Transcript Chapter 18

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Chapter 18
Electromagnetism
Table of Contents
Section 1 Magnets and Magnetism
Section 2 Magnetism from Electricity
Section 3 Electricity from Magnetism
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Chapter 18
Section 1 Magnets and Magnetism
Bellringer
Given the following objects, a piece of aluminum foil, a
nickel, a plastic figure, a piece of wood, a glass vase,
and some paper clips, predict which objects will be
attracted to a magnet. How do you know?
Record your answers in your science journal.
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Chapter 18
Section 1 Magnets and Magnetism
Objectives
• Describe the properties of magnets.
• Explain why some materials are magnetic and some
are not.
• Describe four kinds of magnets.
• Give two examples of the effect of Earth’s magnetic
field.
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Chapter 18
Section 1 Magnets and Magnetism
Properties of Magnets
• Magnetic Poles are points on a magnet that have
opposite magnetic qualities.
• North and South The pole of a magnet that points
to the north is called the magnet’s north pole. The
opposite end of the magnet, called the south pole,
points to the south.
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Chapter 18
Section 1 Magnets and Magnetism
Properties of Magnets, continued
• Magnetic Forces When you bring two magnets
close together, the magnets each exert a magnetic
force on the other. These magnetic forces result from
spinning electric charges in the magnets. The force
can either push the magnets apart of pull them
together.
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Chapter 18
Section 1 Magnets and Magnetism
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Chapter 18
Section 1 Magnets and Magnetism
Properties of Magnets, continued
• Magnetic Fields A magnetic field exists in the region
around a magnet in which magnetic forces can act.
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Chapter 18
Section 1 Magnets and Magnetism
The Cause of Magnetism
• Atoms and Domains In materials such as iron,
nickel, and cobalt, groups of atoms are in tiny areas
called domains. The north and south poles of the
atoms in a domain line up and make a strong
magnetic field.
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Chapter 18
Section 1 Magnets and Magnetism
The Cause of Magnetism, continued
• Losing Alignment When domains move, the
magnet is demagnetized, or loses its magnetic
properties.
• Making Magnets You can make a magnet from
demagnetized material if you line up its domains with
another magnet.
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Chapter 18
Section 1 Magnets and Magnetism
The Cause of Magnetism, continued
• Cutting a Magnet When you cut a magnet in half,
you end up with two magnets.
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Chapter 18
Section 1 Magnets and Magnetism
Kinds of Magnets
• Atoms and Domains Some magnets, called
ferromagnets, are made of iron, nickel, cobalt, or
mixtures of those metals. Another kind of magnet is the
electromagnet. This is a magnet made by an electric
current.
• Temporary and Permanent Magnets Temporary
magnets are made from materials that are easy to
magnetize. But they tend to lose their magnetization
easily. Permanent magnets are difficult to magnetize,
but tend to keep their magnetic properties longer.
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Chapter 18
Section 1 Magnets and Magnetism
Earth as a Magnet
• One Giant Magnet Earth behaves as if it has a bar
magnet running through its center.
• Poles of a Compass Needle The point of a
compass needle is attracted to the south pole of a
magnet. Opposite poles of magnets attract each other.
• South Magnetic Pole near North Geographic Pole
A compass needle points north because the magnetic
pole of Earth that is closest to the geographic North
Pole is a magnetic south pole.
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Chapter 18
Section 1 Magnets and Magnetism
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Chapter 18
Section 1 Magnets and Magnetism
Earth as a Magnet, continued
• The Core of the Matter Scientists think that the
Earth’s magnetic field is made by the movement of
electric charges in the Earth’s core.
• A Magnetic Light Show Earth’s magnetic field plays
a part in making auroras. An aurora is formed when
charged particles from the sun hit oxygen and nitrogen
atoms in the air.
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Chapter 18
Section 1 Magnets and Magnetism
Earth’s Magnetic Field
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Visual Concept
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Chapter 18
Section 2 Magnetism from
Electricity
Bellringer
Suppose you had a machine that could levitate heavy
objects with the flick of a switch. Write a paragraph in
your science journal about how this machine would
make your life easier.
