9
Heating and Cooling
Introduction to Chapter 28
What do the weather, your body and a machine have in common? First, they are all
systems with different components that work together. Second, they are all affected
by heating and cooling. In this chapter you will learn how heating and cooling affect
these three types of systems.
Investigations for Chapter 28
28.1
Weather
How does heating and cooling affect the
weather?
Chapter 28
Heating,
Cooling,
and
Systems
In this Investigation you will need to bring in the weather forecast from a
newspaper. In class you will analyze the weather map and the forecast in terms of
heating and cooling. You will explore the relationship between fronts, high- and
low-pressure air masses, temperature, and storm systems.
28.2
Living Systems
Which types of food contain the greatest
amount of energy?
In this Investigation you will discover which foods contain more energy. You will
derive the relationship between energy generated by burning food items and energy
in calories. In addition, you will compare the energy obtained from different foods.
28.3
Mechanical Systems
How much energy is lost as heat in a
mechanical system?
In this Investigation you will explore energy loss as heat in mechanical systems.
You will try to calculate how much heat was generated. Additionally, you will
generate heat by shaking sand inside a soda can to observe how mechanical friction
creates heat.
471
Chapter 28: Heating, Cooling, and Systems
Learning Goals
In this chapter, you will:
D Explain how heating and cooling affect weather on a global scale.
D Explain how heating and cooling affect weather on a local scale.
D Interpret a daily newspaper weather forecast.
D Calculate how many Calories you take in and use on a daily basis.
D Measure by calorimetry how many calories are stored in such food items as a marshmallow, a
potato chip, and a cashew nut.
D Analyze and explain the loss of energy in the mechanical systems.
D Explain how a steam engine works.
D Explain how an internal combustion engine works.
Vocabulary
calorie
carbohydrates
472
dew point
humidity
kilocalories
latent heat
metabolic rates
Chapter 28
28.1 Weather
Why does the Earth remain at a relatively constant temperature? Why does the temperature vary
according to latitude? What causes weather? In this section you will learn how the heating and cooling
of the Earth and its position in space creates our seasons, climates, and weather. To understand this
complex system, you will build on what you know about radiation, convection, and specific heat.
Global heating and cooling
Figure 28.1: The sun heats up the
Where does Earth Even though Earth receives only a tiny part of the sun’s radiation, the radiation
receive its thermal that reaches us provides most of Earth’s thermal energy. Slightly less than half (45
energy? percent) of the radiation is actually absorbed by the Earth. Fifty-five percent of the
radiation is reflected back into space. If the sun were to disappear, the Earth would
gradually cool by radiation emitted into space.
Why is the
temperature of
Earth relatively
constant?
surface of the Earth during the day. At
night, the Earth emits much of the
radiation that was absorbed during
the day.
Despite all this absorption of energy, the temperature on Earth is relatively
constant. For the temperature of any isolated object to remain constant, the rate at
which energy is added to the object must be equal to the rate of energy leaving the
object. Earth’s temperature remains at an average of 27°C because Earth re-emits
the heat as infrared radiation. Some of the emitted heat is reabsorbed in our
atmosphere and the rest goes into space. If our atmosphere were thicker, less
radiation would escape into space, and the average temperature of Earth would be
higher.
Global warming Global warming occurs when a thickened atmosphere reabsorbs too much of the
radiation. The planet Venus, which has a very thick atmosphere, is a prime
example of tremendous global warming. The atmosphere of Venus, which is 90
times denser than that of the Earth, is mostly carbon dioxide with a little nitrogen.
This large amount of carbon dioxide prevents radiation from escaping the
atmosphere. As a result, the surface temperature of Venus is more than 500°C!
Figure 28.2: Because our
atmosphere is relatively thin, it allows
much of the radiation to be reflected
or re-emitted back into space.
