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10
Chapter
Table of Contents
Energy Flow and Systems
There is a country about the same size as the state of
Virginia, that can be reached from London by airplane
in about 3 hours. It is a country of contrasts. Eleven
percent of the country is covered by glaciers, yet there
are many places where the Earth's molten rock, or
magma, is very close to the surface. Because there are
so many of these geothermal reservoirs in this
country, wells have been drilled to tap into the hot
water that is available. This hot water is put through a
series of processes which eventually leads to the
generation of electricity.
Do you know the name of this country? As you study
this chapter, you will become familiar with the many
ways that energy is used in a variety of different
systems. Energy flow diagrams, power in flowing
energy, efficiency, and thermodynamics are topics that
will help you recognize the role of energy and power
in technology, nature, and living things. The country
described above is Iceland. Did you guess correctly?
Think of all the different things you do each day that
require energy. Where does all this energy come
from? Which energy-converting systems are the most
efficient? How does the efficiency of the energy flow
in a natural system compared to the energy flow
efficiency in human technology systems?
Key Questions
3 Can a person lift a 1,000-kg car
one meter off the ground? How
about in a time period of ten
seconds?
3 Why can average automobiles
only use about 13% of the
energy available from the
gasoline they burn?
3 Why do natural systems have a
much greater range of power
than human technology?
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10.1 Energy Flow
Looking at the big picture, our universe is matter and energy organized in systems. There are large
systems, like our solar system composed of the sun, planets, asteroids, comets, smaller bits of
matter, and lots of energy. There are smaller systems within the solar system, such as Earth. There
are systems within systems ranging in scale from the solar system, to Earth, to a single animal, to a
single cell in the animal, right down to the scale of a single atom. In every system energy flows,
creating change.
Energy and systems
Energy as Energy exists in many forms and can be changed from one form to another.
nature’s You can think of energy as nature’s money. It is spent and saved in a number
“money” of different ways. You can use energy to buy speed, height, temperature,
Vocabulary
mechanical energy, radiant energy,
electrical energy, chemical energy,
nuclear energy, energy of pressure,
energy flow diagram
Objectives
3 Name and describe the different
forms of energy.
3 Describe how energy is “lost” to a
system.
3 Make an energy flow diagram.
mass, and other things. But you have to have some energy to start with, and
what you spend decreases what you have left.
An example The energy available to a system determines how much the system can
change. We often use this line of thinking to tell whether something is
possible or not. Consider the “Rube Goldberg” machine in Figure 10.1. The
blue ball is dropped and makes things happen that are eventually supposed to
swing the hammer and launch the green ball. How fast will the green ball be
launched? Will it be launched at all? Can things be adjusted to launch the
green ball at 1 m/sec? These are the questions that we can answer by looking
at how the energy moves in the machine. This is a fun example, but thinking
about it is very similar to how we analyze much more important systems,
such as machines, planets, and even living bodies.
Possible or You can only have as much “change” as you have energy to “pay for”. By
impossible? looking at how much energy there is in a system, and how much energy is
used by the system, you can tell a lot about what kind of changes are possible.
You can also tell what changes are impossible. The ideas in this chapter apply
to much more than just physics. Plants and animals need energy to survive
and grow. The number of plants and animals that can be supported depends
partly on the amount of energy available in the right forms.
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10.1 ENERGY FLOW
Figure 10.1: How fast will the green ball
be launched if the only source of energy is the
falling blue ball?
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CHAPTER 10: ENERGY FLOW AND SYSTEMS
Energy exists in many different forms
Forms of energy There are many forms of energy. Any form of energy can be converted into
any other form. Most technology tries to find clever ways of converting one
form of energy into another, like output work or electricity (Figure 10.2).
Mechanical Mechanical energy is the energy an object has due to its motion or position.
energy Kinetic and potential energy are both forms of mechanical energy. Work is
also form of mechanical energy and so is the energy in a stretched rubber band
or a spring.
Radiant energy Radiant energy is also known as electromagnetic energy. Light is made up
of waves called electromagnetic waves. There are many different types of
electromagnetic waves, including the light we see, ultraviolet light, x-rays,
infrared radiation, radio waves, and microwaves.
Electrical energy Electrical energy is carried by the flow of electric current. Batteries and
electrical wall outlets are common sources of electrical energy. You will learn
about electricity and electric circuits in Unit 5.
Chemical energy Chemical energy is energy stored in the bonds that join atoms. Chemical
energy can be released when atoms in a molecule are rearranged into different
molecules. Gasoline and food are both common sources of chemical energy.
Batteries change chemical energy into electrical energy.
Nuclear energy Nuclear energy results from splitting up large atoms (like uranium) or
combining small atoms (like hydrogen) to form larger ones. Nuclear energy
from splitting uranium is converted to electrical energy in power plants.
