3
3
Overview
READI NG WARM-U P
This section introduces the law
of conservation of energy. Students will learn how all energy
is continuously being converted
into other forms, and they will
learn why the principle of energy
conversion makes perpetual
motion impossible.
Objectives
•
•
•
•
Bellringer
Explain how energy is conserved
within a closed system.
Explain the law of conservation of
energy.
Give examples of how thermal
energy is always a result of energy
conversion.
Explain why perpetual motion is
impossible.
Terms to Learn
friction
law of conservation of energy
Pose the following questions
to students:
READI NG STRATEGY
• Where does the energy that
makes a roller coaster car move
come from?
Paired Summarizing Read this
section silently. In pairs, take turns
summarizing the material. Stop to
discuss ideas that seem confusing.
• Where does the energy go?
• What does “All of the energy
put into a process still exists
somewhere when the process
has ended” mean?
Conservation of Energy
Many roller coasters have a mechanism that pulls the cars
up to the top of the first hill. But the cars are on their own
for the rest of the ride.
As the cars go up and down the hills on the track, their
potential energy is converted into kinetic energy and back
again. But the cars never return to the same height at which
they started. Does energy get lost somewhere along the way?
No, it is just converted into other forms of energy.
Where Does the Energy Go?
To find out where a roller coaster’s original potential energy
goes, you have to think about more than just the hills of the
roller coaster. Friction plays a part too. Friction is a force that
opposes motion between two surfaces that are touching. For
the roller coaster to move, energy must be used to overcome
friction. There is friction between the cars’ wheels and the
track and between the cars and the air around them. As a
result, not all of the potential energy of the cars changes into
kinetic energy as the cars go down the first hill. Likewise, as
you can see in Figure 1, not all of the kinetic energy of the
cars changes back into potential energy.
Figure 1
a PE is greatest at the
top of the first hill.
Energy Conversions in a Roller Coaster
Not all of the cars’ potential energy (PE) is converted into kinetic
energy (KE) as the cars go down the first hill. In addition, not all
of the cars’ kinetic energy is converted into potential energy as
the cars go up the second hill. Some of it is changed into thermal
energy because of friction.
Demonstration --------------g
Where Does the Energy Go?
Show students a high-density
rubber ball. Explain that it was
designed to bounce for a long
time but that it must eventually
stop. Allow the ball to begin
bouncing. As it bounces, ask students to observe both the height
and the number of bounces. Ask
them to theorize why the ball
eventually stops. Ask them what
happens to the kinetic energy of
the ball’s movement. l Logical
b KE at the bottom of the
first hill is less than the
PE at the top was.
CHAPTER RESOURCES
MISCONCEPTION
ALERT
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• Lesson Plan
• Directed Reading A b
• Directed Reading B s
Technology
Transparencies
• Bellringer
• Energy Conversions in a Roller Coaster
254
c PE at the top of the second hill is less than KE
and PE from the first hill.
Chapter 9 • Energy and Energy Resources
Energy Lost as Friction Students
may assume that all processes involving work and energy are 100% efficient. Remind students that all motion
and all processes involving work are
opposed by friction. Friction converts
kinetic energy into thermal energy. Let
students rub their palms together for
30 s. They will feel the thermal energy
produced by friction.
Energy Is Conserved Within a Closed System
A closed system is a group of objects that transfer energy only
to each other. For example, a closed system that involves a
roller coaster consists of the track, the cars, and the air around
them. On a roller coaster, some mechanical energy (the sum
of kinetic and potential energy) is always converted into
thermal energy because of friction. Sound energy also comes from
the energy conversions in a roller coaster. If you add together
the cars’ kinetic energy at the bottom of the first hill, the
thermal energy due to overcoming friction, and the sound
energy made, you end up with the same total amount of
energy as the original amount of potential energy. In other
words, energy is conserved and not lost.
friction a force that opposes
v--------------------------g
motion between two surfaces
that are in contact
Energy is conserved in all cases. Because no exception to this
rule has been found, this rule is described as a law. According
to the law of conservation of energy, energy cannot be created
or destroyed. The total amount of energy in a closed system is
always the same. As Figure 2 shows, energy can change from
one form to another. But all of the different forms of energy
in a system always add up to the same total amount of energy.