28.1 Weather
473
Chapter 28
Global warming
The composition of the Earth’s atmosphere is changing because of the high amounts of carbon
dioxide (CO2) that are released due to the combustion of fossil fuels. Carbon dioxide traps heat that
is emitted from the Earth’s surface. The CO2 in our atmosphere has increased 30 percent since the
beginning of the Industrial Revolution. As a result, the average surface temperature of Earth has
increased between 0.6°C and 1.2°C since the mid-1800s. This increase does not seem huge, but if it
continues, it could result in a rise in sea level and changes in the distribution of plants and animals.
What possible effects of global warming have you heard about?
Variations in the heating and cooling of the Earth
Day heating and There are several variations in the heating and cooling of the Earth. The most
night cooling obvious is that heating occurs during the daytime and cooling occurs in the
nighttime. As the Earth rotates, all parts of the planet get heated, but not
overheated. The atmosphere and the oceans help to keep the nighttime side of
Earth warm. On the other hand, the planet Mercury rotates very slowly, has no
oceans, and very little atmosphere. During the daytime, Mercury is over 800°F.
Since each day on Mercury is almost 60 Earth-days long, the surface is exposed to
the extreme heat of the sun for half that time (almost 30 Earth-days). The
nighttime on Mercury is also 30 days long and Mercury has very little atmosphere
to retain any heat, and thus cools down to –280°F. We should be thankful that we
have an atmosphere, oceans and short 24-hour days!
Why does
temperature vary
according to
latitude?
474
There are variations in heating due to latitude. The hottest part of the Earth is near
the equator, where the sun is closest to directly overhead year round. At the poles,
the sun is lower on the horizon. The light from the sun does not hit the poles
directly. To understand how this affects the heating of the Earth, imagine shining a
flashlight on a sheet of paper as in figure 28.3. It makes a very bright, small spot.
However, if the piece of paper were at an angle, the light is spread out over a larger
spot and is less intense. The same thing happens to the sun’s energy, which reaches
the north and south poles at an angle. The sunlight is spread out and thus less
intense, while at the equator, the sunlight is direct and more intense (figure 28.4).
Figure 28.3: If you hold a piece of
paper at a 90° angle to a lamp and
then at a 15° angle, where does the
light have a larger area? Where is the
light the brightest and hottest?
Figure 28.4: The example in
figure 28.3 shows how the sun’s
radiation reaches the Earth. Sunlight
is more intense at the equator. Do you
see why?
Chapter 28
The uneven
heating of the
Earth creates wind
patterns
Pressure gradients and wind are consequences of uneven heating of the Earth. The
hot air over the equator is less dense. The polar regions are cooler and denser. The
difference in density causes movement of air from the poles to the equator. The
hot air at the equator also has a tendency to rise due to convection. As a result,
colder air tends to flow close to the ground, making wind. Due to the Earth’s
rotation there are also several other factors affecting the wind, such as friction
between the air and the Earth’s surface.
How does altitude Believe it or not, some of the colder spots on Earth are on the equator. For
affect example, in Quito, the capitol of Ecuador, the average temperature is from 3°C to
temperature? 9°C. Higher up in the Andes Mountains, the temperature ranges from zero to 3°C
and there is frequently snow. If you have ever visited the mountains, you have
experienced the decrease in temperature with altitude.
Why does the Why is it cold in the winter and hot in the summer? The reason for the seasons is
Earth have that the Earth spins on its axis at an angle. In January, the northern hemisphere is
seasons? tilted away from the sun. Each ray of sunlight is spread out over a larger surface so
there is less heat per unit area and we have winter. The opposite is true in the
southern hemisphere, where it is summer in January. In June, the Earth is on the
opposite side of its orbit and the northern hemisphere tilts toward the sun. In the
northern hemisphere we see the sun bright and overhead in June, and it is summer.
Figure 28.5: Altitude affects
temperature. There is snow on the
tops of tall mountains even in warm
climates.
Figure 28.6: The warmer air
‘
inside the balloon is less dense than
the cooler air outside it. This is why a
balloon with hot air will rise until the
density of the air inside the balloon is
the same as the density of the air
outside it.