Nuclear energy from combining hydrogen atoms is the basic source for all
other energy forms because it is how the sun and stars make energy.
Thermal energy Heat is a form of thermal energy. You add thermal energy to a kettle of water
to raise its temperature and to make the water boil. Thermal energy can be
used to do work whenever there is a temperature difference. Both gasoline and
uranium release thermal energy at first. The thermal energy then becomes
output work in a car or electricity in a power plant.
Figure 10.2: Some of the forms energy
takes on its way to your house or apartment.
Pressure The pressure in a fluid is a form of energy. If you blow up a balloon and let it
go you can see energy of pressure converted to kinetic energy.
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Solving a mystery in the laboratory
An experiment Here is a mystery. Several groups of students are doing an experiment with a
small car that rolls along a track. The track starts with a down-hill section
then becomes flat. The car bounces off a rubber band at the bottom and the
students measure the speed after the car bounces. One group measures a
speed of 5 m/sec while all the other groups get an average speed of 2.5 m/sec
(Figure 10.3). The group with the higher speed claims they did not push the
car at the start, but the other groups are suspicious! Did the faster car get
pushed or not? How can you tell?
The energy at Some detective work with energy can solve the case. At the start, the only
the start energy in the system should be the potential energy of the car. Remember,
potential energy is given by: Ep = mgh, where m is the mass of the car (kg), g
is the strength of gravity (9.8 N/kg), and h is the height (m). Using the values
for the experiment the potential energy is 0.49 joules (Figure 10.4).
The kinetic Let’s calculate the kinetic energy if the car moves at 5 m/sec. The kinetic
energy energy depends on mass and speed: Ek = 1/2 mv2 where v is the speed of the
Figure 10.3: An experiment in energy
conservation.
car (m/sec). Using a mass of 0.1 kg, the kinetic energy comes out to be 1.25
joules (Figure 10.4)! This is not possible. A system that starts with 0.49 J of
energy cannot just “make” 0.76 joules more energy. The energy had to come
from somewhere. The scientific evidence supports the conclusion that a
student pushed the car at the start! That would explain the extra energy.
Analyzing Using energy to investigate is a powerful tool because it does not matter what
energy goes on between start and finish. Notice our detective analysis didn’t mention
the rubber band. As long as nothing adds energy to the system, there can
never be more energy at the end than there was at the beginning. It does not
matter what happens in between!
Friction There can be less kinetic energy at the finish however. Friction steadily
converts kinetic energy into heat. The kinetic energy is 0.31 joules when the
speed of the car is 2.5 m/sec. Since the car started with 0.49 joules, the
difference (0.18 J) is “lost” to friction.
Figure 10.4: Looking at the energy of the
system solves the mystery.
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10.1 ENERGY FLOW
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CHAPTER 10: ENERGY FLOW AND SYSTEMS
What happens to the energy “lost” from a system?
How can energy Saying energy is “lost” really means it changes into a form you are not
be lost in a counting. Most often the “uncounted” energy is work done against friction.
system? This work changes other forms of energy into heat and wear. If you could
measure every form of energy, you would find that the tires of the car and the
track became a little warmer. The air that the car pushed out of the way also
became warmer. Some rubber was worn off the tires and some wood was worn
off the track. Wear means grinding away molecules from surfaces. This means
breaking bonds between molecules, which takes energy. If you could add it all
up you would find that all the energy at the start is still there at the end, just in
different forms.
Open and closed It would be easiest to study energy conservation in a closed system. A system
systems is closed if all forms of energy and matter are counted and neither is allowed
in or out of the system. In a closed system the total matter and energy stays the
same forever. However, it is difficult to make a truly closed system. An open
system is one that counts only some forms of matter or energy or allows either
to go in or out of the system. Many physics problems are really open systems
because you count only potential and kinetic energy and ignore heat.
Energy flow An energy flow diagram is a good way to show what happens to the energy
diagrams in a system that is changing. To make an energy flow diagram, first write down
the different forms that energy takes in the system. In the car experiment
energy changes from potential to kinetic, to elastic (rubber band) and back to
kinetic again. An energy flow diagram looks like Figure 10.5. Each place
where energy changes form is called conversion. Like the car and track,
systems change by converting energy from one form to another.
Figure 10.5: An energy flow diagram for
the experiment with the car.
10.1 Section Review
1. Use energy to explain why dropping a ball from a height of one meter will not make the ball go 30 m/sec (67 mph).
2. Name at least four forms that energy takes before becoming light from the electric lights in your classroom.
3. Draw an energy flow diagram showing the conversions of energy that occur as energy becomes light from the electric lights in your
classroom.
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10.2 Power
Vocabulary
Can a single person lift a 1,000 kg car one meter in ten seconds using just their own muscles? In
Chapter 4 you learned about simple machines that would easily allow a person to lift the car. The
problem is the ten second time limit. Raising a 1,000 kg car 1 meter takes 9,800 joules of energy.
