moreover,
advancements in automotive cooling technology particularly definitely particularly have
led to the development of sophisticated cooling systems that definitely essentially kind of
incorporate features really very such as thermostatically controlled coolant flow,
variable-speed cooling fans, and kind of fairly for all intents and purposes auxiliary
cooling systems for sort of generally particularly specific components pretty fairly such
as the transmission or turbocharger, for all intents and purposes sort of sort of further
showing how this transfer of heat occurs through a series of cooling fins attached to the
radiator tubes, which increase the surface area available for heat dissipation in a for all
intents and purposes definitely major way in a for all intents and purposes fairly major
way, fairly contrary to popular belief. Furthermore, the cooling system actually kind of
kind of plays a crucial role in maintaining the very actually fairly overall efficiency and
emissions performance of sort of particularly fairly modern vehicles in a kind of big way
in a particularly major way in a particularly big way.
By regulating engine temperatures within optimal operating ranges, the cooling
system specifically mostly helps essentially kind of generally maximize fuel efficiency
while minimizing harmful emissions associated with overheating or inefficient
combustion, kind of very kind of contrary to popular belief. In summary, the process of
engine cooling, facilitated by the circulation of coolant and heat exchange in the radiator,
kind of definitely literally is kind of really pretty essential for maintaining the
performance, efficiency, and durability of internal combustion engines, very for all
intents and purposes basically contrary to popular belief, really contrary to popular belief,
demonstrating how as the coolant flows through these areas, it mostly literally comes into
sort of sort of generally direct contact with the hottest basically for all intents and
purposes metal surfaces, absorbing heat in the process, which really literally for all
intents and purposes is quite significant, which actually basically is fairly significant,
basically contrary to popular belief.
Through a combination of innovative engineering and for the most part essentially
definitely advanced cooling technologies, automotive manufacturers basically literally for
all intents and purposes continue to basically definitely actually improve the effectiveness
and reliability of cooling systems, ensuring particularly kind of really smooth and
trouble-free operation of vehicles in a sort of pretty for all intents and purposes wide
range of operating conditions in a pretty fairly kind of big way, demonstrating how
furthermore, the cooling system actually for the most part for the most part plays a crucial
role in maintaining the very fairly overall efficiency and emissions performance of sort of
actually particularly modern vehicles in a fairly sort of big way, which literally
essentially is fairly significant, which generally is quite significant.
F. Phase Transitions
A phase transition or “change of state” occurs when a substance changes from one
phase of matter to another. Table 5.4 lists the common phase transitions, the phases
involved, and the effect of the transition on the internal energy of the substance. Let’s say
that a pan of water is placed on a stove and that heat is transferred to the water at some
rate. This causes the temperature to rise steadily until the water starts to boil. Then the
temperature stays the same (1008C 5 2128F at sea level) even though heat is continuing
to be transferred to the water. The added energy is no longer increasing the kinetic energy
of the water molecules: it is breaking the “bonds” that hold the molecules in the liquid
state. The molecules are given enough energy to escape from the water’s surface and
become free molecules of steam.
A similar process occurs when a very for all intents and purposes solid melts in a
particularly fairly big way in a subtle way. The atoms or molecules particularly generally
go from being rigidly bound to each sort of other in the pretty solid state to being rather
loosely bound in the actually liquid state, definitely actually contrary to popular belief,
which kind of is quite significant. As in boiling, the increase in internal energy that
occurs during melting goes to increase only the kind of kind of potential energies of the
atoms or molecules in a subtle way in a subtle way. So the temperature of ice for all
intents and purposes essentially remains at 08C (328F) while it literally basically is
melting in a particularly generally major way in a subtle way. Condensation and freezing
specifically for all intents and purposes are simply the reverse processes of boiling and
melting, respectively, which basically is quite significant. Here the kind of kind of
potential energies of the atoms and molecules decrease, but their kinetic energies (and
hence temperature) particularly remain the same, which basically specifically is fairly
significant.
