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Physical Sciences
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Standard Set 1. Physical Sciences
1. Electricity and magnetism are
related effects that have many useful
applications in everyday life. As a
basis for understanding this concept:
1.b. Students know how to build a
simple compass and use it to detect
magnetic effects, including Earth’s
magnetic field.
1.c. Students know electric currents
produce magnetic fields and know
how to build a simple electromagnet.
Genre
Nonfiction
Comprehension Skill
Main Idea and Details •
•
•
•
1.d. Students know the role of
electromagnets in the construction of
electric motors, electric generators,
and simple devices, such as doorbells
and earphones.
1.e. Students know electrically charged
objects attract or repel each other.
1.f. Students know that magnets
have two poles (north and south) and
that like poles repel each other while
unlike poles attract each other.
1.g. Students know electrical energy can
be converted to heat, light, and motion.
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Science Content
Magnetism
Scott Foresman Science 4.2
ISBN 0-328-23543-1
ì<(sk$m)=cdfedi< +^-Ä-U-Ä-U
by Marcia K. Miller
Vocabulary
electromagnet
generator
magnetic field
magnetic poles
magnetism
by Marcia K. Miller
Picture Credits
Illustrations
5 Peter Bollinger; 23 Tony Randazzo.
Photographs
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Researchers, Inc.; 17 ©Jeremy Walker/Photo Researchers, Inc.; 19 (C) ©Clive Streeter/Courtesy of The Science Museum,
London/DK Images; 22 (B) ©Philip James Corwin/Corbis.
ISBN: 0-328-23543-1
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What are
magnetic fields?
Magnetism
Magnets are objects that attract iron, steel, and
some other metals. All magnets have magnetism.
Magnetism is a force that acts on moving electric
charge. It also acts on magnetic materials that are
near a magnet.
How do magnets make a train seem to float above
the tracks? Electric current flows through the tracks.
It produces enough magnetic force to lift the train!
Pushing and pulling forces also move the train down
the track.
Magnetic forces lift and
move this train. It is called
a maglev train.
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Magnetic Field
A magnet is surrounded by a magnetic field. It is
the space around a magnet in which magnetic forces
operate. A magnetic field reaches in all directions from
a magnet. Its shape is related to shape of the magnet.
Iron filings can show a magnetic field’s shape. The
filings will align with a magnetic field’s lines of force.
The lines of force curve out from a magnet’s ends.
Magnetic Poles
The two ends of a magnet are its magnetic poles.
Every magnet has a north pole and a south pole.
One pole points north. The other pole points south. A
magnetic field is strongest at the magnetic poles. The
field gets weaker as you move away from the poles.
Iron filings show the shape
of the magnetic field
surrounding this magnet.
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Fields, Forces, and Filings
Look at the magnetic field of the bar magnet in the
picture shown below. There are filings surrounding the
magnet. Most of the filings are at the magnets’ poles.
The pattern of filings around each kind of magnet is
a bit different. But each pattern curves from one pole
toward the other pole.
How Magnetic Poles Behave
The poles of a magnet act in the same ways that
electric charges do. Like poles and like charges push
each other apart. They repel. Unlike poles and unlike
charges pull each other together. They attract.
Place two magnets near each other. Try to put the
two north poles together. You will feel their magnetic
fields pushing apart. Now try to put a north pole near
a south pole. The opposite poles attract each other.
You can experience repelling and attracting with
two magnetic sheets. These sheets are used to make
magnets that look like cards. Each sheet is made up of
narrow magnetic strips. If you slide one sheet over the
other, you can feel them push apart and stick together.
The like poles repel. The unlike poles attract.
Broken Magnets
What happens when you break a magnet in two?
You get two magnets. Each new magnet will have both
a north and a south pole. North and south magnetic
poles are always paired.
Each piece of a broken
magnet has its own north
pole and south pole.
This bar magnet’s curved lines
of force are closest together
near its ends.
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5
What are magnetic
effects?
Compasses and the World’s Biggest
Magnet
A compass needle always moves to point north.
Christopher Columbus used a compass when he
crossed the Atlantic Ocean more than 500 years ago.
About 400 years ago, English scientist William Gilbert
developed the idea that Earth is a gigantic magnet,
with a magnetic field all around it.
Why does Earth act like a magnet? Scientists think
that Earth’s outer core is made of iron. The iron is a very
hot liquid. As Earth spins, electric currents flow in the
liquid iron. The flowing current
creates a magnetic field.
Earth’s inner core is
probably solid iron
that is also very hot.
It stays solid due to
very high pressure.
Magnetic Minerals
Some rocks and minerals are natural magnets.
Magnetite, or lodestone, is highly magnetic. Pieces
of magnetite will point north and south. In the past,
sailors used lodestones as compasses.
