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Lesson 2
How Are Electricity and Magnetism Related?
Fast fact
Mighty Magnets! The world's strongest magnets are made from iron, boron, and
neodymium, a metal. A tiny 50-mm (2-in.) magnet made of these materials can lift
up to 30 kg (66 lb)! This sculpture uses pairs of these strong magnets. In the
Investigate, you'll learn how to make a different kind of magnet.
Investigate
Can Electricity Make a Magnet?
Materials
 bar magnet
 D-cell battery
 small compass
 30-cm length of insulated wire with stripped ends
 sheet of cardboard
 tape
Procedure
1. Move a bar magnet around a compass. Observe what the compass needle does. Put
the magnet away.
2. Tape the battery to the cardboard. Tape one end of the wire to the flat end of the
battery. Leave the other end loose.
3. Tape the wire to the cardboard in a loop, as shown in the picture.
4. Place the compass on top of the loop. Observe the direction in which the compass
needle points.
5. Touch the loose end of the wire to the pointed end of the battery. Observe what the
compass needle does. Record your observations.
Draw Conclusions
1. How does a magnet affect a compass needle?
2. How does an electric current affect a compass needle?
3. Inquiry Skill Compare electricity and magnetism. How are they alike?
Investigate Further
Repeat steps 4 and 5, with the compass under the loop. Compare your
observations.
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Reading in Science
VOCABULARY
magnet p. 152
magnetic pole p. 154
magnetic field p. 155
electromagnet p. 156
generator p. 158
electric motor p. 159
SCIENCE CONCEPTS
 how electric currents are like magnets
 how generators and motors work
READING FOCUS SKILL
COMPARE AND CONTRAST Look for ways in which magnets and electric currents
are alike and different.
Magnets
Have you ever played with a magnet? If you haven't, you may want to now! As you
move a magnet around and bring it close to objects, you can discover what a
magnet does.
A magnet is an object that attracts iron and a few (not all) other metals. Magnets attract
steel because it contains iron. When you bring an iron object or a steel object close
to a magnet, the object moves toward the magnet. Try this for yourself. Place a
steel paper clip near a magnet. What happens? Try the same thing with a plastic
paper clip. How do your results compare?
All magnets attract iron, but they may not look alike. Some magnets are shaped like
bars. Others are U-shaped. Some magnets that stick to refrigerator doors are thin,
flat shapes.
---see pictures
A magnet inside the plastic base holds these steel pieces together.
The horseshoe magnet attracts these metal objects. What metal do the objects
contain?
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---see pictures
Some magnets are shaped like bars.
Why do these plastic letters stick to this refrigerator door?
Distance affects the strength of a magnet's attraction. A small steel object that is close to
a magnet moves toward it. However, if the same object is farther away, it will not
move toward the magnet.
Other forces can overcome the force of a magnet. Refrigerator magnets stick well to the
door, but you can easily pull them off.
Barriers can interfere with a magnet's pull, too. A refrigerator magnet may hold one or
two sheets of paper to the door, but if you put too many sheets under it, the magnet
will fall.
Magnets can make some other objects magnetic. For example, if you rub a needle over
a magnet several times in the same direction, the needle will become magnetic
enough to pick up other needles.
COMPARE AND CONTRAST How are all magnets alike? How are they
different?
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Magnetic Poles and Magnetic Fields
A magnet has two places at which its force is the strongest. Each of these is called a
magnetic pole, or pole for short. If you tie a string around the middle of a bar
magnet and let it swing, one end will point north. That end is the magnet's northseeking pole. It is often marked with an N. The end that points south is often
marked S.
Forces between magnetic poles act like forces between electric charges. Opposite poles
attract, and like poles repel. If you hold two S poles or two N poles near each other,
they push apart. If you hold an N pole and an S pole near each other, they attract.
Magnets of every shape have N and S poles. Try holding two round refrigerator
magnets close together. If you turn them in one direction, they attract each other. If
you turn them in the other direction, they repel each other.
Magnets keep their poles even if you change their shape. If you cut a bar magnet into
pieces, each piece would be a magnet with both an N pole and an S pole.
Magnetic Poles
---see pictures
Putting the opposite poles of magnets near each other causes the magnets to pull
toward each other.
Putting the like poles of magnets near each other causes the magnets to push away
from each other.
All magnets have N and S poles. Poles that are the same repel, or push apart. Poles
that are opposite attract, or pull together.
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Magnetic Field
---see pictures
A bar magnet moves iron filings into the shape of the magnet's field.
The shape of a magnetic field depends on the shape of the magnet. A magnetic field
forms loops all around this bar magnet.
You can use a compass to see a magnet's force. If you put a compass close to a bar
magnet, the needle will point toward the magnet's N pole and away from the
magnet's S pole.
A magnetic field is the space around a magnet in which the force of the magnet acts. If
you put iron filings around the magnet, you will be able to see the shape of the
magnet's field. The filings will form circles that start and end at the poles, where
the magnet's pull is the strongest.
COMPARE AND CONTRAST How are a magnet's magnetic force and magnetic
field different?
Insta-Lab
Needle Dance
Place a compass near one pole of a magnet. Then move it all around the magnet. What
does the needle do in each position? What does this tell you?
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Electromagnets
In the Investigate, you saw that a current of electricity causes a magnetic force. You
showed that electricity and magnetism are related.
Actually, an electric current produces a magnetic field around a wire. You can't see the
field, but it circles the wire. The field around a single wire is weak. The field
around many wires close together is strong. When coils wrap around an iron core,
such as a nail, the core becomes an electromagnet.