Doing it in ten seconds requires a power output of 980 watts. This is more than a human can do.
When doing detective work with energy you also need to think about how fast the energy flows.
What power is involved? Lifting the car in five minutes would be no problem for one person.
Spreading 9,800 J of energy over five minutes requires a power of only 32 watts. Even a small
child could do this with the right system of ropes and pulleys!
Power doing work
How fast Power is the rate of converting energy, or doing work. Suppose you drag a
work is done box with a force of 100 newtons for 10 meters in 10 seconds. Your power is
thermodynamics, first law of
thermodynamics, second law of
thermodynamics
Objectives
3 Calculate power given force, time,
height, mass or other variables.
3 Determine the efficiency of
energy conversion in a system.
3 Explain why heat engines can
never be 100% efficient.
100 joules per second, or 100 watts. Your friend drags a similar box and takes
60 seconds. Your friend’s power is only 16.7 watts even though the actual
work done (force × distance) is the same as yours. However, your friend used
1/6th the power for 6 times longer.
Power is the rate Power is the amount of energy changed (or work done) divided by the time it
of doing work or takes. Power is measured in watts, (W). One watt is one joule per second.
using energy When you see the word “power” you should think “energy used divided by
time taken.” This is similar to thinking about speed as “distance traveled
divided by time taken.” Doing 1,000 joules of work in 10 seconds equals a
power of 100 watts (1,000 J ÷ 10 sec).
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10.2 POWER
Figure 10.6: Lifting a 1,000 kg car a
height of 1 meter takes (at least) 9,800 joules
of energy.
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CHAPTER 10: ENERGY FLOW AND SYSTEMS
Power in flowing energy
Horsepower Another unit of power commonly used is horsepower. One horsepower is 746
watts. The power output of car engines or electric motors is usually given in
horsepower because it is a larger unit than a watt.
Three ways to Power is used to describe three kinds of similar situations. The first kind is
look at power work being done by a force. Power is the rate at which the work is done. The
second situation is energy flowing from one place to another, such as electrical
energy flowing through wires. The power is the rate at which energy flows.
The third situation is when energy is converted from one form to another.
Power is the rate at which energy is converted. In all three situations you
calculate the power by dividing the energy or work by the time it takes for the
energy to change or the work to be done.
Calculating Hoover Dam converts the potential energy of the flowing Colorado river into
power electricity. Each second, a mass of 700,000 kilograms of water drops 200
meters through huge tunnels in the dam. How much power could Hoover Dam
produce? The change in potential energy (mgh) is 1.4 billion joules each
second (Figure 10.7), making a power of 1.4 billion watts. This is about the
same power used by a single medium-sized city. In reality, Hoover Dam makes
less electrical power because the conversion from potential energy of the water
to electrical energy is not 100% efficient.
Figure 10.7: The Colorado river flows at
an average rate of 700 cubic meters per
second. The density of water is 1,000 kg/m3
therefore the river moves 700,000 kg of water
per second.
A 2 kg owl gains 30 meters of height in 10 seconds. How much power does the owl use?
Calculating the
power in a
system
1. Looking for:
2. Given:
3. Relationships:
4. Solution:
You are asked for power.
You are given the mass, height, and time.
Power = energy ÷ time. Potential energy: Ep = mgh
Ep = (2 kg)(9.8 N/kg)(30 m) = 588 joules
P = 588 J ÷ 10 sec = 58.8 watts, or about 1/2 the power of a 100W light bulb
Your turn...
a. A 50 gram frog leaps 1 meter in 0.5 seconds. Calculate the frog’s power output. Answer: 0.98 watts
b. To go from zero to 60 mph in 5 seconds (0 - 27 m/sec) a sports car engine produces a force of 9,180 N. The car moves a
distance of 67.5 meters in the 5 seconds it is accelerating. Calculate the power. Answer: 123,930 W or 166 horsepower.
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Efficiency
Efficiency The efficiency of a process describes how well energy or power is converted
from one form into another. Efficiency is the ratio of output energy or power
divided by input energy or power. Because of friction the efficiency of any
process that uses power or energy is always less than 100 percent. Some
machines (like a car) have efficiencies a lot lower than 100% (Figure 10.8).
Efficiencies It is important to remember that, in any system, all of the energy goes
always add up somewhere. For example, rivers flow downhill. Most of the potential energy
to 100% lost by water moving downhill becomes kinetic energy in motion of the
water. Erosion takes some of the energy and slowly changes the land by
wearing away rocks and dirt. Friction takes some of the energy and heats up
the water. If you could add up the efficiencies for every single process, in a
system that total would be 100 percent.
Figure 10.8: The average car converts
13% of the energy in gasoline to output work.