Earth’s Magnetic Poles
Earth’s magnetic field is strongest at its poles. But
Earth’s magnetic poles and its geographic poles are
not the same. Earth’s geographic poles are found on
its axis. The axis is the invisible line around which
our planet rotates. Earth’s magnetic north pole is in
Canada. It is about 1,000 kilometers (600 miles) from
the geographic North Pole. Its magnetic south pole is
in the ocean near Antarctica.
Scientists think that Earth’s
magnetic field is made by its
outer core.
A compass needle normally
points north and south. The
needle lines up with Earth’s
magnetic field.
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Compasses
Earth’s Magnetic Field
A compass is a tool people use to find directions.
A compass needle is a tiny magnet. It can turn freely
inside the compass. One end of a compass needle
always points to Earth’s magnetic north pole. But why?
Earth’s magnetic field attracts the needle. The lines
of the field run north and south between the poles. A
compass needle lines up with the magnetic field.
Suppose you head east with a compass. You keep
the N on the compass under the end of the needle that
points north. Suddenly the needle swings away. Why?
Any magnet can detect Earth’s magnetic field. Just
hang the magnet so it can swing freely. The magnet
will soon line up with Earth’s magnetic lines of force.
Its ends will point north and south.
But how do you know which pole points north and
which points south? Compare your hanging magnet
with a compass. Keep the compass far enough away
from the magnet so it does not pull the compass
needle. The north end of the magnet points the same
way as the north end of the compass. Label that end of
the magnet N. Now you can use it as a compass.
A Magnetic Rock
You backtrack. Soon you find a large rock that
causes the compass needle to move. You
found magnetite! The magnetite’s
magnetic field was stronger than
Earth’s. So the compass needle
swung around.
A donut magnet
does not have ends.
But its poles still
line up with Earth’s
magnetic field.
Hang a magnet
so it turns freely.
Its poles will line
up with Earth’s
magnetic field.
The bar magnet’s
magnetic field
causes the compass
needle to swing.
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How to Make a Compass
Finding Your Way
A compass can be made with a needle, a magnet,
a bowl of water, and a sponge. First, rub the needle on
the magnet. Rub it quickly and always in the same
direction. Rubbing the needle on the magnet gives it a
magnetic field.
Then, put the needle on the sponge. Place the
needle parallel to the water’s surface. Let the sponge
float in a bowl of water. This lets the needle turn freely.
Magnetized needles line up with Earth’s magnetic
field. They point north and south. A real compass will
show the direction of magnetic north. Magnetic north
is the north end of the needle in a floating compass.
How does a floating compass show directions? Label
north, south, east, and west on the edges of the bowl.
Turn the bowl so that the north end of the needle, or
magnet, points to the “north” mark on the bowl. It is
hard to use this kind of compass on a hike. But it works
just like a real compass does.
A piece of cork works just as
well as sponge does for making
a floating compass.
A magnetized needle floats on a
piece of sponge. It will point toward
Earth’s magnetic north pole.
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How are electric
currents and
magnetic fields
related?
In 1820, the Danish scientist Hans Christian
Oersted was running electric current through a wire. A
nearby compass’s needle moved each time he turned
on the current. Oersted wondered why. He realized that
the flow of electric current made a magnetic field.
These pictures show how an electric current affects
compasses. When the current flows, the compass
needles line up along the lines of a magnetic field.
When the current is not
flowing, the compass
needles point north.
Moving Magnets Produce Electricity
In 1831, British scientist Michael Faraday found
that you can make an electric current by moving
a magnet inside a wire coil. He had invented the
dynamo. It uses a moving magnet to make electricity.
The meter shown here measures electric current. A
magnet sitting in the coil of wire does not seem to do
much. The meter reading is zero. No current is flowing.
What if the magnet is moved back and forth? The
meter’s needle swings away from zero. The moving
magnet makes electricity that flows through the meter.
Electric charges in motion cause magnetism. They
also cause electric current. So you can
conclude that magnetism
and electricity are
related.
When current is flowing, the
compass needles line up with
the magnetic field.
The meter’s needle
swings away from zero
as the magnet is moved
back and forth.
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Electromagnets
An electromagnet is a coil of wire with many
loops. Electric current passes through the loops. The
moving current creates a magnetic field.
You can change the strength of an electromagnet.
You can make its magnetic field stronger. One way is
to put an iron or steel bar inside the coil of wire. You
can also add more coils of wire. You can wrap the coils
more tightly to make them closer together.
You can connect more batteries
or use a more powerful
battery. The pictures on these
pages show two electromagnets
of different strengths.
A more powerful battery
makes more electric current. So
its magnetic field is stronger.
This battery makes a small
amount of current, which
makes a weak magnetic field.
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Current flows through the coils that are
attached to the batteries. It changes some
energy to heat. The heat made by the currents
changes the mat’s color.
15
Applications of Electromagnets
If you have used a computer recently, then you
have used an electromagnet. If you looked inside a
computer, you would not see an iron bar with wire
wrapped around it. But the computer’s hard drive uses
electromagnets just the same.