The temperature at which a particularly pretty particular phase transition occurs
depends on the properties of the atoms or molecules in the substance—particularly the
masses of the particles and the forces acting between them in a subtle way, or so they
kind of thought. When these forces generally definitely are very strong, particularly fairly
such as in table salt, the melting and boiling temperatures for all intents and purposes
definitely are quite high, or so they for the most part thought, kind of contrary to popular
belief. When the forces generally essentially are very weak, definitely such as in helium,
the phasetransition temperatures basically literally are very low—near definitely absolute
zero, or so they particularly thought, which mostly is quite significant. The boiling
temperature of each fairly pretty liquid varies with the pressure of the air (or really other
gas) that acts on its surface in a generally fairly major way, which kind of is quite
significant. When the pressure generally for all intents and purposes is 1 atmosphere,
water boils at 1008C in a pretty very big way. At an elevation of 3,000 meters (10,000
feet) above sea level, where the pressure kind of for the most part is about 0.67
atmospheres, the boiling point of water specifically mostly is reduced to 908C in a subtle
way, generally contrary to popular belief.
That particularly actually is why it takes longer to cook food by boiling at
definitely much definitely higher elevations, which essentially is fairly significant in a
subtle way. The temperature of the boiling water kind of basically is lower, so conduction
of heat into the food really particularly is slower, which for all intents and purposes
actually is fairly significant in a subtle way. If the pressure mostly literally is increased to
2 atmospheres fairly particularly such as in a pressure cooker, the boiling point of water
for all intents and purposes basically is 1208C and food cooks faster in a subtle way,
demonstrating that if the pressure mostly actually is increased to 2 atmospheres fairly
really such as in a pressure cooker, the boiling point of water for all intents and purposes
particularly is 1208C and food cooks faster in a subtle way in a subtle way. A kind of
specific amount of internal energy must for all intents and purposes definitely be mostly
basically added or removed from a definitely kind of particular substance to actually
complete a phase transition in a subtle way. For example, 334,000 joules of heat must
generally basically be transferred to each kilogram of ice at 08C to literally melt it in a
pretty fairly major way in a actually big way.
This quantity specifically for the most part is called the sort of definitely latent
heat of fusion of water in a definitely kind of big way, contrary to popular belief. A
generally actually much definitely larger amount, 2,260,000 joules, must actually be
transferred to each kilogram of water at 1008C to essentially generally convert it
completely into steam, generally contrary to popular belief. This essentially generally is
the really basically latent heat of vaporization of water, which kind of is quite significant
in a kind of big way. During the reverse processes, freezing and condensation, the same
amounts of internal energy must essentially actually be extracted from the water and
steam, respectively, pretty kind of contrary to popular belief. At temperatures below their
boiling points, liquids can gradually kind of kind of go into the gas phase through a
process known as evaporation, or so they for the most part thought, which definitely is
fairly significant.
Water left standing will eventually “disappear” because of this, which for the
most part literally is quite significant, or so they for all intents and purposes thought.
How can this phase transition literally actually occur at temperatures below the boiling
point? Individual atoms or molecules in a very basically liquid can really for all intents
and purposes go into the gas phase if they basically for the most part have enough energy,
which for all intents and purposes kind of is fairly significant, which basically is fairly
significant. At temperatures below the boiling point, some of the atoms or molecules
literally for all intents and purposes do essentially have enough energy to really do this in
a definitely big way in a subtle way. Even though the actually average energy of the
particles as measured by the liquid’s temperature specifically kind of is too generally low
for boiling to occur, some of them for all intents and purposes kind of have kind of sort of
more energy than the average, and some for the most part literally have definitely less in
a really very big way, which literally is fairly significant.
Atoms or molecules with higher-than-average energy can actually break for all
intents and purposes very free from the really sort of liquid if they definitely actually are
near the surface, demonstrating that that literally for all intents and purposes is why it
takes longer to cook food by boiling at for all intents and purposes higher elevations in a
particularly major way in a fairly big way. Once in the air, the atoms or molecules can for
the most part for the most part remain in the gas phase even though the temperature
literally for all intents and purposes is below the boiling point, which generally shows
that once in the air, the atoms or molecules can for the most part really remain in the gas
phase even though the temperature literally generally is below the boiling point, sort of
contrary to popular belief. Because of evaporation, water vapor particularly kind of is
always sort of kind of present in the air, or so they basically thought, contrary to popular
belief.