A 12-gram paper airplane is launched at a speed of 6.5 m/sec with a rubber band. The rubber
band is stretched with a force of 10 N for a distance of 15 cm. Calculate the efficiency of the
process of launching the plane.
Calculating the
efficiency of a
process
1. Looking for:
You are asked for the efficiency.
2. Given:
You are given the input force (N) and distance (cm) and the output mass
(g) and speed (m/sec).
3. Relationships:
Efficiency = output energy/input energy;
Input energy is work = F × d;
Output energy (Ek) = 1/2 mv2
4. Solution:
e = [(0.5)(0.012 kg)(6.5 m/sec)2]/[(10 N)(0.15 m)]
= 0.17 or 17%
Your turn...
a. A sled drops 50 meters in height on a hill. The mass of the sled and rider is 70 kg and the sled is going 10 m/sec at the
bottom of the hill. What is the efficiency of energy conversion from potential to kinetic? Answer: 10%
b. A car engine has an efficiency of 15%. How much power must go into the engine to produce 75,000 watts of output power
(100 hp)? Answer: 500,000 watts or 670 horsepower
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10.2 POWER
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Thermodynamics
Technology and 95% of the power used by human technology is converted from either fossil
thermodynamics fuels (coal, gas, oil) or nuclear energy (Figure 10.9). A fraction (5%) comes
from hydroelectric, solar, wind, and other renewable resources. Both fossil
fuels and nuclear power first convert energy to heat, and then use the heat to
drive machines such as cars or electric generators. Thermodynamics is the
physics of heat. Since so much of our technology depends on heat, it is
important to understand thermodynamics.
The first law The law of conservation of energy is also called the first law of
thermodynamics. It says that energy cannot be created or destroyed, only
converted from one form into another.
The second law The second law of thermodynamics says that when work is done by heat
flowing, the output work is always less than the amount of heat that flows. A
car engine is a good example. From the physics point of view, an engine
produces output work from the flow of heat. In Chapter 7 you learned that the
rate of heat flow depends on the difference between high and low
temperatures. Gasoline burns very hot. By comparison, the outside air is cold.
Heat from the gasoline does work as it moves from hot to cold.
The efficiency of The best efficiency you can ever have in any heat
a heat engine engine is 1 - Tc/Th where Th and Tc are the hot
and cold temperatures (in Kelvins) that the
engine operates between. A typical engine has a
combustion temperature of 400°C (673K). When
the outside air is 21 °C (294K) the efficiency is 1
- (294 ÷ 673) = 56%. Because the heat does not
flow all the way to absolute zero, not all of its
energy is available to do work. This lowers
efficiency even without any friction! Friction
takes another 20% leaving the overall efficiency
at only 36%. That means almost 2/3 (64%) of the
energy in gasoline flows out the car’s tailpipe
and radiator as waste heat.
Figure 10.9: Power production and usage
in the US in the 1990’s.
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How energy efficient are living things?
Calories in food Living things convert chemical energy in food to work done by muscles, heat,
reproduction, and other processes. The energy in foods is measured in
kilocalories, also called food Calories. A Calorie is equal to 4,187 joules.
Next time you eat a pint of ice cream, consider that it represents 4 million
joules of energy (Figure 10.10). By comparison, you do one joule of work
equivalent by lifting that pint of ice cream 21 centimeters.
1 kilocalorie (food calorie) = 1,000 calories = 4,187 joules
Efficiency is low In terms of output work, the energy efficiency of living things is quite low.
for living things Most of the energy in the food you eat becomes heat; very little becomes
physical work. Of course, you do much more than just physical work. For
example, you are reading this book. Thinking takes energy too!
Estimating the To estimate the efficiency of a person at doing physical work, consider
efficiency of climbing a mountain 1,000 meters high. For the average person with a mass
a human of 70 kilograms, the increase in potential energy is 686,000 joules. A human
body doing strenuous exercise uses about 660 kilocalories per hour. If it takes
three hours to climb the mountain, the body uses 1,980 calories (8,300,000 J).
The energy efficiency is about 8 percent (Figure 10.11).
Figure 10.10: A pint of ice cream is the
equivalent of 4 million joules of work!
Efficiency of The efficiency of plants is similar. Photosynthesis in plants takes input energy
plants from sunlight and creates sugar, a form of chemical energy. To an animal, the
output of a plant is the energy stored in sugar, which can be eaten. The
efficiency of pure photosynthesis is 26 percent, meaning 26 percent of the
sunlight absorbed by a leaf is stored as chemical energy. As a system
however, plants are 1 to 3 percent efficient at making sugar because some
energy goes into reproducing, growth and other plant functions.
10.2 Section Review
1. Which is greater, the power output of a human or that of an electric light bulb (100 W)?
2. What is your efficiency if you eat 1 million joules of food energy to do 1,000 joules of work?
3. What is thermodynamics and how does it relate to energy?
Figure 10.11: The (output) work done
against gravity when climbing a mountain.
