Download Electromagnetic waves have unique traits.

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KEY CONCEPT
Electromagnetic waves
have unique traits.
BEFORE, you learned
NOW, you will learn
• Waves transfer energy
• Mechanical waves need a
medium to travel
• How electromagnetic waves
differ from mechanical waves
• Where electromagnetic waves
come from
• How electromagnetic waves
transfer energy
VOCABULARY
EXPLORE Electromagnetic Waves
electromagnetic
wave p. 553
radiation p. 555
How does the signal from a remote
control travel?
MATERIALS
PROCEDURE
1
Turn the TV on and off using the
remote control.
2 Work with a partner to try to turn on the TV
by aiming the remote control at the mirror.
• TV with remote
control unit
• mirror
with
stand
WHAT DO YOU THINK?
How did you have to position the remote control and
the mirror in order to operate the TV? Why do you
think this worked?
An electromagnetic wave is a disturbance
in a field.
Did you know that you are surrounded by thousands of waves at this
very moment? Waves fill every cubic centimeter of the space around
you. They collide with or pass through your body all the time.
VOCABULARY
Create a frame game
diagram for the term
electromagnetic wave.
Most of these waves are invisible, but you can perceive many of
them. Light is made up of these waves, and heat can result from them.
Whenever you use your eyes to see, or feel the warmth of the Sun on
your skin, you are detecting their presence. These waves also allow radios,
TVs, and cell phones to send or receive information over long distances.
These waves have the properties shared by all waves, yet they are different
from mechanical waves in important ways. This second type of wave
is called an electromagnetic wave. An electromagnetic wave (ih-LEHKtroh-mag-NEHT-ihk) is a disturbance that transfers energy through a
field. Electromagnetic waves are also called EM (EE-EHM) waves.
Chapter 17: Electromagnetic Waves 553
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A field is an area around an object where the object can apply a
force—a push or a pull—to another object without touching it. You
have seen force applied through a field if you have ever seen a magnet
holding a card on the door of a refrigerator. The magnet exerts a pull
on the door, even though it does not touch the door. The magnet
exerts a force through the magnetic field that surrounds the magnet.
When a disturbance occurs in an electric or magnetic field rather than
in a medium, the wave that results is an electromagnetic wave.
How EM Waves Form
EM waves occur when electrically
VISUALIZATION
CLASSZONE.COM
charged atomic particles move.
Learn more about the
Charged particles exert an electric
nature of EM waves.
force on each other, so they have
electric fields. A moving charged
particle creates a magnetic force, so
a moving charge also has a magnetic
field around it.
electric field
magnetic
field
direction of
wave
When electrically charged
particles move quickly, they can
start a disturbance of electric and
magnetic fields. The fields vibrate
at right angles to each other, as shown in the diagram above. The
EM wave travels in the form of these vibrating fields. As you read
in Chapter 15, waves have the properties of wavelength and frequency.
In an EM wave, both the electric and the magnetic fields have
these properties.
check your reading
What are the two types of fields that make up an EM wave?
Sources of EM Waves
Many of the EM waves present in Earth’s environment come from the
Sun. The Sun’s high temperature allows it to give off countless EM
waves. Other stars give off as many EM waves as the Sun, but because
these bodies are so far away, fewer of their EM waves reach Earth. In
addition to the Sun, technology is a source of EM waves that humans
use for a wide variety of purposes.
EM waves from the Sun provide most of the energy for the environment on Earth. Some of the energy goes into Earth’s surface,
which then gives off EM waves of different wavelengths.
check your reading
554 Unit 4: Waves, Sound, and Light
What are two sources of EM waves on Earth?
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Electromagnetic waves can travel in
a vacuum.
Energy that moves in the form of EM waves is called radiation
(RAY-dee-AY-shuhn). Radiation is different from the transfer of energy
through a medium by a mechanical wave. A mechanical wave must
vibrate the medium as it moves, and this uses some of the wave’s energy.
Eventually, every mechanical wave will give up all of its energy to the
medium and disappear. An EM wave can travel without a material
medium—that is, in a vacuum or space empty of matter—and does
not lose energy as it moves. In theory, an EM wave can travel forever.
reading tip
EM waves are also called
rays. The words radiation
and radiate come from the
Latin word radius, which
means “ray“ or “spoke
of a wheel.“
How EM Waves Travel in a Vacuum
Because they do not need a medium, EM waves can pass through outer
space, which is a near vacuum. Also, because they do not give up energy
in traveling, EM waves can cross the great distances that separate stars
and galaxies. For example, rays from the Sun travel about 150 million
kilometers (93 million mi) to reach Earth. Rays from the most distant
galaxies travel for billions of years before reaching Earth.
