Name
CHAPTER 20
Class
Date
The Energy of Waves
SECTION
1 The Nature of Waves
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
PS 3a
• What is a wave, and how does it transmit energy?
• How do waves move?
• What are the different types of waves?
What Is Wave Energy?
A wave is any disturbance that transmits energy
through matter or empty space. Energy can be carried
away from its source by a wave. However, the material
through which the wave moves is not transmitted. For
example, a ripple caused by a rock thrown into a pond
does not move water out of the pond.
A wave travels through a material or substance called
a medium. A medium may be a solid, a liquid, or a gas.
The plural of medium is media.
STUDY TIP
As you read the section,
make a table of the types of
waves. Have columns for the
type of wave, what it moves
through, its direction of motion, and how it transmits
energy.
READING CHECK
1. Identify What does a
wave move through?
Wave motion
A wave travels through the medium, but the medium does not travel. In
a pond, lake or ocean, the medium through which a wave travels is the
water. The waves in a pond travel towards the shore. However, the water
and the leaf floating on the surface only travel up and down.
How Can Waves Do Work?
As a wave travels, it does work on everything in its
path. The waves traveling through a pond do work on
the water. Anything floating on the surface of the water
moves up and down. The fact that any object on the
water moves indicates that the waves are transferring
energy. Waves can transfer energy through a medium or
without a medium.
READING CHECK
2. Describe What indicates
that a water wave transfers
energy to a floating object?
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The Energy of Waves
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The Nature of Waves continued
WAVES CAN TRANSFER ENERGY THROUGH A MEDIUM
When a particle vibrates (moves back and forth), it
can pass its energy to the particle next to it. The second
particle will vibrate like the first particle and may pass
the energy on to another particle. In this way, energy is
transmitted through a medium.
TAKE A LOOK
3. Describe How did the
last ball in the figure on the
right gain energy?
A particle can pass energy to the particle next to it. The particle receiving
the energy will vibrate like the first particle. This is shown by the Newton’s
pendulum above. When the moving steel ball collides with another steel
ball, its energy is given to that ball. Notice that the first ball stops, but its
energy is passed on to a third ball.
Waves that require a medium are called mechanical
waves. Mechanical waves include sound waves, ocean
waves, and earthquake waves. For example, consider a
radio inside a jar. If all of the air from inside the jar is
removed to create a vacuum, the radio can not be heard.
WAVES CAN TRANSFER ENERGY WITHOUT A MEDIUM
STANDARDS CHECK
PS 3a Energy is a property
of many substances and is
associated with heat, light,
electricity, mechanical motion,
sound, nuclei, and the nature of
a chemical. Energy is transferred
in many ways.
4. Identify What kinds of
waves need a medium to
transfer their energy? What
kinds of waves don’t need
a medium to transfer their
energy?
Waves that transfer energy without a medium are
called electromagnetic waves. Examples of electromagnetic waves include visible light, microwaves, TV and
radio signals, and X-rays used by dentists and doctors.
Electromagnetic waves may also go through matter, such
as air, water, or glass. Light waves travel from the sun through
space toward Earth. Light waves then travel through the air
in the atmosphere to reach the surface of Earth.
Atmosphere:
a medium
Sun
Earth
Vacuum of space: no medium
To reach the Earth, light travels from the sun, through the vacuum
of space. The light then travels through the particles of the atmosphere before reaching the surface of the earth.
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The Energy of Waves
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The Nature of Waves continued
What Are the Different Types of Waves?
Waves transfer energy through vibrations. However,
the way particles in a wave vibrate depends on the type
of wave. Waves are classified based on the direction in
which wave particles vibrate compared with the direction in which waves move. There are two main types of
waves, transverse waves and longitudinal waves.
5. Identify What are the two
main types of waves?
TRANSVERSE WAVES
Waves in which the particles vibrate in an up-anddown motion are called transverse waves. Particles in
a transverse wave move at right angles relative to the
direction of the wave. See the figure below. The highest
point in a transverse wave is a crest. The lowest point in
a transverse wave is a trough.
Motion of a Transverse Wave
Crests
The points along
the rope vibrate
up and down.
READING CHECK
The wave travels to the right.
READING CHECK
6. Identify What is the
direction of a transverse
wave relative to its direction
of motion?
Troughs
A wave traveling down a length of rope is an example of a transverse wave.
The wave travels to the right. The particles in the medium, the rope, travel
up-and-down. The particles in the wave and the medium are moving at right
angles to each other.
All electromagnetic waves are transverse waves.
Remember, electromagnetic waves can travel through
space or through a medium. Electromagnetic waves are
transverse waves because the wave vibrations are at right
angles to the direction the wave is traveling.
READING CHECK
7. Describe Why is an
electromagnetic wave
identified as a transverse wave?