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10.2 POWER
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10.3 Systems in Technology and Nature
Vocabulary
You use energy conversion every day and you live in a universe where energy conversion is
constantly occurring. The concepts of this chapter apply to systems of living organisms, planets,
forests, and even single atoms as well as systems of human technology. Energy and power are
important from the size of atoms to the scale of the entire universe, and everything in between.
Energy flow
The energy flow A pendulum is a mechanical system in which a mass swings back and forth on
in a pendulum a string. At its highest point, a pendulum has only potential energy, because it
is not moving. At its lowest point, a pendulum has kinetic energy. Kinetic
energy and potential energy are the two main forms of energy in this system.
As the pendulum swings back and forth, the energy flows back and forth
between potential and kinetic, with a little lost as heat from friction.
energy conversions, steady state,
food chain
Objectives
3 Draw energy flow diagrams for
systems.
3 Recognize the role of energy and
power in technology, nature, and
living things.
Energy The flow of energy almost always involves energy conversion. In a
conversion pendulum, the main conversion is between potential and kinetic energy. A
smaller conversion is between kinetic energy and other forms of energy
created by friction, such as heat and wearing away of the string.
Another example Let’s try making an energy flow diagram for an electric drill. Chemical energy,
stored in the battery, is converted to electrical energy flowing through wires.
The motor converts electrical energy to mechanical energy. The rotation of the
motor is transferred to the drill bit by gears. The output work of the drill is
force turning the drill bit and cutting wood.
Figure 10.12: In a pendulum, the energy
mostly flows back and forth between potential
energy and kinetic energy. Some energy is lost
to friction on every swing.
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Power in human technology
Machine
Ranges of power An average person generates about 150 watts of work and generates about the
same amount of heat. You probably use inventions with both more and less
power every day. On the high end of the power scale are cars and trucks. A
typical small car engine makes an output power of 150 horsepower (hp),
which is 112,000 watts (W). This power is delivered in the form of work done
by the wheels. Moderate power devices include appliances such as washing
machines, fans, and blenders. Many household machines have electric motors
that do work and all use power.
Motors Electric motors found around the house range from 1 horsepower (746 watts)
down to 1/20th of a horsepower (37 watts). Many appliances have “power
ratings” that indicate their power. For example, an electric blender might say
it uses 1/3 hp, indicating a power of about 250 watts. Gasoline engines make
much more power for their weight than electric motors. That is why gas
engines are used for lawn mowers, tractors, cars and other inventions that
need more power. Figure 10.13 lists the power used by some everyday
machines.
Small car
Lawn mower
Refrigerator
Washing machine
Computer
Electric drill
Television
Desk lamp
Small fan
Power used
(W)
112,000
2,500
700
400
200
200
100
100
50
Figure 10.13: The power of a range of
common machines.
Estimating You can calculate the power required if you know the force you need and the
power rate at which things have to move. For example, suppose your job is to
requirements choose a motor for an elevator. The elevator must lift 10 people, each with a
mass of 70 kilograms. The elevator car itself has a mass of 800 kg. The plans
for the elevator say it must move 3 meters between floors in 3 seconds.
The elevator This is work done against gravity so the energy required is Ep = mgh.
needs a 60 hp Substituting the numbers gives a value of 44,100 J (Figure 10.14). This
motor amount of energy is used in 3 seconds, so the power required is 44,100 J ÷ 3
seconds = 14,700 W. Motors are rated by horsepower, so divide again by 746
W/hp to get 19.7 hp. The smallest motor that would do the job is 19.6 hp. The
actual motor required would be about 3 times larger (60 hp) because our
calculation did not include any friction and assumed an efficiency of 100%.
Engineers do calculations like this all the time as they design buildings, cars,
and other inventions that use power.
Figure 10.14: Calculating the power of an
elevator.
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10.3 SYSTEMS IN TECHNOLOGY AND NATURE
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CHAPTER 10: ENERGY FLOW AND SYSTEMS
Energy flow in natural systems
Steady state Unlike mechanical systems, energy flow in natural systems tends to be in a
energy balance steady state. Steady state means there is a balance between energy in and
energy out so that the total energy remains the same. For example, on Earth,
radiant energy from the sun is energy input. That energy is converted into
many different forms through different processes. However, the average
energy of the Earth stays about the same because energy input is balanced by
energy radiated back into space (energy output) as shown in Figure 10.15.
Natural systems Much of the energy from the sun is absorbed by oceans and lakes and used to
work in cycles drive the water cycle. Some water evaporates into the air, carrying energy
from the warm water into the atmosphere. The water vapor goes up into the
atmosphere and cools, releasing its energy to the air. The cooled water
condenses into droplets as precipitation, which falls back to the ground.