Usually, EM waves spread outward in all directions from the
source of the disturbance. The waves then travel until something
interferes with them. The farther the waves move from their source,
the more they spread out. As they spread out, there are fewer waves in
a given area and less energy is transferred. Only a very small part of
the energy radiated from the Sun is transferred to Earth. But that
energy is still a great amount—enough to sustain life on the planet.
The Speed of EM Waves in a Vacuum
In a vacuum, EM waves travel at a constant speed, and they travel very
fast—about 300,000 kilometers (186,000 mi) per second. In 1 second,
an EM wave can travel a distance greater than 7 times the distance
around Earth. Even at this speed, rays from the Sun take about 8 minutes to reach Earth. This constant speed is called the
speed of light. The vast distances of space are
often measured in units of time traveled at this
speed. For example, the Sun is about 8 lightminutes away from Earth. The galaxy
shown in the photograph is 60 million
light-years from Earth.
check your reading
The light and other EM
waves from this galaxy
took approximately 60
million years to reach
Earth.
How are EM waves used to measure distances in space?
Chapter 17: Electromagnetic Waves 555
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Electromagnetic waves can interact with a
material medium.
When EM waves encounter a material medium, they can interact with
it in much the same way that mechanical waves do. They can transfer
energy to the medium itself. Also, EM waves can respond to a change
of medium by reflecting, refracting, or diffracting, just as mechanical
waves do. When an EM wave responds in one of these ways, its direction
changes. When the direction of the wave changes, the direction in
which the energy is transferred also changes.
reminder
Potential energy comes
from position or form;
kinetic energy comes
from motion.
Transferring Energy
A mechanical wave transfers energy in two ways. As it travels, the
wave moves potential energy from one place to another. It also
converts potential energy into kinetic energy by moving the medium
back and forth.
In a vacuum, EM waves transfer energy only by moving potential
energy from one place to another. But when EM waves encounter
matter, their energy can be converted into many different forms.
check your reading
In what form do EM waves transfer energy in a vacuum?
Wave Behavior
SKILL FOCUS
How do EM waves interact with matter?
Designing
experiments
PROCEDURE
1
Observe the radiometer on a table or desk.
MATERIALS
2 Write a hypothesis in the form of an “If . . . ,
radiometer
then . . . , because . . .” statement to answer
the question: What makes the radiometer
vanes move?
3 Develop an experiment to test your hypothesis.
TIME
30 minutes
vanes
WHAT DO YOU THINK?
• How does light affect the vanes?
dark side
• Based on your observation of the vanes, does light affect
the light and dark surfaces differently? If so, how?
• How would you modify your design now that you have seen
the results?
CHALLENGE Based on your observations, what does a
radiometer measure? Explain your answer.
556 Unit 4: Waves, Sound, and Light
radiometer
light
side
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Converting Energy from One Form to Another
How EM waves interact with a medium depends on the type of the
wave and the nature of the material. For example, a microwave oven
uses a type of EM wave called microwaves. Microwaves pass through
air with very little interaction. However, they reflect off
the oven’s fan and sides. But when microwaves
1
encounter water, such as that inside a potato,
their energy is converted into thermal
energy. As a result, the potato gets cooked,
but the oven remains cool.
1
A device on the oven produces
microwaves and sends them toward
the reflecting fan.
reflecting
fan
microwave
source
2
4
microwaves
3
2
Microwaves are reflected in many directions by the blades of the fan and then
again by the sides of the oven.
3
Microwaves move through the air without
transferring energy to the air.
4
Microwaves transfer energy to the water molecules inside
the potato in the form of heat, cooking the potato.
EM waves usually become noticeable and useful when they transfer energy to a medium. You do not observe the microwaves in a
microwave oven. All you observe is the potato cooking. In the rest
of this chapter, you will learn about different types of EM waves,
including microwaves, and about how people use them.
check your reading
How does microwave cooking depend on reflection?
KEY CONCEPTS
CRITICAL THINKING
1. How are EM waves different
from mechanical waves?
4. Predict What would happen
to an EM wave that never
came into contact with matter?
2. What are two sources of EM
waves in Earth’s environment?
3. How can EM waves transfer
energy differently in a material
medium as compared to a
vacuum?
5. Infer What might be one
cause of uneven heating in a
microwave oven?
CHALLENGE
6. Synthesize EM waves can
interact with a medium. How
might this fact be used to
make a device for detecting
a particular type of EM
radiation?
Chapter 17: Electromagnetic Waves 557