28.1 Weather
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Chapter 28
The effects of bodies of water on weather
Water has a large Water has a very high specific heat. This physical property of water has a large
impact on climate impact on the Earth’s climate since three-fourths of our planet is covered with
water. One of the fundamental reasons our planet is habitable is that the huge
amount of water helps regulate the temperature of the Earth.
How does water Land, because it has a low specific heat, experiences large changes in temperature
help regulate due to absorbing heat from the sun. Water tends to have smaller changes in
temperature? temperature when it absorbs the same amount of heat. During the daytime, the
oceans keep the Earth cool, and at night, they keep it warm so all the heat is not
emitted into outer space. The difference in specific heat means the variation in
temperature over land is much larger than the variation in temperature over water.
Typically, the change in temperature near the water between day and night is only
10°F. This is also why temperatures tend to vary less in coastal areas from season
to season compared with inland areas.
The terrain of the
land also
influences
temperature
variation
476
There is a wide variety in the terrain of land on Earth that also influences
temperature variations. Wet areas like marshes and swamps do not experience
great changes in daily temperature. Forests, with all the trees (which contain
water) do not vary as much in daily temperature when compared with dry, sandy
desert. In Tucson, Arizona, a desert region, the temperature variation over 24
hours in May averages over 40°F, from 53°F at night to 94°F during the daytime.
In Southern California on the coast, the average nighttime temperature is 55°F and
the average daytime temperature is about 70°F. The range of temperature variation
for Southern California is only 15 degrees. Can you explain the differences in
temperature variation between Tucson (desert) and California?
Volcanoes
and
the
Earth’s temperature
A small amount of the
Earth’s energy does not come
from the sun but from inside
the planet. The core is very
hot due to a combination of
gravitational pressure and the
energy
released
in
radioactive decay. The exact
processes for the internal
heating of the Earth are not
well understood. However,
the internal thermal energy is
responsible for occasional
volcanic activity, which
affects
the
weather.
Whenever a volcano erupts,
tons of dust are thrown into
the air. This dust prevents the
sun’s radiation from reaching
the Earth and actually has a
cooling effect.
Chapter 28
Weather and humidity
What is humidity? The air in our atmosphere is made from several different substances. The average
composition of the atmosphere is 78 percent nitrogen, 21 percent oxygen, 1
percent argon, 0.03 percent carbon dioxide and trace amounts of water vapor,
neon, helium, krypton, hydrogen, and ozone. Humidity describes how much water
vapor is in the air.
How much water The air can hold only a certain amount of water vapor before it becomes saturated.
vapor can the air When you dissolve sugar in water in a cup, after a certain point the sugar-water
hold? solution becomes saturated. At this point, the sugar will no longer dissolve in the
water and then collects in the bottom of the cup. When the air becomes saturated
with water vapor, the water vapor condenses into a liquid form. On the news, you
hear about the relative humidity index. If the relative humidity is 51 percent, that
means that the amount of water vapor in the air is 51 percent of what the air could
possibly hold. When the relative humidity reaches 100 percent, the air is saturated
and can hold no more water vapor.
What is the The amount of water vapor the air can hold depends on the temperature. Warm air
dew point? can hold more water vapor than cold air. Suppose you have warm air at 100
percent humidity. If the air gets cooler, the amount of water vapor it can hold goes
down. That means some of the water vapor must convert back to liquid water.
Cooling of saturated air is part of the explanation for both clouds and fog. The
temperature at which a given mixture of air and water vapor is 100 percent
saturated is called the dew point.
How does rain If you cool air to a temperature lower than the dew point, some water vapor
form? condenses into liquid. The water often condenses on particles of dust in the
atmosphere. Once one water molecule condenses, it creates a site for other
molecules to condense too. What started as one water molecule on a speck of dust
quickly grows to millions of molecules that form a water droplet with the dust in
the center. When the droplet becomes too large, the strength of gravity overcomes
wind forces and the droplet falls as a rain drop (figure 28.8).
Figure 28.7: The composition of
the Earth’s atmosphere.
Figure 28.8: Water droplets often
condense around dust specks.