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The Energy of Waves
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The Nature of Waves continued
LONGITUDINAL WAVES
READING CHECK
8. Describe How do the
particles of a longitudinal
wave vibrate?
Waves in which the particles of the medium vibrate
back and forth along the path of the wave are called
longitudinal waves. For example, pushing together two
ends of a spring causes the coils to crowd together. When
you let go, a longitudinal wave is created in the spring
that travels along the length of the spring.
In a longitudinal wave, a compression is the location
of the crowded particles. A rarefaction is where the
particles are spread apart.
Compressions and rarefactions are similar to crests
and troughs in a transverse wave. See the figure below.
Comparing Longitudinal and Transverse Waves
Rarefactions
Compressions
Critical Thinking
9. Describe How could you
produce a transverse wave in
a spring?
Longitudinal wave
A wave traveling along the length of a spring is an example of a longitudinal
wave. The wave travels to the right. The particles in the medium, the spring,
move back-and-forth. The particles in wave and the medium are moving
along the same direction as each other.
Troughs
Crests
Transverse wave
The troughs and crests of a transverse wave represent an up-and-down motion around a central point. Similarly, the rarefactions and compressions of a
longitudinal wave represent a back-and-forth motion around a central point.
A sound wave is an example of a longitudinal wave.
Sound waves travel by compressions and rarefactions of
air particles.
TAKE A LOOK
10. Identify Circle the
compression part of the
wave in the second figure.
When the drumhead moves out
after being hit, a compression is
created in the air particles.
When the drumhead moves
back in, a rarefaction is
created.
Sound energy is carried away from a drum by a longitudinal wave
through the air.
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The Energy of Waves
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The Nature of Waves continued
SURFACE WAVE
When waves move at or near the surface between
two media a surface wave may form. For example, this
occurs when an ocean wave comes into shallow water
at the shore. Surface waves travel in both transverse and
longitudinal motion. A particle in a surface wave will
appear to move in a circular motion.
Wave Motion
Ocean waves are surface waves. A floating bottle shows the circular motion of
particles in a surface wave.
Critical Thinking
11. Identify An ocean wave
forms a surface wave as it
comes into shallow water.
What are the two media involved in forming the surface
wave?
Summary of Wave Types and Their Motion Through Space
wave
needs a
medium
mechanical wave
particles
travel at right
angles to
the direction
of the wave
motion
particles
travel backand-forth in
the direction
of the wave
motion
transverse wave longitudinal wave
does not need
a medium
electromagnetic wave
particles travel
both at right
angles to, and
back-and-forth in
the direction of
the wave motion
surface wave
particles travel
at right angles
to the direction
of the wave
motion
transverse wave
TAKE A LOOK
12. Identify Which type of
wave must have a medium in
order to travel?
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The Energy of Waves
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Section 1 Review
NSES
PS 3a
SECTION VOCABULARY
medium a physical environment in which
phenomena occur
longitudinal wave a wave in which the particles
of the medium vibrate parallel to the direction
of wave motion
transverse wave a wave in which the particles
of the medium move perpendicularly to the
direction the wave is traveling
wave a periodic disturbance in a solid, liquid, or
gas as energy is transmitted through a medium
1. Describe How does energy travel through a wave in a medium?
2. Identify Determine the method of energy transfer, and the wave type for each
wave source.
Wave source
Wave energy transfer
(electromagnetic wave or
mechanical wave)
Wave type (transverse
wave, longitudinal wave,
or surface wave)
Light emitted from a light bulb
Sound coming from a violin
Rock dropped in a pond
3. Apply Concepts A ribbon is tied to the first loop of a spring as a marker. The
spring is pulled and then released to create a longitudinal wave. Where is the
ribbon after three complete vibrations?
4. Recall Label each wave part as a crest, trough, compression, or rarefaction
according to its description.
Wave Part
Description
particles are crowded toward
each other
particles are at their highest
point
particles are at their lowest
point
particles are spread away from
each other
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The Energy of Waves
Physical Science Answer Key continued
7. Digital signals are pulse or no pulse; analog
8.
9.
10.
11.
12.
13.
14.
15.
9. Input: mouse, keyboard, microphone
signals are continuous.
They are not continuous.
by taking more sample values per second
a laser
electromagnetic waves
No, he hears sound waves. The EM waves
reach the radio receiver, which separates
them from the analog signal. The speakers
convert the analog signal into sound.
Electric current is the carrier of signals to
electronic devices such as TVs.
electromagnetic
It uses pixel cells filled with gases, instead
of electron tubes.
10.
11.
12.
13.
14.
15.
16.
17.
Review
1. Turning on a light switch allows a current to
2.
3.
4.
5.
6.
flow; turning it off stops the current. Pulses
of current and non-pulses are also how digital signals are sent.
The second graph is closer to the original
sound wave because it looks like a continuous wave, without big jumps up or down.
the telegraph
electromagnetic waves
electric current, electromagnetic waves
Morse code is a digital signal because it
consists of on/off sequences.
