Download 21.1 Magnets and Magnetic Fields

Section 21.1
21.1 Magnets and Magnetic Fields
1 FOCUS
Objectives
21.1.1 Describe the effects of
magnetic forces and magnetic
fields and explain how
magnetic poles determine the
direction of magnetic force.
21.1.2 Interpret diagrams of
magnetic field lines around one
or more bar magnets.
21.1.3 Describe Earth’s magnetic field
and its effect on compasses.
21.1.4 Explain the behavior of
ferromagnetic materials in
terms of magnetic domains.
Key Concepts
◆
How can a magnetic
field affect a magnet
that enters the field?
◆
Why are some materials
magnetic while others
are not?
◆
◆
◆
Reading Strategy
Using Prior Knowledge Copy the diagram
below and add what you already know about
magnets. After you read, revise the diagram
based on what you learned.
a.
?
b.
?
d.
?
Properties
of magnets
?
A
ncient Greeks observed that magnetite, or lodestone, attracts iron.
Some time before 200 A.D., the Chinese sculpted magnetite into spoonshaped compasses. They called these stones “south pointers.” By 1150
A.D., Chinese navigators used compasses with magnetized iron needles.
But properties of magnets were not well explained until 1600. In that
year, the English physician William Gilbert published De Magnete.
L2
Word-Part Analysis Have students
research the origin of the word magnet
and write a short paragraph explaining
how the word originated. (The word
magnet is derived from the name
Magnesia, a region that was once
part of ancient Greece. This area was
known for its magnetite ore mines.)
Reading Strategy
◆
magnetic force
magnetic pole
magnetic field
magnetosphere
magnetic domain
ferromagnetic
material
c.
Reading Focus
Build Vocabulary
Vocabulary
How do magnetic
poles interact?
Magnetic Forces
L2
a. Can be temporary or permanent
b. Have north and south poles; like poles
repel, unlike poles attract c. Only a few
materials can be magnets. d. Magnets
affect objects with iron but don’t affect
most materials, such as paper, cotton,
and so on.
2 INSTRUCT
Figure 1 The green magnet and
lower red magnet attract each
other. The lower red magnet and
the yellow magnet repel each
other. Predicting What would
happen if the upper red magnet
on the pencil were flipped over?
You can explore properties of magnets on your own. Either side of a
magnet sticks to a refrigerator. Yet if you push two magnets together,
they may attract or repel. Magnetic force is the force a magnet exerts
on another magnet, on iron or a similar metal, or on moving charges.
Recall that magnetic force is one aspect of electromagnetic force.
Magnetic forces, like electric forces, act over a distance. Look at the
suspended magnets in Figure 1. If you push down on the top two magnets, you can feel the magnets repel. Push harder, and the force
increases. Magnetic force, like electric force, varies with distance.
Gilbert used a compass to map forces around a magnetite sphere. He
discovered that the force is strongest at the poles. All magnets have two
magnetic poles, regions where the magnet’s force is strongest. One end
of a magnet is its north pole; the other end is its south pole. The direction
of magnetic force between two magnets depends on how the poles face.
Like magnetic poles repel one another, and opposite magnetic
poles attract one another.
Magnetic Forces
Build Reading Literacy
L1
KWL (Know/Want to Know/Learned)
Refer to page 124D in Chapter 5,
which provides the guidelines for
a KWL strategy.
Have students label three columns on
a sheet of paper K, W, and L. Have
them write in the K column what they
know about magnetic forces, and in
the W column questions they would like
answered about the forces exerted by
magnets. Then, have students read the
paragraphs on this page and record in
the L column the answers to as many of
their questions as possible.
Verbal, Interpersonal
630 Chapter 21
630 Chapter 21
Section Resources
Print
• Laboratory Manual, Investigation 21B
• Guided Reading and Study Workbook
With Math Support, Section 21.1
• Transparencies, Chapter Pretest and
Section 21.1
Technology
• iText, Section 21.1
• Presentation Pro CD-ROM, Chapter Pretest
and Section 21.1
Magnetic Fields
Magnetic Fields
Integrate Earth Science
A magnetic field surrounds a magnet and can exert magnetic forces. In
Figure 2, iron filings are used to show the shape of the magnetic field
around a bar magnet.
A magnetic field, which is strongest near a
magnet’s poles, will either attract or repel another magnet that enters
the field. The field lines begin near the magnet’s north pole and extend
toward its south pole. The arrows on the field lines indicate what direction a compass needle would point at each point in space. Where lines are
close together, the field is strong. Where lines are more spread out, the
field is weak.
Earth’s magnetic field is produced by
the motions of hot, liquefied iron within
its core. Induced electric currents in
the iron give rise to magnetic fields,
which affect the flow of the iron and
cause the resulting magnetic fields to
become stronger.