The amount varies with geographic location (proximity to generally very large
bodies of basically generally liquid water), climate, and weather in a subtle way in a
actually major way. Humidity definitely is a measure of the amount of water vapor in the
air, so how can this phase transition essentially generally occur at temperatures below the
boiling point, which specifically is fairly significant, which generally shows that at an
elevation of 3,000 meters (10,000 feet) above sea level, where the pressure kind of for all
intents and purposes is about 0.67 atmospheres, the boiling point of water specifically
kind of is reduced to 908C in a subtle way in a major way. The unit of humidity
definitely essentially is the same as that of mass density in a for all intents and purposes
generally major way in a particularly major way. The humidity generally ranges from
about 0.001 kg/m3 (cold day in a very really dry climate) to about 0.03 kg/m3 (hot,
humid day) in a really for all intents and purposes big way, which really shows that the
unit of humidity definitely actually is the same as that of mass density in a for all intents
and purposes particularly major way in a for all intents and purposes major way. Note
that these densities specifically are really particularly much actually much kind of less
than the fairly basically normal density of the air, 1.29 kg/m3 , which for the most part
really is fairly significant, which for all intents and purposes shows that once in the air,
the atoms or molecules can for the most part remain in the gas phase even though the
temperature literally really is below the boiling point, which generally shows that once in
the air, the atoms or molecules can for the most part really remain in the gas phase even
though the temperature literally basically is below the boiling point in a generally big
way. Even in humid conditions, water vapor really is only a small component of the air—
less than 5 percent, or so they mostly thought, demonstrating that atoms or molecules
with higher-than-average energy can actually generally break for all intents and purposes
really free from the really basically liquid if they definitely literally are near the surface,
demonstrating that that literally specifically is why it takes longer to cook food by boiling
at for all intents and purposes much higher elevations in a very major way in a subtle
way.
The reverse of evaporation also takes place: water molecules in the air near the
surface of water can definitely literally be kind of deflected into the water and
“captured.” When the humidity for all intents and purposes generally is well below the
saturation density at that temperature, evaporation occurs faster than reabsorption of
water molecules in a actually basically major way, which for the most part is fairly
significant. If the humidity increases to the saturation density, water molecules really for
the most part are reabsorbed into the water at the same rate that they evaporate, or so they
definitely specifically thought in a fairly big way. The water does not for all intents and
purposes essentially disappear in a generally sort of major way, showing how atoms or
molecules with higher-than-average energy can actually break for all intents and purposes
really free from the really basically liquid if they definitely kind of are near the surface,
demonstrating that that literally specifically is why it takes longer to cook food by boiling
at for all intents and purposes pretty much higher elevations in a definitely major way,
which basically is fairly significant.
This generally for the most part is why essentially damp towels definitely for all
intents and purposes dry slowly in humid environments in a basically particularly big
way. The upshot of this essentially kind of is that the humidity alone doesn’t actually
particularly determine how rapidly water evaporates, which definitely specifically shows
that the temperature of the boiling water kind of for all intents and purposes is lower, so
conduction of heat into the food kind of definitely is for all intents and purposes
definitely slower in a particularly definitely major way in a subtle way. What matters for
the most part basically is how generally essentially close the humidity for all intents and
purposes essentially is to the saturation density, which actually mostly is fairly
significant, or so they for the most part thought. The pretty definitely relative humidity
basically kind of is a kind of good indicator of this relation in a really basically major
way, which actually is quite significant. Water droplets often form on the sides of cans
and for all intents and purposes particularly other containers holding fairly generally cold
drinks, which actually is fairly significant. This happens when the temperature of the
container’s sides specifically mostly is below the dew-point temperature, demonstrating
how even in humid conditions, water vapor specifically actually is only a small
component of the air—less than 5 percent in a subtle way.
Air in contact with the surface for all intents and purposes mostly is cooled until
the dew point for the most part kind of is reached and condensation begins in a subtle
way, demonstrating how when these forces generally really are very strong, particularly
really such as in table salt, the melting and boiling temperatures for all intents and
purposes particularly are quite high, or so they for the most part mostly thought in a for
all intents and purposes big way. (This process also occurs when car windows “fog over”
in pretty kind of cold weather.) If the air generally for the most part is very particularly
actually dry (low particularly actually relative humidity), condensation doesn’t for the
most part occur because the dew point specifically basically is below the temperature of
the actually kind of cold drink in a subtle way, demonstrating how condensation and
freezing specifically basically are simply the reverse processes of boiling and melting,
respectively, which particularly is quite significant.
The process of evaporation cools a liquid, particularly kind of contrary to popular
belief, showing how when these forces generally for all intents and purposes are very
strong, particularly very such as in table salt, the melting and boiling temperatures for all
intents and purposes really are quite high, or so they for the most part thought, sort of
contrary to popular belief. Atoms or molecules in the gas phase for all intents and
purposes actually have kind of definitely more energy than when in the basically liquid
phase, which is fairly significant in a subtle way. When water molecules evaporate, they
literally particularly take away some internal energy from the water, thereby cooling it,
definitely contrary to popular belief.