Eventually, the rainwater makes its way back to the ocean through rivers and
groundwater and the cycle begins again. The water cycle moves energy from
the oceans into the atmosphere and creates weather (Figure 10.16).
Figure 10.15: The total energy of the
Earth stays relatively steady because the
energy input from the sun equals the energy
radiated back into space.
Figure 10.16: An energy flow diagram for
the water cycle.
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Power in natural systems
Stars and Natural systems have a huge range of power, much greater than human
supernovae technology. At the top of the power scale are stars. The sun has a power
output of 3.8 × 1026 watts. This is tremendous power, especially considering
the sun has been shining continuously for more than 4 billion years. A
supernova is the explosion of an old star at the end of its normal life. These
explosions are among the most powerful events in the known universe,
releasing 10 billion times the power of the sun. Fortunately, supernovae are
rare, occurring about once every 75 years in the Milky Way galaxy.
Energy Almost all of the sun’s power comes to the Earth as radiant energy, including
from the sun light. The top of the Earth’s atmosphere receives an average of 1,373 watts
per square meter. In the summer at northern latitudes in the United States,
about half that power (660 W/m2) makes it to the surface of the Earth. The
rest is absorbed by the atmosphere or reflected back into space. In the winter,
the solar power reaching the surface drops to 350 W/m2. About half of the
power reaching the Earth’s surface is in the form of visible light. The
remaining power is mostly infrared and ultraviolet light.
Figure 10.17: Estimating the power in a
gust of wind
Estimating the The power received from the sun is what drives the weather on Earth. To get
mass of a gust of an idea of the power involved in weather, suppose we estimate the power in a
wind gust of wind. A moderate wind pattern covers 1 square kilometer and
involves air up to 200 meters high (Figure 10.17). This represents a volume
of 200 million cubic meters (2 × 108 m3). The density of air is close to
1 kg/m3, so the mass of this volume of air is 200 million kilograms.
Estimating the Assume the wind is moving at 10 m/sec (22 mph) and it takes 3 minutes to
power get going. The power required to start the wind blowing is the kinetic energy
of the moving air divided by 180 seconds (3 minutes). The result is 56 million
watts, nearly the power to light all the lights in a town of 60,000 people.
Compared with what people use, 56 million watts is a lot of power. But
1 square kilometer receives 1.3 billion watts of solar power. A 10 m/sec wind
gust represents only 4 percent of the available solar power. A storm delivers
much more power than 56 million watts because much more air is moving
(Figure 10.18).
252
10.3 SYSTEMS IN TECHNOLOGY AND NATURE
Figure 10.18: A powerful storm system
like this hurricane has a power of billions of
watts. Photo courtesy NASA.
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CHAPTER 10: ENERGY FLOW AND SYSTEMS
Energy flow in living systems
Producers and A food chain is a way of describing the flow of energy between living things
food chains (Figure 10.19). Organisms at the bottom of the food chain are producers. A
producer is a plant or one-celled organism that converts energy from the sun
into chemical energy in molecules like sugar. Grass and trees are producers, as
are corn, wheat, and all the crops we grow for food.
Herbivores Next up on the food chain are the herbivores. An herbivore gets energy by
eating plants. Herbivores include rabbits, most insects, and many land and sea
animals. Herbivores concentrate energy from plants into complex molecules
but they also use energy for living. It takes many producers to support one
herbivore. Think of how many blades of grass a single rabbit can eat!
Carnivores Carnivores get their energy by eating herbivores or other carnivores. A
primary carnivore eats herbivores. A hawk is an example of a primary
carnivore. Hawks eat mice and other small herbivores. Secondary carnivores
eat other carnivores as well as herbivores. A shark is an example of a
secondary carnivore. The food chain is often drawn as a pyramid to show that
producers are the basis for all other life and also the most abundant.
Figure 10.19: The energy pyramid is a
good way to show how energy moves through
an ecosystem.
Decomposers The last important group in the food chain are decomposers. Decomposers
break down waste and bodies of other animals into simple molecules that can
be used by plants. Earthworms, fungi, and many bacteria are examples of
decomposers. You can think of decomposers as recycling raw materials such
as carbon and nitrogen so they can be used by producers again.
10.3 Section Review
1. Draw an energy flow diagram for a person who eats then runs a race.
2. Describe at least three examples of energy and power in a natural system.
3. What categories of plants and animals make up a food chain?
Figure 10.20: Energy flow in an
ecosystem.
UNIT 4 ENERGY AND CHANGE
253
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Energy in the Ocean
Tidal power
Governments and engineering companies are
heading to the beach in large numbers. With
them they bring millions of dollars in the
hopes of being able to harvest the renewable
energy of the moving ocean. In theory it is
simple; use the moving water found in
currents, waves, and tides to turn electrical
generating turbines. Most believe that those
who are successful will be able to provide a
significant portion of the energy demands of
the future. Scientists estimate that the kinetic
energy of waves crashing on the shores of the
world each day release enough energy to
power the world many times over.