It does not matter how many energy conversions take place.
Chemical Energy to Thermal
Energy Divide the class into
three groups, and go outside as
a class. This activity will explore
how the body converts chemical
energy to thermal energy. One
group should simply stand or
sit for 5 min. Meanwhile, the
second group should walk at a
comfortable pace, and the third
group should engage in a running activity. After 5 min, have
students discuss the amount
of thermal energy produced
by their bodies. Ask students,
“Which group produced the
most thermal energy? Which
produced the least? Why?”
✓
l Kinesthetic
law of conservation of energy
the law that states that energy
cannot be created or destroyed
but can be changed from one
form to another
Law of Conservation of Energy
Reading Check Why is the conservation of energy considered a
scientific law? (See the Appendix for answers to Reading Checks.)
Answer to Reading Check
Figure 2
Energy Conservation in a Light Bulb
Some energy is converted into thermal
energy, which makes
the bulb feel warm.
Conservation of energy is considered a scientific law because
no exception to it has ever been
observed.
Some electrical
energy is converted
into light energy.
As electrical energy is
carried through the wire,
some of it is converted
into thermal energy.
Is That a Fact!
At extremely low temperatures, some
materials become superconductors,
materials that have no resistance to the
flow of electrical energy. These materials
are used to create giant electromagnets
that generate strong magnetic fields
with very little thermal-energy loss.
Q: What happened when the sandpaper
and the wood got together to settle
their differences?
A: The discussion got heated! There was
just too much friction between them.
Section 3 • Conservation of Energy 255
No Conversion Without Thermal Energy
Reteaching -------------------------------------b
Conservation of Roller-Coaster
Energy Revisit Figure 1 by
showing students the transparency of that figure. Ask students why each successive hill is
smaller than the last, and make
sure that they know where the
energy went. l Visual
Quiz ---------------------------------------------------------------------g
1. Think of an example other
Energy Conversions
With a parent, find three
examples of energy conversions that take place in your
home. In your science journal, write down the kinds
of energy that go into each
conversion and the kinds of
energy that result. For each
type of energy that is output,
indicate whether the energy
is useful.
than the ones given in this
section that illustrates the
law of conservation of energy.
(Answers may vary but should
reflect an understanding of
energy conservation.)
Any time one form of energy is converted into another form,
some of the original energy always gets converted into thermal
energy. The thermal energy due to friction that results from
energy conversions is not useful energy. That is, this thermal
energy is not used to do work. Think about a car. You put
gas into a car. But not all of the gasoline’s chemical energy
makes the car move. Some wasted thermal energy will always
result from the energy conversions. Much of this energy leaves
through the radiator and the exhaust pipe.
Perpetual Motion? No Way!
People have sometimes tried to make a machine that would
run forever without any additional energy. This perpetual (puhr
PECH oo uhl) motion machine would put out exactly as much
energy as it takes in. But that’s impossible, because some waste
thermal energy always results from energy conversions. The
only way a machine can keep moving is to have a constant
supply of energy. For example, the “drinking bird” shown in
Figure 3 uses thermal energy from the air to evaporate the water
from its head. So, it is not a perpetual motion machine.
✓Reading Check
2. What condition would have
Figure 3
to exist for perpetual motion
to be possible? (no production
of waste thermal energy; no
friction)
The “Drinking Bird”
1 When the bird “drinks,” the felt
covering its head gets wet.
Alternative
Assessment ---------------------------g
2 When the bird is upright, water
evaporates from the felt, which
decreases the temperature and
pressure in the head. Fluid is
drawn up from the tail, where
pressure is higher, and the bird
tips downward.