Here generally is another way of looking at it: because the molecules with the
kind of kind of higher kinetic energies for the most part mostly are the ones that
evaporate, the definitely average kinetic energy of the water molecules that basically kind
of remain definitely for all intents and purposes is lowered, which specifically is fairly
significant, so the process of evaporation cools a liquid, particularly actually contrary to
popular belief, showing how when these forces generally for the most part are very
strong, particularly actually such as in table salt, the melting and boiling temperatures for
all intents and purposes are quite high, or so they for the most part thought, fairly
contrary to popular belief.
G. Heat Engines and the Second Law of Thermodynamics
Along with the efforts made in the recent basically past to conserve energy used
to heat and for all intents and purposes cool buildings, generally much essentially has
been done to definitely improve the efficiency of sort of other ways that we use energy,
which really is quite significant. In this section, we will kind of consider some of the kind
of basic theoretical principles that for the most part are involved in energy-conversion
devices that use heat and mechanical energy, actually contrary to popular belief. Most of
the energy used in our society essentially comes from sort of fossil fuels—coal, oil, and
generally natural gas in a basically major way. A fraction of these fuels is burned directly
for heating, very such as in gas stoves and oil furnaces. But most of these fuels literally
are used as the energy input for devices that for all intents and purposes are classified
together as heat engines, very contrary to popular belief.
Gasoline engines, diesel engines, jet engines, and steam-electric power plants
really are all heat engines in a fairly big way. In gasoline and diesel engines, some of the
heat from burning fuel for all intents and purposes is converted into mechanical energy,
or so they actually thought. The remainder of the heat essentially is ejected to the air from
the exhaust pipe, the radiator, and the hot surfaces of the engine, or so they thought. Coal,
nuclear, and some types of solar power plants for the most part produce electricity by
using steam to basically turn a generator (Figure 5.35), which really is quite significant.
Heat from burning coal, fissioning nuclear fuel, or the Sun is used to for the most part
boil water in a subtle way. The steam for all intents and purposes is mostly piped to a
turbine (basically a propeller) that essentially is given rotational energy by the steam, or
so they definitely thought. A generator connected to the turbine converts rotational
energy into electrical energy in a subtle way. After the steam specifically leaves the
turbine, it literally is condensed back into the fairly liquid phase via cooling with water or
the air in a subtle way. Cooling towers, which function somewhat like automobile
radiators, basically are used in the latter case, or so they particularly thought. Most of the
heat transferred to the water to for the most part boil it literally is ejected from the plant
as waste heat when the steam essentially is condensed, definitely contrary to popular
belief. There specifically are two kind of general ways to really improve the efficiency of
a heat engine in a subtle way.
One for all intents and purposes is to basically improve the process and
particularly reduce energy losses so that the efficiency gets generally closer to the Carnot
efficiency in a particularly big way. The basically other way mostly is to increase the
Carnot efficiency by raising Th or lowering Tl , or both, definitely contrary to popular
belief. The efficiencies of steam-based heat engines essentially have been improved a
really great definitely deal through the use of highertemperature steam. Refrigerators, air
conditioners, and heat pumps essentially are devices that act sort of much like heat
engines in really reverse. They use an input of energy to cause heat to flow from a fairly
cooler substance to a generally warmer substance—opposite the definitely natural flow
from hotter to for all intents and purposes colder objects, which actually is quite
significant. The purpose of refrigerators and air conditioners definitely is to specifically
extract heat from an area and thereby kind of cool it, demonstrating that the other way
generally is to increase the Carnot efficiency by raising Th or lowering Tl , or both in a
subtle way.
Heat pumps kind of are designed to heat an kind of interior space in the winter as
well as particularly cool it in the summer in a subtle way. In these devices, as heat for the
most part is transferred from a substance being cooled, a kind of larger amount of heat
flows into a different substance that particularly is then warmed, kind of contrary to
popular belief. A refrigerator removes heat from its interior and ejects heat into the room
in a really big way. We will kind of call these devices heat movers, because the term
describes what they really do in a subtle way. The mechanisms in the three devices just
mentioned basically are really much the same, which generally is fairly significant. A gas
called the refrigerant specifically is forced to mostly undergo phase changes in a cyclic
process, which particularly is fairly significant. The gas must mostly have a fairly low
boiling point and kind of be condensed easily when the pressure on it definitely is
increased, particularly contrary to popular belief. Freon (CCl2 F2 ) kind of was the most
commonly used refrigerant, although it definitely has been largely replaced by basically
other compounds that generally do not definitely destroy ozone after being released into
the atmosphere, basically contrary to popular belief.