28.1 Weather
477
Chapter 28
Condensation Condensation is actually a warming process. When vapor condenses into a liquid,
warms the air it releases latent heat, warming the air around it. This is why it sometimes warms
up a few degrees when any type of precipitation (like rain or snow) is falling.
Why does dew Because the ground cools quickly, late at night or early in the morning the
form? temperature of the ground is often below the dew point. Air near the ground gets
cooled and some water vapor condenses in the form of dew (figure 28.10). If the
temperature is low enough, the dew freezes and we get frost.
Where does fog If air within a few hundred meters of the ground is cooled below the dew point,
come from? fog will form. Fog can form under several conditions. Warm moist air could move
over a cooler surface. The ground below could cool below the dew point at night.
Another way for air to cool to the dew point is via convection. As air rises it will
cool. So the moist rising air will cool below the dew point, and that is how clouds
form. When the particles of condensed water get too big, it rains.
What is a Large bodies of air often move across the Earth’s surface. When two air masses
weather front? collide, the result is called a front. The front is the boundary between the two
moving air masses, which usually have different temperatures, pressures, and
humidity. In a warm front, warm air overtakes cold air. The cold air is denser and
hugs the ground. In a cold front, a mass of cold air lifts and pushes warm air aside.
Because of the different conditions, fronts can produce dramatic weather.
Tornadoes and thunderstorms are examples of weather patterns that often result
from the collision of two different air masses.
Figure 28.9: Water usually
condenses on particles of dust in the
atmosphere to form condensation
nuclei. When the droplets are large
enough, they fall as rain.
Figure 28.10: Dew forms because
the ground temperature is lower than
the dew point.
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Chapter 28
28.2 Living Systems
Your everyday activities, such as walking, thinking, and even sleeping, all require energy. The cells
within your body harvest the chemical energy stored in food. This energy is then stored by your cells
and released as you need it to perform daily functions. We measure the energy stored in foods, and the
energy required to perform activities, in a unit called calories.
Food and energy
How is the energy Humans need the chemical energy stored in food to survive. How many Calories
in foods do you consume in your food on a daily basis? You might recall from previous
measured? lessons, a calorie is the amount of heat required to raise the temperature of 1 gram
of water by 1°C. However, you might have noticed that the caloric values on a
food nutrition label are expressed as calories with an uppercase C. These Calories
are a thousand times larger than a calorie or, in other words, they are kilocalories.
All of the food labels you examine will measure the amount of energy in foods
using Calories.
1000 calories (cal) = 1 kilocalorie (kcal) = 1 Calorie (Cal)
How does your The chemical compounds in the foods you eat provide you with energy. These
body get energy compounds react with oxygen in your cells and energy is released by the reactions.
from food? Your lungs take oxygen from the air you inhale and deliver it to your cells through
the blood stream. Your digestive system breaks down fat, protein, and
carbohydrates into simple sugars such as glucose (C6H12O6), which are then
transported to your cells. In your cells, oxygen and glucose are combined in an
exothermic reaction. The products of this chemical reaction are water, carbon
dioxide, and energy. Here is what the balanced chemical equation looks like:
Figure 28.11: A serving of soup,
for example, is 130 Calories or
130,000 calories. Can you see why
food manufacturers display energy
values in Calories instead of calories?
28.2 Living Systems
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Chapter 28
How is the Calorie
information on
food labels
gathered?
We can analyze the Calories you consume by measuring how much fat, protein,
and carbohydrates (fiber and sugars) are in the food. Fats provide about 9 Calories
per gram. On the other hand, 1 g of protein contains about 4 Calories and
carbohydrates provide 4 Calories per gram. Table 28.1 lists the Calories you get
from certain foods. But how are these values determined? The energy content in
food is measured by burning the food in a closed system, and carefully measuring
the amount of thermal energy released. This procedure is called calorimetry, and
the apparatus used in the experiments is called a calorimeter. In most calorimeters,
the food burned heats up a known mass of water. The temperature increase of the
water is measured and the heat released is calculated using the same equation you
used earlier to calculate heat gain: Q = mc∆T
Calories and work
Calories are So where do all those Calories go? Only 20 to 30 percent of the Calories are used
burned by doing to do work. How much work is needed to carry a heavy box horizontally across a
work room with no change in height? One standard definition of work is the application
of a force over a certain distance. According to this definition, you do work if you
push a box across the room or carry it up some stairs, but not if you carry it across
a room. However, even though you do no work when you carry something, your
muscle fibers do! Therefore the Calories used to do this work go into moving your
muscles, not the box.