Review
1. Hardware is all the pieces of equipment
2.
3.
SECTION 3 COMPUTERS
1. input, process, store, output
2. the sound that wakes you up
3. Today’s are smaller, cheaper, and made by
4.
5.
6.
7.
8.
Processing: CPU
Storage: CD, RAM, ROM
Output: speaker, monitor, printer
connects a computer to the Internet
A CD-R can be used only once. A CD-RW
can be used more than once because it is
erasable.
a set of instructions that tells a computer
what to do
operating system and application
word processing, interactive instruction
a computer network that connects computers and smaller networks around the world
a collection of Web pages
a part of the Internet that can be accessed
using a browser
4.
computer companies, and they don’t need to
be kept in air-conditioned rooms.
3 million
an integrated circuit that controls and
carries out a computer’s instructions
Input: keyboard, mouse, microphone
Processing: CPU
Storage: hard disk, floppy disk, CD, ROM, RAM
Output: monitor, speaker
the computer’s central processing unit, the
microprocessor
the hard disk or RAM, or to an external
memory
5.
that make up the computer. Software is the
group of programs that make the computer
run.
Clockwise from bottom left: storage,
processing, output
Numbers are the input. You put them in by
using the keypad. Processing (doing the
math problem) takes place in the calculator.
The calculator stores information on how to
do mathematical calculations. You see the
output (the answer to the math problem) on
the digital display.
A laser shines on different parts of the
CD. This causes ink on the CD to become
opaque, so laser light does not reflect from
it. This produces the equivalent of pits and
lands on the CD.
Possible answer: Yes, because many computers use hardware other than phone lines
to connect to the Internet.
Chapter 20 The Energy of
Waves
SECTION 1 THE NATURE OF WAVES
1. a medium
2. The object moves.
3. The ball on the far left transferred its energy
to the middle ball when they collided. The
middle ball immediately collided with the
ball on the far right, causing it to move.
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Physical Science
Physical Science Answer Key continued
4. Mechanical waves need a medium to
5.
6.
7.
8.
9.
10.
11.
12.
7. those with the highest frequency
8. v f 30 m 20/s 600 m/s
transfer their energy, while electromagnetic
waves don’t.
transverse and longitudinal waves
at right angles to the direction of motion
The wave vibrations are at right angles to
the direction the wave is traveling.
back and forth along the path that the wave
moves
Shake the spring up and down.
Densely packed group of particles should be
circled.
water and sand in the shallow water
mechanical wave
Review
1. The amplitude is 0.5 meters.
2. Wave description
Wave energy
Wave energy
transfer
(electromagnetic
wave or
mechanical
wave)
Wave type
(transverse
wave,
longitudinal
wave, or
surface wave)
Light emitted
from a light
bulb
electromagnetic
wave
transverse wave
Sound coming
from a violin
mechanical wave
longitudinal
wave
Rock dropped
in a pond
mechanical wave
transverse wave
Low wavelength
high
SECTION 3 WAVE INTERACTIONS
1. It looks like the opposite. When you raise
2.
3.
4.
5.
6.
3. The ribbon is in the same position on the
7.
first loop of the spring.
4. Compression: Particles are crowded toward
each other.
Crest: Particles are at their highest point.
Trough: Particles are at their lowest point.
Rarefaction: Particles are spread away from
each other.
8.
9.
10.
11.
12.
SECTION 2 PROPERTIES OF WAVES
1. the maximum distance the particles of a
2.
3.
4.
5.
6.
low
(make short-wavelength waves) before
releasing the spring. The other is to make
the frequency high.
4. v f 100 m 25/s 2,500 m/s
5,000 m/s
5. _v_ _________ 40 m
f
125/s
away from the wave source. Energy is transferred from one particle to the particle next
to it.
Wave source
high
Low frequency
3. One way is to compress coils closer together
Review
1. Energy moves in the direction of the wave
2.
High amplitude
13.
wave vibrate away from their rest position
It took more energy to make the wave.
one-half wavelength
the group of waves with short wavelengths
30 hertz, or 30 Hz
0:20
your right arm, it looks like you raised your
left arm.
the bending of waves around a barrier or
through an opening
It would become straighter (have less
diffraction).
It slows down and bends to travel in a new
direction.
spreads out
A line should be drawn between red
and yellow.
interference
The medium returns to its rest, or undisturbed, position.
crests and troughs
interference between a wave and a reflected
wave
Arrows should be drawn at the two cancellation points, not at the end points.
when two objects naturally vibrate at the
same frequency
reflection and diffraction
Review
1. When the crests meet, a wave with larger
amplitude is formed. This is constructive
interference.
2. When the crest and trough meet, a wave
with smaller amplitude is formed. This is
destructive interference.
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