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Chapter 18
Section 2 Magnetism from
Electricity
Objectives
• Identify the relationship between an electric current
and a magnetic field.
• Compare solenoids and electromagnets.
• Describe how electromagnetism is involved in the
operation of doorbells, electric motors, and
galvanometers.
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Chapter 18
Section 2 Magnetism from
Electricity
The Discovery of Electromagnetism
• Hans Christian Oersted Danish physicist Hans
Christian Oersted discovered the relationship between
electricity and magnetism in 1820.
• More Research From his experiments, Oersted
concluded that an electric current produces a magnetic
field. His research was the first research in
electromagnetism—the interaction between electricity
and magnetism.
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Chapter 18
Section 2 Magnetism from
Electricity
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Chapter 18
Section 2 Magnetism from
Electricity
Using Electromagnetism
• A solenoid is a coil of wire that produces a magnetic
field when carrying an electric current.
• An electromagnet is made up of a solenoid wrapped
around an iron core.
• Turning Electromagnets On and Off
Electromagnets are very useful because they can be
turned on and off as needed. The solenoid has a field
only when there is electric current in it.
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Chapter 18
Section 2 Magnetism from
Electricity
Applications of Electromagnetism
• Doorbells Two solenoids in a doorbell allow the
doorbell to work.
• Magnetic Force and Electric Current An electric
current can cause a compass needle to move. The
needle is a small magnet. This property is useful in
electric motors.
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Chapter 18
Section 2 Magnetism from
Electricity
Applications of Electromagnetism, continued
• An electric motor is a device that changes electrical
energy into mechanical energy. All electric motors have
an armature—a loop or coil of wire that can rotate.
• Galvanometers A galvanometer measures current. A
galvanometer has an electromagnet placed between
the poles of a permanent magnet.
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Chapter 18
Section 2 Magnetism from
Electricity
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Chapter 18
Section 3 Electricity from Magnetism
Bellringer
Have you ever discovered something by accident?
Maybe you looked in a dictionary for the definition of
an unknown word, only to find the definition of another
word you didn’t know.
Write a short paragraph in your science journal
describing how you have discovered something by
accident.
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Chapter 18
Section 3 Electricity from Magnetism
Objectives
• Explain how a magnetic field can make an electric
current.
• Explain how electromagnetic induction is used in a
generator.
• Compare step-up and step-down transformers.
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Chapter 18
Section 3 Electricity from Magnetism
Electric Current from a Changing
Magnetic Field
• Faraday’s Experiment Michael Faraday conducted
an experiment trying to get the magnetic field of the
electromagnet to make an electric current in a second
wire.
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Chapter 18
Section 3 Electricity from Magnetism
Electric Current from a Changing
Magnetic Field, continued
• Success for an Instant Faraday realized that
electric current in the second wire was made only
when the magnetic field was changing. The process
by which an electric current is made by changing a
magnetic field is called electromagnetic induction.
• Inducing Electric Current Look at the next slide to
see electromagnetic induction.
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Chapter 18
Section 3 Electricity from Magnetism
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Chapter 18
Section 3 Electricity from Magnetism
Electric Generators
• Alternating Current The electric current produced
by the generator shown on the next slide changes
direction each time the coil makes a half turn.
Because the electric current changes direction, it is an
alternating current.
• Generating Electrical Energy The energy that
generators convert into electrical energy comes from
different sources such as fossil fuels and nuclear
energy.
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Chapter 18
Section 3 Electricity from Magnetism
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Chapter 18
Section 3 Electricity from Magnetism
Transformers
• A transformer increases or decreases the voltage of
alternating current.
• Step Up, Step Down The number of loops in the
primary and secondary coils of a transformer
determines whether it increases or decreases the
voltage.
• Electrical Energy for Your Home The electric
current that brings electrical energy to your home is
usually transformed three times.