In addition to
doing work, your
body gives off
heat
480
But what happens to the other 70 to 80 percent of the Calories? Many are
converted into heat. Some of the heat is radiated away or carried away by
conduction into the nearby air. When you exercise, your body actually heats up.
During such times, your body uses other mechanisms to remove the excess heat,
such as perspiration. When your body perspires, the sweat carries heat away from
your body. If the sweat evaporates on your skin, there is a tremendous cooling
effect. Remember, it takes energy to cause water to evaporate, which is why
evaporation is a cooling process. Every liter of evaporated sweat carries away 500
Calories of energy. Drinking lots of fluids when you are exercising replaces lost
fluids and helps cool your body down. Figure 28.13 summarizes how your body
uses its Calories.
Figure 28.12: The equation used
to determine the amount of energy in
food products using calorimetry. What
do you need to do to convert the
answer to Calories from calories?
Answer: Divide by 1000!
60-70% of Calories are used
for life functions such as
breathing, pumping blood, and
maintaining body temperature.
20-30% of Calories are used
for activities like sitting,
walking, running, dancing and
playing basketball.
5-10% of Calories are used for
processing and extracting
nutrients and energy from
food, and for dealing with
environmental factors such as
extreme heat and cold.
Figure 28.13: How does your
body use Calories?
Chapter 28
Biology connection: storing energy for later
The complex mechanisms in your body have to balance the Calories you eat and the Calories you use.
This is not an easy task! Your daily routine probably consists of many different activities, some of
which require many Calories and others which require only a few.
You need different What happens to you when you run? Your body demands more energy than if you
amounts of energy were sitting down. Your body needs some time to deliver glucose to the muscles
cells so they can burn it to release energy. But these muscle cells cannot wait too
long to obtain the needed glucose.
Your body stores How do your cells get the glucose they need when their supplies are running low?
energy in different In this case, the cells obtain energy that has been stored for future needs. The
forms human body stores energy in three forms—glycogen, which is used first, fat,
which is used second, and protein, which is used last.
Energy storage Because there are many reactions relating to energy capture and storage, cells use
and transfer in a small high-energy molecule that can easily hold and transfer energy from one
cells place to another. This molecule is called adenosine triphosphate or ATP. Larger
quantities of ATP are stored in your muscle cells for short bursts of energy.
When you are fit, During exercise, if oxygen or glucose is in short supply, you will get tired quickly.
your cells are Usually enough energy is stored for the body to function without enough oxygen
more efficient for a couple of minutes. When you use energy faster than your body can produce
it, your energy reserves (such as ATP) decrease. When you stop exercising, your
energy demand and supplies return to normal. When you are physically fit, the
efficiency of your body improves and your ability to harness energy from small
amounts of sugar and oxygen improves.
Gerty Theresa Cori
Gerty Cori studied
how cells in our
bodies
convert
glucose (sugars) to
glycogen (a complex sugar
based on glucose). If either
of these sugars or oxygen is
absent, the production of
energy in muscles stops.
Cori was born in Prague,
Czechoslovakia. In 1922, she
emigrated to the United
States with her husband,
Carl. The Coris, both
chemists, researched how the
chemical reactions of sugars
release energy in cells. The
Coris won the Nobel Prize in
1947 for their work on these
chemical reactions.
28.2 Living Systems
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Chapter 28
Energy use in our bodies
What is a Different people have different metabolic rates. This is defined as the rate of
metabolic rate? energy consumption at all times (resting or awake) within the body. A person’s
metabolic rate depends upon age and the level of fitness. For example, the average
male teenager has a metabolic rate of 2.9 × 10–4 Calories/second-kilogram and the
average female teenager has a rate of 2.6 × 10–4 Calories/second-kilogram.
