KEY CONCEPT
Current can produce
magnetism.
Sunshine State
STANDARDS
SC.C.2.3.1: The student
knows that many
forces (e.g., gravitational, electrical, and
magnetic) act at a distance (e.g., without
contact).
SC.H.3.3.4: The student
knows that technological design should
require taking into
account constraints
such as natural laws,
the properties of the
materials used, and
economic, political,
social, ethical, and
aesthetic values.
VOCABULARY
BEFORE, you learned
NOW, you will learn
• Electric current is the flow
of charge
• Magnetism is a force exerted
by magnets
• Magnets attract or repel
other magnets
• How an electric current can
produce a magnetic field
• How electromagnets are used
• How motors use electromagnets
EXPLORE Magnetism from Electric Current
What is the source of magnetism?
PROCEDURE
1
Tape one end of the wire to the battery.
2 Place the compass on the table. Place the
wire so that it is lying beside the compass,
parallel to the needle of the compass.
Record your observations.
MATERIALS
•
•
•
•
electrical tape
copper wire
AA cell (battery)
compass
3 Briefly touch the free end of the wire to
the other end of the battery. Record your
observations.
electromagnetism p. 421
electromagnet p. 422
4 Turn the battery around and tape the other
end to the wire. Repeat steps 2 and 3.
WHAT DO YOU THINK?
• What did you observe?
• What is the relationship between the direction of the
battery and the direction of the compass needle?
An electric current produces a magnetic field.
reminder
Current is the flow of electrons through a conductor.
Like many discoveries, the discovery that electric current is related to
magnetism was unexpected. In the 1800s, a Danish physicist named
Hans Christian Oersted (UR-stehd) was teaching a physics class.
Oersted used a battery and wire to demonstrate some properties of
electricity. He noticed that as an electric charge passed through the
wire, the needle of a nearby compass moved.
When he turned the current off, the needle returned to its original
direction. After more experiments, Oersted confirmed that there is a
relationship between magnetism and electricity. He discovered that an
electric current produces a magnetic field.
420 Unit 3: Electricity and Magnetism
Electromagnetism
The relationship between electric current and magnetism plays an
important role in many modern technologies. Electromagnetism
is magnetism that results from an electric current. When a charged
particle such as an electron moves, it produces a magnetic field.
Because an electric current generally consists of moving electrons,
a current in a wire produces a magnetic field. In fact, the wire acts as
a magnet. Increasing the amount of current in the wire increases the
strength of the magnetic field.
VOCABULARY
Remember to record
electromagnetism in your
notebook.
magnetic
You have seen how magnetic
field
field lines can be drawn around
a magnet. The magnetic field
lines around a wire are usually
illustrated as a series of circles.
The magnetic field of a wire
current-carrying
wire
actually forms the shape of
a tube around the wire. The
direction of the current determines the direction of the magnetic field.
If the direction of the electric current is reversed, the magnetic field
still exists in circles around the wire, but is reversed.
If the wire is shaped into a loop, the magnetism becomes concentrated inside the loop. The field is much stronger in the middle of the
loop than it is around a straight wire. If you wind the wire into a coil,
the magnetic force becomes stronger with each additional turn of wire
as the magnetic field becomes more concentrated.
coil
currentcarrying wire
S
N
magnetic
field
A coil of wire with charge flowing through it has a magnetic field
that is similar to the magnetic field of a bar magnet. Inside the coil,
the field flows in one direction, forming a north pole at one end.
The flow outside the coil returns to the south pole. The direction of
the electric current in the wire determines which end of the coil
becomes the north pole.
Check Your Reading
How is a coil of wire that carries a current similar to
a bar magnet?
Chapter 12: Magnetism 421
Making an Electromagnet
Recall that a piece of iron in a strong magnetic field becomes a magnet
itself. An electromagnet is a magnet made by placing a piece of iron
or steel inside a coil of wire. As long as the coil carries a current, the
metal acts as a magnet and
iron
coil
increases the magnetic field of
core
the coil. But when the current
is turned off, the magnetic
domains in the metal become
S
N
random again and the magnetic field disappears.
By increasing the number
of loops in the coil, you can
increase the strength of the electromagnet. Electromagnets exert a much
more powerful magnetic field than a coil of wire without a metal core.
They can also be much stronger than the strongest permanent magnets
made of metal alone. You can increase the field strength of an electromagnet by adding more coils or a stronger current. Some of the most
powerful magnets in the world are huge electromagnets that are used
in scientific instruments.
check your reading
How can you increase the strength of an electromagnet?
Electromagnets
How can you make an electromagnet?
SKILL FOCUS
Observing
PROCEDURE
1
Starting about 25 cm from one end of the wire, wrap the wire in tight coils
around the nail. The coils should cover the nail from the head almost to
the point.
2 Tape the two batteries together as shown. Tape one end of the wire to a free
battery terminal.
3 Touch the point of the nail to a paper clip and record your observations.
4 Connect the other end of the wire to the other battery terminal. Again touch
the point of the nail to a paper clip. Disconnect the wire from the battery.
Record your observations.
WHAT DO YOU THINK?
• What did you observe?
• Did you make an electromagnet? How do you know?
CHALLENGE Do you think the result would be different
if you used an aluminum nail instead of an iron nail? Why?
422 Unit 3: Electricity and Magnetism
MATERIALS
•
•
•
•
•
insulated wire
large iron nail
2 D cells
electrical tape
paper clip
TIME
20 minutes
Uses of Electromagnets
Because electromagnets can be turned on and off, they have more uses
than permanent magnets. The photograph below shows a powerful
electromagnet on a crane. While the electric charge flows through the
coils of the magnet, it lifts hundreds of cans at a recycling plant. When
the crane operator turns off the current, the magnetic field disappears
and the cans drop from the crane.