Most hydroelectric generators use a dam
built across a river or stream. Flowing
water spins a turbine that in turn spins a
generator to produce electricity. But how
do you capture the energy of tides which
ebb and flow in a rhythm instead of
flowing in one direction only?
Tidal power uses a huge dam (called a
"barrage") built across a river estuary.
When the tide goes in and out, the water
flows through tunnels in the dam. The
ebb and flow of the tides turns a turbine.
Large lock gates, like the ones used on
canals, allow ships to pass.
The race is on
The race is indeed on and the stakes are very,
very high! Companies around the world are
offering their engineers the opportunity to
develop new and more futuristic machines to
harness the renewable energy of the moving
ocean. Looking to the ocean as a renewable
source of energy makes sense because
oceans cover roughly 71% of Earth’s surface.
The gravitational pull of the sun and the
moon do the hard work! The challenge is to
be able to develop the technology that
harnesses this energy safely, efficiently at or
below the current cost of producing
electricity, and without harming the
environment.
254
Wave power
On the Scottish Isle of Islay, there is a
unit called a Land Installed Marine
Powered Energy Transformer (LIMPET).
The LIMPET is installed onshore with a
chamber that extends into the sea. As a
wave approaches, it forces captured air in
the chamber through a turbine. As the
wave retreats, the pressure inside the
chamber drops and air is drawn through
the turbine in the opposite direction. The
turbine generates electricity as the wave
approaches and when it retreats (picture
shown on the facing page.)
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CHAPTER 10: ENERGY FLOW AND SYSTEMS
Engineers are
pleased since the
movement of the
water is very
predictable, unlike
solar and wind
energy, so they can
depend on the
generation of the
needed electricity.
Clean energy for
the future
One of the difficult
lessons learned from
our production of
electricity by nuclear and fossil fuels is that we cannot trade a
perfectly clean environment for electrical power. Initial studies
show that careful planning might be able to utilize the movement of
the oceans to generate electricity without harming the environment.
Environmental scientists are working very closely with today's
engineers as they develop new and exciting ways to harness the
energy of our oceans.
Undersea currents
In September 2003, an undersea “windmill” with 10 meter diameter
blades began sending electricity to the tiny town of Hammerfest,
Norway. The windmill is deep enough not to disturb shipping in the
area and the company believes that the blades turn slowly enough
so as not to disturb the local sea life. When completed there will be
a field of as many as 20 such windmills generating power for the
town. The current of the channel travels around 3 m/sec, and the
windmills are able to rotate in order to follow the current.
Questions:
1. Make an energy flow diagram that traces the energy flow in
hydroelectric power.
2. Make a table that summarizes three ways to harness the
energy of the moving ocean.
3. What are the positive and negative aspects of generating
electricity in the oceans?
UNIT 4 ENERGY AND CHANGE
255
Chapter 10 Review
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Understanding Vocabulary
Reviewing Concepts
Select the correct term to complete the sentences.
Section 10.1
chemical energy
nuclear energy
horsepower
watt
joule
mechanical energy
electrical energy
power
efficiency
energy conversion
steady state
thermal energy
second law of thermodynamics
food chain
radiant energy
1.
Why can energy be thought of as “nature’s money?”
2.
Chemical energy is sometimes thought of as a form of potential
energy. Explain this statement.
3.
Explain what is meant by the “energy of pressure.”
4.
Is a stretched rubber band a form of potential or kinetic energy?
Explain.
Why is work done against friction “lost” to a system?
Section 10.1
1.
Light is a form of ____.
5.
2.
____ results from splitting or fusing atoms.
Section 10.2
3.
Energy stored in a candy bar is an example of ____.
6.
Describe the meaning of power and how it is calculated.
4.
____ can be used to do work whenever there is a temperature
difference.
7.
Name two units of power and an application for each.
8.
Describe two different ways to describe power.
5.
Kinetic and potential energy are both forms of ____.
9.
What is efficiency and how is it calculated?
Section 10.2
6.
____ is the amount of energy changed divided by the time it takes.
7.
One ____ is equal to one joule per second.
8.
One ____ is equal to 746 watts.
9.
When work is done by heat flowing, the output work is always less
than the amount of heat that flows. This is a statement of the ____.
Section 10.3
10. The flow of energy almost always involves ____.
11. ____ means there is a balance between energy in and energy out so
the total energy remains the same.
12. A ____ is a way of describing the flow of energy between living
things.
10. Use the example of a car engine to explain the meaning of the second
law of thermodynamics.
11. Explain, in terms of work output, why the efficiency of living things
is quite low.