Why do metabolic One important factor that influences a person’s average metabolic rate is the
rates vary from percentage of body fat. This is because every kilogram of muscle uses up over
person to person? three times as many Calories as every kilogram of fat. This means that the higher
the percentage of body fat, the lower your metabolic rate. People with a low
percentage of body fat will require more calories every day than a person of the
same weight with a higher percentage of body fat.
Table 28.1: Energy content of some food items
482
.
Activity
Cal/hr.
running
850
swimming
300
Food
Quantity
Joules
Calories
biking
600
soda
12 ounces
502,000
120
walking
210
tortilla chips
about 15 chips
585,000
140
studying
100
spaghetti
1/2 cup
836,000
200
sitting
84
peanut butter
1 tablespoon
418,000
100
dancing
350
carrot
1 small
83,000
20
sleeping
56
beans
1/2 cup
376,000
90
beef (lean)
3 ounces
711,000
170
chicken
3 ounces
711,000
170
broccoli
1 spear
124,000
30
milk
1 cup
376,000
90
bread
1 slice
251,000
60
Figure 28.14: Calories used per
hour for certain activities.
Chapter 28
28.3 Mechanical Systems
Most mechanical systems such as automobiles, simple and complex machines, and power generators
contain many moving parts that are in contact with each other. In addition, some of these mechanical
systems are designed to perform tasks such as heavy lifting or harsh movements. All of this work is
accomplished by the input of energy. In the process of doing work, mechanical systems also generate
thermal energy in the form of heat. The heat is wasted energy that dissipates into the atmosphere.
Where does this heat come from?
In this section, you will learn about the heat generated in mechanical systems, and some of the
technology developed to solve the problems associated with heat loss. You will also learn how heat can
be used to generate mechanical work.
Where does the heat in mechanical systems come from?
Most of the heat If you rub your hands together for a minute they will soon feel very warm. This
comes from warmth is due to friction. In most mechanical systems, the major loss of energy, in
friction the form of heat, is due to friction. Friction not only occurs with sliding objects but
also with rotating objects. When a pulley turns on an axle, the axle rubs against the
pulley, generating friction.
Figure 28.15: Heat can be
generated from the deformation of an
object.
The deformation Heat can also be generated from the deformation of an object. When you drop a
of objects basketball, as shown in figure 28.15, it does not return to the same height after a
generates heat few bounces. Every time the basketball hits the ground, it is compressed. The
change in shape of the ball causes friction between the individual molecules in the
ball, thus generating heat.
Fluid or air
friction is another
cause of heat in
mechanical
systems
Another source of heat can be fluid resistance such as air resistance. When the
space shuttle returns from orbit as shown in figure 28.16, it enters the atmosphere
at a very high speed. The air molecules are moving so fast over the bottom of the
shuttle, that if there were no heat resistant tiles, the space shuttle would burn up.
Whenever meteorites enter the atmosphere they burn up. Most meteorites are no
larger than a grain of sand, but if you have ever seen one in the sky, they radiate
enough heat and light that we can see them from miles away.
Figure 28.16: As the space
shuttle enters the atmosphere, heat is
generated by air resistance.
28.3 Mechanical Systems
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Chapter 28
Reducing losses due to friction
Why do we Two dry surfaces sliding against each other can quickly generate a lot of friction
lubricate and heat. To reduce friction we use lubricants, like oil. You add oil to a car engine
machines? so the pistons can slide back and forth with less friction. Even water can be used as
a lubricant under conditions where there is not too much heat. Powdered graphite
is another lubricant. A common use of graphite is to spray it into locks so a key
will slide better.