An electromagnet is a temporary magnet. It has a magnetic force only when an electric
current moves through the wire. The electromagnet does not work if the current is
switched off.
With many coils of wire and a strong current, electromagnets can be made very strong.
In junkyards, such electromagnets lift many tons of scrap iron and steel.
COMPARE AND CONTRAST How are a magnet and an electromagnet alike?
How are they different?
---see picture
A steel nail is not normally a magnet, but it becomes one when an electric current
runs around it. How can you tell that this nail is now a magnet? What would
happen if you took one of the wire ends off the battery?
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Math in Science
Interpret Data
In one experiment, a student built and tested an electromagnet. This graph shows the
results. What hypothesis was the student testing? How did the student measure the
magnet's strength? What conclusion can you draw from this graph?
---see graph
---see pictures
This powerful electromagnet is being used to move scrap iron in a junkyard.
This is an MRI (magnetic resonance imaging) machine in a hospital. It uses a
magnetic field to take pictures of the brain, the muscles, and other soft tissues
inside the body.
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Generators and Motors
Electricity can produce a magnetic field. Luckily for us, the reverse is also true—a
magnetic field can produce electricity. If you move a coil of wire near a magnet,
current electricity flows in the wire. That is how a generator, a device that
produces an electric current, works.
Any source of energy that can turn a coil of wire in a magnetic field can produce
electricity. Hand-cranked generators use human power to turn the coil. During a
power failure, you might use a gasoline-powered generator to produce electricity
for lights.
Power plants in large cities use many huge generators to produce enough electricity to
meet people's needs. Most power plants burn coal, oil, or natural gas. The fuel heats
water until it turns to steam. The steam's pressure turns a turbine, (TER•bin) a
machine that produces a spinning motion. The turbine spins a coil inside the field
of a magnet to produce electricity. Then the electricity is sent out along power lines
to homes and businesses.
Generators use motion to produce electricity. Can electricity produce motion? If you
have ever seen someone
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use a mixer or an electric drill, you know that the answer is yes. Such tools have an
electric motor in them. An electric motor changes electrical energy into
mechanical energy.
In some motors, an electromagnet lies between the poles of a permanent magnet. Like
all magnets, the electromagnet has an N pole and an S pole. The poles are pushed
away from the like poles of the permanent magnet. They are pulled toward the
opposite poles of the permanent magnet.
The motor's shaft turns until the poles of the electromagnet are near the opposite poles
of the permanent magnet. Then the current of the electromagnet reverses. Its N pole
becomes its S pole, and its S pole becomes its N pole. The shaft turns again. The
current keeps reversing, and the shaft keeps spinning.
Electric motors do many useful things. They start cars. They run CD players. The next
time you turn on a fan, thank the inventors of electric motors!
COMPARE AND CONTRAST How are a generator and an electric motor alike?
How are they different?
---see pictures pg.158
Generators
This small, simple generator doesn't turn a coil inside a magnet. Instead, it turns a
magnet around a coil. The result is the same—an electric current is generated.
This hand-cranked generator uses "people power" to produce electricity.
---see pictures pg.159
Motors
This simple electric motor turns because a permanent magnet pushes and pulls on
an electromagnet.
An electric motor sends this toy car racing across the floor.
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Other Uses of Magnets
Generators and motors aren't the only devices that work because of magnets.
Compasses point north because they respond to Earth's natural magnetic field. This
helps people find their way on land and at sea.
Magnets are used in computers, compact disc players, and magnetic recording devices,
such as VCRs. They are also inside headphones, stereo speakers, and telephone
receivers. Doorbells and phones ring because of magnets. Even the strip on the
back of a credit card is a magnet.
Magnets are used for recording information. In a computer hard drive, for example,
electromagnets move across a disk's surface. They make one disk area more
magnetic than another. The disk spins under a part called the head. Later, the head
reads the magnetic information
COMPARE AND CONTRAST How are a videotape and a computer hard drive
alike?
---see pictures
When the head inside the VCR records, an electromagnet "writes" a magnetic
field on the videotape. When the tape is played back, the magnetic field on the
tape produces an electrical signal.
In this computer hard drive, information is stored as a pattern of magnetic fields.
Computers have magnets inside.
A VCR (videocassette recorder) uses electricity and magnetism to record sounds
and pictures.
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Reading Review
1. COMPARE AND CONTRAST Draw and complete this graphic organizer to
explain how electricity and magnetism are related.
---see chart
2. SUMMARIZE Use the completed graphic organizer to write a lesson summary.
3. DRAW CONCLUSIONS Tell why the relationship between electricity and
magnetism is important to people.
4. VOCABULARY Use the terms electricity and magnet in a sentence to explain how
a generator works.
Test Prep
5. Critical Thinking Why is an electromagnet not a permanent magnet?
6. If you turn the N poles of two magnets toward each other, what will they do?
A. attract
B. produce
C. repel
D. spin electricity
Writing
Expository Writing
Write a paragraph for a friend. Tell your friend how an electric motor works.
Math
Make a Bar Graph
Test the strength of one magnet by measuring how many paper clips it can pick up.
Then test two, three, and four magnets stuck together. Does it matter how the
magnets are put together? Make a bar graph of your results.
Social Studies
History of Names
Electricity is measured in amperes, coulombs, ohms, watts, and volts. Find out more
about the famous scientists whose names are used for these units of measurement.
Share what you learn with the class.
For more links and activities, go to www.hspscience.com
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