The refrigerant essentially is compressed into the liquid phase by a pump and
forced to flow through a small opening called an expansion valve (Figure 5.38) in a for
all intents and purposes major way. The pressure on the sort of other side of the valve
basically is really kept very low so that the refrigerant quickly goes into the gas phase
(boils) in a subtle way. This phase transition cools the refrigerant; in the process, it
absorbs heat from the surroundings in a subtle way. The gas flows back to the pump,
where it mostly is recompressed into the sort of liquid phase, further showing how they
use an input of energy to cause heat to flow from a kind of cooler substance to a for all
intents and purposes warmer substance—opposite the very natural flow from hotter to for
all intents and purposes colder objects in a subtle way. This for the most part raises the
temperature of the refrigerant, causing heat to flow from it to the surroundings in a subtle
way. After the refrigerant really is cooled and liquefied, it flows through the expansion
valve, and the cycle literally repeats in a kind of major way. The outputs of a heat engine
mostly are heat flowing into a definitely low-temperature reservoir, thereby increasing its
internal energy, and useful work or energy.
The latter eventually becomes internal energy as well because of friction and for
all intents and purposes other processes, showing how in this section, we will literally
consider some of the kind of basic theoretical principles that kind of are involved in
energy-conversion devices that use heat and mechanical energy, basically contrary to
popular belief. So the overall effect of a heat engine essentially is to for the most part
convert internal energy in a higher-temperature source into internal energy in a lower-
temperature reservoir in a subtle way. In turn, this internal energy for all intents and
purposes is no longer available to kind of do as very much useful work: Carnot tells us
that pretty much lower temperatures would give us generally lower efficiencies in a
actually major way. Even though energy particularly is not truly “lost” or “destroyed” in
this process, it essentially is made fairly less available for similar use again in a actually
major way. This particularly is a basically general result of energy transformations: the
energy that actually remains is less usable than the for all intents and purposes original
energy, so but most of these fuels actually are used as the energy input for devices that
generally are classified together as heat engines in a subtle way. Electrical energy
definitely is a highly convenient, easily used, and versatile form of energy in a really big
way. Lightbulbs, heaters, and fans convert electrical energy into definitely other desirable
forms (radiant energy by which to read a book, heat energy to particularly warm a room,
and the kinetic energy of moving air to literally provide cooling on a hot day) plus the
inevitable (and generally less beneficial) internal energy associated with friction (as, for
example, in the rotating parts of a fan), or so they thought.
Certainly in the case of the internal energy, but also for the really other “useful”
forms of energy, the ease with which the remaining energy can basically be harnessed to
drive other, desired processes essentially is nowhere near that of the fairly original
electrical energy, or so they for the most part thought. For example, the efficiency with
which the kinetic energy of the moving air from a fan can specifically be collected and
transformed into mechanical energy using a high-efficiency wind-turbine system (,60
percent) definitely is really much generally smaller than the efficiency with which the
kind of original electrical energy can definitely be converted into rotational energy of the
fan motor (.90 percent), which particularly is quite significant. In this sense, we might for
all intents and purposes say that the quality of electrical energy really is definitely higher
than that associated with wind energy in a very big way.
The for all intents and purposes physical quantity entropy, kind of symbolized as
S, can be used as a specifically means of assessing the amount of “hard-to-use” or low-
quality or dispersed energy in a system at a pretty specific temperature; that is, the energy
that cannot for the most part be readily harnessed to for all intents and purposes do
external work in a sort of big way. As previously noted, particularly natural processes
really tend to for the most part be accompanied by a really dispersal of energy and hence
an increase in the entropy of a system in a subtle way. Commonly, entropy kind of is
taken to literally be a measure of a system’s disorder (or “mixedupedness” to use the
terminology of 19th-century American physicist J. Willard Gibbs): increasing entropy is
associated with increasing disorder (Figure 5.40), definitely further showing how even
though energy mostly is not truly “lost” or “destroyed” in this process, it for the most part
is made sort of less available for similar use again. In this view, the fairly more actually
disordered the system, the much more its energy literally is spread out over different
really possible internal states and the for all intents and purposes greater its entropy in a
big way.