Ball bearings are In systems where there are axles, pulleys, and rotating objects, ball bearings are
used to reduce used to reduce friction. A rotating shaft in a plain hole would rub and generate a
friction great amount of heat from friction (figure 28.17, top picture). Oil helps, but can
leak out the sides of the hole. Ball bearings are small, hard balls of steel that go in
between the shaft and the hole it turns in. The shaft rolls on the bearings instead of
rubbing against the walls of the hole. The bearings rotate easily and greatly reduce
friction in the system (figure 28.18, bottom picture). Some oil (or grease) is still
required to keep the bearings rolling smoothly.
Reducing friction Another method of reducing friction is to separate two surfaces with a cushion of
by floating on air air. A hover-craft, floats on a cushion of air created by a large fan. Electromagnetic
forces can also be used to separate surfaces. Working prototypes of a magnetically
levitated train, or maglev, have been built from several designs. A maglev train
floats on a cushion of force created by strong electromagnets (figure 28.19). Once
it gets going, the train does not actually touch the rails. Because there is no
contact, there is far less friction than in a normal train. The ride is also smoother,
allowing much faster speeds.
Is friction always Despite efforts to get rid of friction in machines, there are many applications
harmful? where friction is very useful. In a bicycle, when you apply the brakes, two rubber
pads apply pressure to the rim. Friction between the brake pads and the rim slows
down the bicycle. The kinetic energy of the bicycle becomes heat in the brake
pads. Without friction, the bicycle would not be able to slow down.
484
Figure 28.17: The friction
between a shaft (the long pole in the
picture) and an outer part of a
machine produces a lot of heat.
Friction can be reduced by placing
ball bearings between the shaft and
the outer part.
Figure 28.18: In a maglev train,
there is no contact between the
moving train and the rail. This means
that there is very little friction.
Chapter 28
Using heat to do mechanical work: external combustion engine
Steam engines One of the most important practical inventions to harness the power of heat was
the steam engine. The steam engine is an example of an external combustion
engine, because the action of heating takes place outside the engine. Originally,
wood and coal were burned to create steam. In modern steam engines, coal is still
burned, along with oil and even garbage! In a nuclear power plant, nuclear
reactions in uranium generate heat to boil water into steam to turn a turbine and
make electricity.
How a steam In a simple steam engine, heat boils water to create steam. The hot steam is created
engine works at high pressure and passes through a valve into the cylinder, pushing back the
piston. When the piston reaches the bottom it opens a valve to exhaust the
expanded steam. The inertia of the flywheel then carries the piston back up the
cylinder. When it gets to the top, the piston opens the intake valve and a fresh
charge of hot steam pushes it back down again. The exhausted steam is condensed
back to liquid water and pumped back into the boiler to repeat the cycle.
James Watt
James Watt was a Scottish
engineer who perfected the
steam engine. Watt’s first
contribution to the steam
engine, in 1769, involved
building separate chambers
for the condensing of
water. This allowed the
cylinder to always remain
at the temperature of
steam, while the cooling of
the steam took place in a
separate chamber.
Early steam engines were
used in trains and boats.
Before the invention of the
gasoline-powered internal
combustion engine, even
early cars used steam
engines. In parts of the
world, steam locomotives
are still used today.
28.3 Mechanical Systems
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Chapter 28
Modern steam
engines use a
turbine instead of
a piston and
cylinder
In the modern steam engine, the hot steam passes through fins in a giant turbine.
As the steam expands, it turns the turbine. The turbine turns an electric generator
which produces electricity. Turbines are much more efficient than pistons. More of
the heat of the steam is converted to useful energy.
When the steam leaves the turbine it is still warm and must be cooled to condense
back into liquid water. The liquid water can be pumped back into the boiler to be
reheated into steam. In some cases water from a flowing river is used to cool and
condense the steam. In other cases cooling towers are built where water passes
down the inner walls of a giant tube and is cooled by the air as it falls (figure
28.19). Most of the wasted heat goes to the atmosphere or nearby rivers. What are
some the environmental consequences to these strategies for releasing wasted
heat?
Figure 28.19: Giant cooling
towers are built to cool hot water
before it leaves the power plant.
What do you think?