12. Would the efficiency of a motorcycle be higher or lower than the
efficiency of a bicycle? Explain your answer.
Section 10.3
13. Describe the energy conversions that happen in a swinging pendulum.
14. List two examples of technology you use each day that have a high
power rating, and two examples of technology that have a relatively
low power rating. Explain why these particular examples have high
and low power ratings.
15. What is the primary energy input on Earth? What happens to that
energy once it gets to Earth?
16. Why are herbivores more abundant than carnivores?
17. What are decomposers and what is their role in energy transfer?
256
CHAPTER 10: ENERGY FLOW AND SYSTEMS
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CHAPTER 10 REVIEW
Solving Problems
Section 10.2
4.
Michelle takes her 75 kilogram body up a 3.0 meter staircase in 3.0
seconds.
a. What is her power in watts?
b. What is her power in horsepower?
c. How many joules of work did she do?
d. If Michelle uses 10 food Calories to do the work, what is her
efficiency?
5.
A motor pushes a car with a force of 35 newtons for a distance of 350
meters in 6 seconds.
a. How much work has the motor accomplished?
b. How powerful is the motor in watts?
c. How powerful is the motor in horsepower?
6.
How much power is required to do 55 joules of work in 55 seconds?
7.
The manufacturer of a machine says that it is 86 percent efficient. If
you use 70 joules of energy to run the machine, how much output
work will it produce?
8.
Carmen uses 800 joules of
energy on a jack that is 85
percent efficient to raise her
car to change a flat tire.
a. How much energy is
available to raise the car?
b. If the car weighs 13,600
newtons, how high off
the ground can she raise
the car?
9.
Suppose you exert 200 newtons of force
to push a heavy box across the floor at a
constant speed of 2.0 meters per
second.
a. What is your power in watts?
b. What would happen to your power
if you used the same force to push
the box at a constant speed of 1.0
meters per second?
Section 10.1
1.
A 1.0-kilogram ball rolls down a 1.0-meter high hill and reaches a
speed of 20.0 meters per second at the bottom. Was the ball pushed by
someone? Explain your answer using calculations of potential and
kinetic energy.
2.
A ball at the top of a hill has 12.5 joules of energy. After it rolls down,
it converts 11.6 joules to kinetic energy. How much energy is “lost” to
the system? Where did that energy go?
3.
Use the diagram below to answer questions a through d.
a.
b.
c.
d.
Where does the initial energy come from in the apparatus?
What forms of energy are involved in the operation of the
apparatus?
Draw an energy flow diagram.
Do you think the apparatus will work? Why or why not?
UNIT 4 ENERGY AND CHANGE
257
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b.
Section 10.3
10. Suppose your job is to choose a motor
for an escalator. The escalator must be
able to lift 20 people at a time, each
with a mass of 70 kilograms. The
escalator must move between two
floors, 5 meters apart in 5 seconds.
a. What energy is required to do this
work?
b. What is the power rating of the
required motor, in horsepower?
c.
d.
What are the advantages and disadvantages to using solar cells to
power homes?
What current percentage of homes in the United States use solar
cells to generate power some of their power? Is this number on
the rise or decline?
Which state currently leads the nation in using solar cells to
power homes? What are the reasons?
Section 10.2
2.
A typical car is about 13 percent efficient at converting energy from
gasoline to energy of motion. The average car today gets about 25
miles for each gallon of gasoline.
a. Name at least four energy transformations that occur in a car.
b. Name three things that contribute to lost energy and prevent a
car from ever being 100 percent efficient.
c. If a car that currently gets 25 miles per gallon was 100 percent
efficient, what would be its miles per gallon?
3.
Two mountain lions run up a steep hillside. One animal is twice as
massive as the other, yet the smaller animal got to the top of the hill in
half the time. Which animal did the most work? Which delivered the
most power?
4.
Steve lifts a toolbox 0.5 meters off the ground in one second. If he
does the same thing on the moon, does he have to use more power,
less power, or the same amount of power? Explain your answer.
11. Fill in the joules of energy for each box
below. Compute the output work and
total wasted energy. What is the overall efficiency of the model solar
car?
Section 10.3
5.
Applying Your Knowledge
Section 10.1
1.
Solar cells (also called photo voltaic cells) are used to power satellites
in outer space, yet they are not as commonly used to power
households on Earth. Conduct Internet research on the use of solar
cells to generate electricity for homes. Prepare a short report that
answers the following questions:
a. Why is it more difficult to use solar cells on the surface of the
Earth as opposed to outer space?
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CHAPTER 10: ENERGY FLOW AND SYSTEMS
At each level of the food
pyramid, about 90 percent
of the usable energy is
lost in the form of heat.
a. Which level requires
the most overall
input of energy to
meet its energy
needs?
b. Use the diagram to
explain why a pound of steak costs more than a pound of corn.
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