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Chapter 18
Section 3 Electricity from Magnetism
Transformers, continued
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Chapter 18
Electromagnetism
Concept Map
Use the terms below to complete the concept map
on the next slide.
mechanical energy
electromagnetic
induction
transformers
voltage
electrical energy
electric motor
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Chapter 18
Concept Map
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Chapter 18
Concept Map
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End of Chapter 18 Show
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Chapter 18
Standardized Test Preparation
Reading
Read each of the passages. Then, answer the
questions that follow each passage.
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Chapter 18
Standardized Test Preparation
Passage 1 Place a small disk of plastic foam in a
bowl of water. Hold the eye of a sewing needle in
one hand and a bar magnet in the other. Starting
near your fingers, drag one end of the magnet
down the needle from the eye to the point and off the
end of the needle. Drag the magnet down the needle
in the same direction about 20 times. Be sure each
stroke uses the same end of the magnet and moves
in the same direction down the needle. Carefully
place the needle on the plastic foam.
Continued on next slide
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Chapter 18
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Passage 1, continued The needle and foam should
float. Bring the south pole of the magnet near the
needle. Note which end of the needle points toward
the magnet. Remove the magnet, and observe how
the needle turns. The end of the needle that pointed
toward the south pole of the magnet will point in a
northerly direction.
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Standardized Test Preparation
1. What is the purpose of the passage?
A to express
B to instruct
C to convince
D to direct
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Chapter 18
Standardized Test Preparation
1. What is the purpose of the passage?
A to express
B to instruct
C to convince
D to direct
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Chapter 18
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2. What can a person build by following the steps
described in the passage?
F a fishing float
G a bar magnet
H an electromagnet
I a compass
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2. What can a person build by following the steps
described in the passage?
F a fishing float
G a bar magnet
H an electromagnet
I a compass
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Chapter 18
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Passage 2 Frogs have been seen floating in midair!
No, it’s not a magic trick. It’s part of an experiment on
magnetic levitation. Every object, living or nonliving,
contains atoms that act like magnets. These atomic
magnets are millions of times weaker than ordinary
household magnets. But the atomic magnets are
still strong enough to be influenced by other magnets.
If an object, such as a frog, is exposed to a magnet
that is strong enough, the magnetic force between the
object and the magnet can lift the object and make it
float.
Continued on the next slide
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Passage 2, continued A large solenoid is used in
these experiments. A solenoid is a coil of wire that
acts like a magnet when an electric current is in the
wire. The solenoid has a magnetic field with a north
pole and a south pole. The interaction of the
magnetic field of the solenoid and the magnetic
fields of the atoms in the object causes the object to
float.
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1. Why can objects be made to float in a magnetic
field?
A Every object has a strong magnetic field.
B Every object has atoms that have magnetic fields.
C Every object acts like a solenoid.
D Every object makes a magnetic field when it is
exposed to a household magnet.
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Chapter 18
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1. Why can objects be made to float in a magnetic
field?
A Every object has a strong magnetic field.
B Every object has atoms that have magnetic fields.
C Every object acts like a solenoid.
D Every object makes a magnetic field when it is
exposed to a household magnet.
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Standardized Test Preparation
2. Which of the following can be inferred from
information in the passage?
F Household magnets cannot levitate in the solenoid
used in the experiment.
G Atomic magnets are stronger than the solenoid
used in the experiment.
H Household magnets are strong enough to levitate a
frog.
I Household magnets are stronger than atomic
magnets but weaker than the solenoid used in the
experiment.
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Chapter 18
Standardized Test Preparation
2. Which of the following can be inferred from
information in the passage?
F Household magnets cannot levitate in the solenoid
used in the experiment.
G Atomic magnets are stronger than the solenoid
used in the experiment.
H Household magnets are strong enough to levitate a
frog.
I Household magnets are stronger than atomic
magnets but weaker than the solenoid used in the
experiment.
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Interpreting Graphics
The graph below shows current versus rotation angle
for the output of an alternating current generator. Use
the graph below to answer the questions that follow.
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1. Which of the following describes points at which the
generator produces no current?