A bar with a coil of wire is the simplest type of
electromagnet. Electromagnets can come in different
sizes and shapes. They may be made of high-tech
materials. But they can only make a magnetic field
when electric current passes through them.
This computer
hard drive uses
electromagnetic
fields to save and
call up files.
Scrap Metal Lifter
A scrap metal lifter is a giant electromagnet. It
weighs far less than the metal it picks up. The
lifter’s face is one pole of a bar magnet. This
strong magnet needs special parts. The parts stop
the magnetic field after the current stops flowing.
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How are
electromagnets used?
Electromagnets in Sound Devices
Electromagnets are found in electronic devices
that make sound. Electromagnets in speakers form
a magnetic field. The field changes as the amount
of current changes. The changes cause motion, or
vibrations. Vibrations make the sound waves you hear.
Earphones
You can also hear sound in earphones. With
earphones, electric current comes through the
wires. A metal disc and an electromagnet are inside
each earphone. Changes in the current make the
magnetism weaker or stronger. These changes make
each metal disc vibrate. The vibrations are the sounds
that you can hear. This speaker works in a similar way.
magnet
speaker cone
How a Doorbell Works
Pressing the button closes the circuit. This makes
current flow to the transformer, which controls the
amount of current. Current in the wire coil makes an
electromagnet. The electromagnet pulls up a contact
arm. The contact arm moves a clapper to hit the bell.
electromagnet
contact arm
bell
How a Speaker Works
Electromagnets quickly turn on and off inside the
speaker. The magnet pushes and pulls on the speaker
cone. This makes the cone vibrate. The vibrations
make sound waves.
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Electric Energy Changing to Motion
Going Around in Circles
A motor changes electrical energy into mechanical
energy. Mechanical energy is the energy that makes
things move. Computers, CD and DVD players, and
some toy cars have motors.
The rotor is stopped from changing directions
after it turns halfway. Why? Because one of the
wires connected to one side of the rotor has had its
insulation removed. But the wire connected to the
other side of the rotor still has one side of its insulation.
The insulation stops the flow of charges in the spinning
coil. But before the coil stops spinning, the bare wire
closes the circuit. This allows charges to flow again.
So how does the motion of the motor turn into the
spin of a fan or blender? Find the axle in the picture.
The axle gets turned when the rotor spins. In some
objects, the axle connects to something that can spin.
These might be the blades of a fan or a blender. Can
you picture where the blades might attach?
Simple Electric Motor
Look at the simple motor on page 21. It has a
battery, a magnet, and a coil with seven loops. There
are also parts that connect or hold the other parts
together. The rotor is the coil of wire at the center. Its
spins. The battery’s electric current makes a magnetic
field in the rotor. The permanent magnet’s poles attract
and repel the poles of the rotor’s magnetic field. This is
what makes the rotor itself spin.
The fan’s electric motor turns a rod
called an axle. The axle moves the fan
blades. The fan blades and motor spin
at the same speed.
Battery
Rotor
Permanent magnet
Axle
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How Generators Make Electricity
A generator is a machine that turns coils of wire
around very strong magnets. It changes motion into
electrical energy.
Huge generators make the electric power for most
homes, buildings, and factories. They change the
motion of moving water flowing through a dam into
electrical energy. Or they change the wind that blows
past blades into electrical energy. These forces of
motion turn the coiled wire fields around each other.
A wind farm is made up of rows and rows of
wind turbines. A turbine is a machine that turns by
the power of wind or water. A wind turbine has long
blades. The turbine’s axle connects to an electric
generator. California has over 13,000 wind turbines.
The power they make can light San Francisco.
Wind spins the turbine blades.
An axle connects them to the
generator.
As the blades spin, gears make
the generator spin much faster.
More electricity is made as the
generator spins faster.
Wind farms are often located in narrow
mountain passes with strong winds.
The wind turns a rotor. The rotor
is made of coils of wire around an
iron core. The rotor spins inside the
magnetic field of another magnet in
the generator. As the rotor turns, an
electric current is produced.
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The generator is located
on top of a pole. The pole
can be 20 stories high.
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Glossary
What did you learn?
1. Where is a magnetic field strongest?
electromagnet
a coil of wire through which electric
current passes, creating a magnetic
field
generator
a machine that uses moving
magnets to produce electrical
energy
magnetic field
the space around a magnet in
which magnetic forces operate
magnetic poles the two ends of a magnet, called
the north magnetic pole and the
south magnetic pole
magnetism
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a force that acts on either a moving
electric charge or a magnetic
material that is near a magnet
2. What happens if you place one magnet’s south pole near
another magnet’s north pole?
3. Who was William Gilbert? What idea did he develop?
4.
Write a paragraph about
electromagnets, based on what you read on pages 14 and
15. Then edit and revise the paragraph by adding
information about electromagnets from pages 16 and 17.
5.
Main Idea and Details What is the main idea on page 8?
What details support this idea?