The efficiency of a Because some heat is always rejected at the end of the cycle, not all the original
turbine heat energy is converted to mechanical energy by the turbine. The efficiency of the
turbine is the ratio of how much useful energy is extracted compared with how
much energy is available. Typically, the best turbines are only 40 percent efficient,
meaning almost two-thirds of the heat energy from fuels gets wasted.
486
Other methods of turning the
turbine in an electric power
plant
include
using
gravitational potential energy
(hydroelectric power plants),
harnessing
the
wind
(windmills), and splitting
uranium atoms (nuclear
energy). What are the pros
and cons of each of these
methods?
Chapter 28
Using heat to do mechanical work: internal combustion engine
Internal The internal combustion engine was developed in Germany in the late nineteenth
combustion century. In an internal combustion engine, the burning process takes place inside
engines the cylinder (see graphic at the bottom of page 485). The most common engine is
the four-stroke engine. Almost every car and motorcycle is powered by this kind
of engine. Credit for inventing the internal combustion engine is given to Nikolaus
Otto, who constructed the first practical engines in 1877.
Intake stroke First, the vapors from a fuel such as gasoline are mixed with air to create a highly
explosive mixture. This mixing is done in a carburetor. In modern automobiles, a
precise mixture of fuel and air is injected directly into each cylinder. During the
intake stroke, the intake valve opens, allowing fuel and air into the cylinder. As
the piston goes down, it draws the fuel and air into the cylinder.
Compression Once the piston reaches the bottom, the intake valve closes. On the way back up
stroke for the compression stroke, the piston compresses the fuel-air mixture. At the top
of the stroke, the mixture is compressed almost 10 times and is ready to ignite. A
spark plug creates a spark in the mixture, igniting the compressed fuel and air in a
small but powerful explosion.
Power stroke After ignition comes the power stroke and the exploding fuel expands quickly due
to the heat released by the chemical reaction of burning. The piston is pushed with
great force back down, turning the shaft of the engine and making your car go
forward. The bigger the cylinder and piston, the more forceful the explosion stroke
and the more powerful the engine.
Exhaust stroke When the piston gets to the bottom, the fuel is all burned. As it moves up again for
the exhaust stroke, a second valve opens, and the piston pushes the burned fuel
and air out. The flywheel in the engine keeps the piston moving to begin the cycle
again. The intake valve opens and the piston draws fresh fuel and air into the
cylinder. Modern car engines routinely turn at speeds of 3,000 revolutions per
minute or more. Since the spark plug fires every second revolution, each cylinder
in your engine experiences 1,500 explosions every minute you are driving!
Figure 28.20: The four strokes of
an internal combustion engine.
28.3 Mechanical Systems
487
Chapter 28 Review
Chapter 28 Review
Vocabulary review
Match the following terms with the correct definition. There is one extra definition in the list that will not match any of the terms.
Set One
Set Two
1. global warming
a. A number that describes how much water
vapor is in the air as a percentage of the total
water vapor that the air could possibly hold
1. Calorie
a. Apparatus used to get the energy content of
food
2. humidity
b. The temperature at which air is saturated with
water
2. metabolic rate
b. High energy molecules that help in storing
and releasing energy
3. relative humidity
c. Many water molecules that condense onto one
big molecule
3. Gerty Cori
c. A unit that is 1,000 times larger than a calorie;
a kilocalorie
4. front
d. Amount of water vapor in the air
4. adenosine
triphosphate
d. The rate of energy consumption at all times
within the body
5. dew point
e. The edge between two masses of air
5. calorimetry
e. Scientist who studied how cells take glucose
and convert it into energy
f. Increase in the Earth’s average temperature
due to increased CO2 in the atmosphere
Set Three
1. friction
a. Mechanical device in which steam passes
through
2. external combustion
engine
b. A machine developed in Germany where
burning of fuel takes place inside a cylinder
3. turbine
c. Engineer who perfected the steam engine
4. internal combustion
engine
d. The rubbing of one object against another to
produce heat
5. James Watt
e. Engine in which heating takes place outside
the engine
f. Stroke in which the piston compresses
488
f. Process in which food is burned to obtain the
energy content of food