Energy Conservation Diagram
Have students draw a diagram
of a system in which energy
conservation is demonstrated.
The diagram should include
labels that indicate the kinds
of energy involved. l Visual
3 After the bird “drinks,” fluid
returns to the tail, the bird flips
upright, and the cycle repeats.
Answer to Reading Check
Perpetual motion is impossible because energy
conversions always result in the production of
waste thermal energy.
256
Why is “perpetual motion” impossible?
Chapter 9 • Energy and Energy Resources
Making Conversions Efficient
Answers to Section Review
You may have heard that a car is energy efficient if
it gets good gas mileage, and that your home may be
energy efficient if it is well insulated. In terms of energy
conversions, energy efficiency (e FISH uhn see) is a
comparison of the amount of energy before a conversion
with the amount of useful energy after a conversion. A
car with high energy efficiency can go farther than other
cars with the same amount of gas.
Energy conversions that are more efficient end up
wasting less energy. Look at Figure 4. Newer cars tend to
be more energy efficient than older cars. One reason is
the smooth, aerodynamic (ER oh die NAM ik) shape of
newer cars. The smooth shape reduces friction between
the car and the surrounding air. Because these cars move
through air more easily, they use less energy to overcome friction. So, they are more efficient. Improving the
efficiency of machines, such as cars, is important because
greater efficiency results in less waste. If less energy is
wasted, less energy is needed to operate a machine.
1. Sample answer: According
to the law of conservation of
energy, energy is not lost,
although kinetic energy can
be converted into thermal
energy by friction.
2. d
3. Sample answer: At the top of a
“hill,” a roller-coaster car has
maximum potential energy. As
the car rolls down the hill, its
potential energy is turned into
kinetic energy and thermal
energy due to friction. Because
the car “loses” energy because
of friction, the next hill that the
car climbs must be lower than
the first one, and so on.
4. 80 J 60 J ! 20 J
5. The energy that the ball had initially is not “gone” but was converted into thermal energy and
sound energy.
6. No, efficiency depends on the
ratio of energy input to energy
output, and you do not know the
energy input. If the energy input
is much higher than the energy
output, the car is not efficient.
Review
Summary
•
•
•
Because of friction,
some energy is always
converted into thermal
energy during an energy
conversion.
Energy is conserved
within a closed system.
According to the law of
conservation of energy,
energy cannot be created or destroyed.
Perpetual motion is
impossible because
some of the energy
put into a machine is
converted into thermal
energy because of
friction.
Figure 4 The shape of newer cars
reduces friction between the body
of the car and the air.
More aerodynamic car
Less aerodynamic car
Using Key Terms
Critical Thinking
1. Use the following terms in the
same sentence: friction and the
law of conservation of energy.
5. Evaluating Conclusions
Imagine that you drop a ball. It
bounces a few times and then
it stops. Your friend says that
the energy that the ball had is
gone. Where did the energy go?
Evaluate your friend’s statement
based on energy conservation.
Understanding Key Ideas
2. Perpetual motion is impossible
because
a.
b.
c.
d.
things tend to slow down.
energy is lost.
machines are very inefficient.
machines have friction.
6. Evaluating Assumptions If
someone says that a car has high
energy output, can you conclude
that the car is efficient? Explain.
3. Describe the energy conversions that take place on a roller
coaster, and explain how energy
is conserved.
Developed and maintained by the
National Science Teachers Association
Math Skills
4. A bike is pedaled with 80 J of
energy and then coasts. It does
60 J of work in moving forward
until it stops. How much of the
energy that was put into the
bike became thermal energy?
For a variety of links related to this
chapter, go to www.scilinks.org
Topic: Law of Conservation of Energy
SciLinks code: HSM0856
CHAPTER RESOURCES
Chapter Resource File
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• Section Quiz g
• Section Review g
• Vocabulary and Section Summary g
Section 3 • Conservation of Energy 257