A a and b
B b and d
C c and d
D a and e
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1. Which of the following describes points at which the
generator produces no current?
A a and b
B b and d
C c and d
D a and e
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2. Between which points is current increasing?
F between a and b
G between b and c
H between c and e
I between b and d
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2. Between which points is current increasing?
F between a and b
G between b and c
H between c and e
I between b and d
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3. At which points is the same amount of current
produced but the direction of the current reversed?
A a and b
B b and d
C a and c
D b and e
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3. At which points is the same amount of current
produced but the direction of the current reversed?
A a and b
B b and d
C a and c
D b and e
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Chapter 18
Standardized Test Preparation
Math
Read each question, and choose the best answer.
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Standardized Test Preparation
1. The voltage across the secondary coil of a
transformer is found by multiplying the voltage across
the primary coil by the number of turns in the secondary
coil and then dividing by the number of turns in the
primary coil. A certain transformer has a primary coil with
1,000 turns and a voltage of 1,200 V. What is the voltage
across the secondary coil if it has 2,000 turns?
A 600 V
B 1,000 V
C 2,400 V
D 2,400,000 V
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1. The voltage across the secondary coil of a
transformer is found by multiplying the voltage across
the primary coil by the number of turns in the secondary
coil and then dividing by the number of turns in the
primary coil. A certain transformer has a primary coil with
1,000 turns and a voltage of 1,200 V. What is the voltage
across the secondary coil if it has 2,000 turns?
A 600 V
B 1,000 V
C 2,400 V
D 2,400,000 V
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Chapter 18
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2. Laura cuts a magnetic strip into thirds. She then
cuts each third into halves. How many small magnets
does she have?
F 1.5
G3
H5
I6
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Chapter 18
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2. Laura cuts a magnetic strip into thirds. She then
cuts each third into halves. How many small magnets
does she have?
F 1.5
G3
H5
I6
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Chapter 18
Standardized Test Preparation
3. Mrs. Welch is ordering magnets for her science
class. She has 24 students in her class, who work in
pairs. Each pair of lab partners needs three magnets
for the experiment. If Mrs. Welch can order a pack of
six magnets for $5.00, what is the cost for her order?
A $2.50
B $30.00
C $60.00
D $120.00
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Chapter 18
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3. Mrs. Welch is ordering magnets for her science
class. She has 24 students in her class, who work in
pairs. Each pair of lab partners needs three magnets
for the experiment. If Mrs. Welch can order a pack of
six magnets for $5.00, what is the cost for her order?
A $2.50
B $30.00
C $60.00
D $120.00
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Chapter 18
Standardized Test Preparation
4. A disc magnet is 0.5 cm thick and has a diameter of
2.5 cm. The volume of this magnet in cubic
centimeters (cm3 ) can be calculated using which of
the following equations?
F V  (2.5)2  0.5
G V  (2.5/2)2  0.5
H V  (0.5)2  2.5
I V  (0.5/2)2  2.5
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Chapter 18
Standardized Test Preparation
4. A disc magnet is 0.5 cm thick and has a diameter of
2.5 cm. The volume of this magnet in cubic
centimeters (cm3 ) can be calculated using which of
the following equations?
F V  (2.5)2  0.5
G V  (2.5/2)2  0.5
H V  (0.5)2  2.5
I V  (0.5/2)2  2.5
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Chapter 18
Section 1 Magnets and Magnetism
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Chapter 18
Section 1 Magnets and Magnetism
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Chapter 18
Section 1 Magnets and Magnetism
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Chapter 18
Section 1 Magnets and Magnetism
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Chapter 18
Section 1 Magnets and Magnetism
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Chapter 18
Section 2 Magnetism from
Electricity
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Chapter 18
Section 2 Magnetism from
Electricity
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Chapter 18
Section 3 Electricity from Magnetism
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Chapter 18
Section 3 Electricity from Magnetism
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Chapter 18
Section 3 Electricity from Magnetism
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Chapter 18
Section 3 Electricity from Magnetism
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Chapter 18
Standardized Test Preparation
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