MAIN IDEA WEB
Make a main idea
web for the uses of
electromagnets.
A permanent magnet would not be nearly as useful for this purpose.
Although you could use a large permanent magnet to lift the cans, it
would be hard to remove them from the magnet.
This powerful electromagnet can be turned on and
off to collect and move
cans at a recycling plant.
electromagnet
wire
supplying
electric
current
You use an electromagnet every time you store information on a
computer. The computer hard drive contains disks that have billions
of tiny magnetic domains in them. When you save a file, a tiny
electromagnet in the computer is activated. The magnetic field of the
electromagnet changes the orientation of the small magnetic domains.
The small magnets store your file in a form that can be read later by
the computer. A similar system is used to store information on magnetic tape of an audiocassette or videocassette. Sound and pictures are
stored on the tape by the arrangement of magnets embedded in the
plastic film.
Magnetic information is often stored on credit cards
and cash cards. A black strip on the back of the card
contains information about the account number and
passwords. The cards can be damaged if they are frequently exposed to magnetic fields. For example, cards
should not be stored with their strips facing each other, or
near a magnetic clasp on a purse or wallet. These magnetic
fields can change the arrangement of the tiny magnetic
domains on the card and erase the stored information.
423
Motors use electromagnets.
Because magnetism is a force, magnets can be used to move things.
Electric motors convert the energy of an electric current into motion by
taking advantage of the interaction between current and magnetism.
There are hundreds of devices that contain electric motors. Examples
include power tools, electrical kitchen appliances, and the small fans in
a computer. Almost anything with moving parts that uses current has
an electric motor.
VISUALIZATION
CLASSZONE.COM
See a motor in motion.
Motors
Page 425 shows how a simple motor works. The photograph at the top
of the page shows a motor that turns the blades of a fan. The illustration in the middle of the page shows the main parts of a simple
motor. Although they may look different from each other, all motors
have similar parts and work in a similar way. The main parts of an
electrical motor include a voltage source, a shaft, an electromagnet,
and at least one additional magnet. The shaft of the motor turns other
parts of the device.
Recall that an electromagnet consists of a coil of wire with current
flowing through it. Find the electromagnet in the illustration on page
425. The electromagnet is placed between the poles of another magnet.
FLORIDA
Content Review
reminder
Notice how simple
machines, which you read
about in grade 6, are combined to create complicated
machines.
When current from the voltage source flows through the coil, a
magnetic field is produced around the electromagnet. The poles of
the magnet interact with the poles of the electromagnet, causing the
motor to turn.
1
The poles of the magnet push on the like poles of the electromagnet,
causing the electromagnet to turn.
2
As the motor turns, the opposite poles pull on each other.
3
coil of
wire
magnet
shaft
When the poles of the electromagnet line up with
the opposite poles of the magnet, a part of the
motor called the commutator reverses the polarity
of the electromagnet. Now, the poles push on each
other again and the motor continues to turn.
The illustration of the motor on page 425 is
simplified so that you can see all of the parts. If
you saw the inside of an actual motor, it might
look like the illustration on the left. Notice that
the wire is coiled many times. The electromagnet in a strong motor may coil hundreds of
times. The more coils, the stronger the motor.
What causes the electromagnet in a motor to turn?
424 Unit 3: Electricity and Magnetism
How a Motor Works
Although motors may look different from each other,
they all have similar parts and work in a similar way.
motor in fan
electromagnet
shaft
voltage
source
magnet
shaft
commutator
The commutator rotates along
with the electromagnet, causing
the electromagnet’s poles to
switch with every half-rotation.
electromagnet
1
2
Like poles of the magnets push
on each other.
As the motor turns, opposite
poles attract.
3
The electromagnet’s poles are
switched, and like poles again repel.
Would a motor work without an electromagnet? Why or why not?
Chapter 12: Magnetism 425
Uses of Motors
Many machines and devices contain electric motors that may not be
as obvious as the motor that turns the blades of a fan, for example.
Even though the motion produced by the motor is circular, motors
can move objects in any direction. For example, electric motors move
power windows in a car up and down.
Motors can be very large, such as the motors that
power an object as large as a subway train. They draw
electric current from a third rail on the track or wires
overhead that carry electric current. A car uses an
electric current to start the engine. When the key is
turned, a circuit is closed, producing a current from the
battery to the motor. Other motors are very small, like
the battery-operated motors that move the hands of
a wristwatch.
Motor B moves
a laser across
the CD.
These gears change
the rotational motion
of the motor into a
straight motion.
laser
Motor A
turns the CD.
Check Your Reading
The illustration on the left shows the
two small motors in a portable CD player.
Motor A causes the CD to spin. Motor B
is connected to a set of gears. The gears
convert the rotational motion of the
motor into a straight-line motion, or
linear motion. As the CD spins, a laser
moves straight across the CD from the
center outward. The laser reads the
information on the CD. The motion
from Motor B moves the laser across
the CD.
Explain the function served by each motor in a CD player.
KEY CONCEPTS
CRITICAL THINKING
1. Explain how electric current and
magnetism are related.
4. Contrast How does an
electromagnet differ from
a permanent magnet?
2. Describe three uses of
electromagnets.
3. Explain how electrical energy
is converted to motion in
a motor.
426 Unit 3: Electricity and Magnetism
5. Apply Provide examples of
two things in your home that
use electric motors, and
explain why they are easier to
use because of the motors.
CHALLENGE
6. Infer Why is it necessary
to change the direction of
the current in the coil of an
electric motor as it turns?