Magnetic Fields Around Magnets You can use iron filings
to visualize how magnetic fields of two magnets interact. Figure 3A
shows the north pole of one magnet facing the north pole of another
magnet. Notice that there are no iron filings in the gap between the magnets. Iron filings are not attracted to this area because the combined
magnetic field is very weak. Figure 3B shows the combined field of two
magnets with opposite poles facing each other. The field lines start at
the north pole of one magnet and extend to the south pole of the other
magnet. The field in the gap between the magnets is very strong, as you
can see from the dense crowding of iron filings in this area.
L2
Figure 2 A magnetic field
surrounds every magnet. Iron
filings reveal the field lines, which
start near the north pole and
extend toward the south pole.
Interpreting Diagrams In
which two areas of a bar magnet
is the field strongest?
Figure 3 Iron filings reveal the combined magnetic field
of two interacting magnets. A When like poles of two
magnets come together, the magnets repel each other.
B When opposite poles of magnets come together, the
magnets attract each other.
Earth’s magnetic field undergoes sudden
reversals every few hundred thousand
years, although the change does not
happen at regular intervals. Currently
the North Magnetic Pole is a south
pole, which explains why “North” on
a compass points to the north. Evidence
for reversals is found in cooled magma,
where iron atoms have aligned with
Earth’s magnetic field at the time of cooling. Cooled magma in ridges along the
ocean floor provides a continuous record
of magnetic field reversals over time.
Logical
Build Science Skills
L2
Inferring
Purpose Students
are helped to an
understanding of the
shape of the magnetic field
around two bar magnets.
A
Materials 2 bar magnets, a small
magnetic compass
Class Time 20 minutes
Procedure Arrange two magnets as
shown in either part of Figure 3. Have
students place the compass at different
positions about 1–2 cm away from the
magnets and sketch the direction in
which the compass needle points for each
position. Ask them how the direction of
the needle corresponds to the direction of
the field lines in each location. Have them
identify any patterns they notice.
B
Magnetism
631
Customize for English Language Learners
Reinforce Science Concepts
The various concepts, such as magnetic field,
magnetic forces, and magnetic domains, each
use the word magnetic, but mean and refer to
different things. To help English language
learners develop a clear understanding of these
concepts, have them construct a Word Analysis
Chart. Instruct students to write each of the
vocabulary words on a separate chart and give
a definition of each word. Suggest that they
compare and contrast the vocabulary words.
Finally, have them draw a picture or diagram
to illustrate each concept.
Expected Outcome The compass
needle will be parallel to the field lines
at any location. The needle’s south pole
points along the field lines toward the
north pole of a nearby magnet.
Logical, Visual
Answer to . . .
Figure 1 The red magnet would float
above the blue magnet.
Figure 2 The field is strongest at
the poles.
Magnetism 631
Geographic
North Pole
Section 21.1 (continued)
Observing Magnetic
Field Lines
Figure 4 Earth is surrounded by
magnetic field lines. These lines
are densest at the poles.
Skill Focus Observing
small container of iron filings,
2 bar magnets, paper,
2 textbooks, masking tape
Class Time 20 minutes
Safety Students should wear safety
goggles, not inhale the iron filings, and
wash their hands when finished.
Teaching Tips
• Dropping or banging magnets causes
them to lose their strength.
• Suggest that students tape the paper
in place, so that the pattern is not
accidentally disturbed.
Expected Outcome Students will
realize that the interaction of the field
lines of two magnets depends on how
the magnets are positioned.
Analyze and Conclude
1. The field was strongest in the gap
between opposite poles and weakest in
the gap between like poles.
2. The field lines of two like poles spread
apart and had a gap between the poles
with very few field lines. The field lines
of two unlike poles extended in lines
connecting the north and south pole.
3. No pattern would appear, because
sawdust is not magnetic. Visual, Group
L3
Have students repeat the experiment
using different separations between the
magnets. Ask, What happens to the
iron filings and to the field strength
when two opposite poles are moved
apart? (The filings are less crowded,
indicating the field is weaker.)
Visual, Logical
Magnetic Materials
FYI
Magnetic domains are quite small and
can only be imaged using microscopes.
A variety of instruments are used, such
as scanning tunneling microscopes,
magnetic force microscopes, and light
microscopes with polarizing filters.
Magnetic
South Pole
Observing Magnetic
Field Lines
Materials
Prep Time 10 minutes
632 Chapter 21
Magnetic
field
L2
Objective
After completing this activity, students
will be able to
• recognize how the magnetic fields
of two magnets combine.
For Enrichment
Magnetic
North Pole
Procedure
1. Place two textbooks side by
side, about 7 cm apart.
2. Place the magnets between
the books, with north poles
facing, about 2 cm apart.
Tape the magnets in place.
Magnetic Field Around Earth Earth is like a giant magnet
surrounded by a magnetic field. The area surrounding Earth that is influenced by this field is the magnetosphere (mag NET oh sfeer).
A compass points north because it aligns with Earth’s magnetic
field. However, as Figure 4 shows, Earth’s magnetic poles are not at the
geographic poles. The geographic North Pole is at 90° N latitude, but
the magnetic North Pole is at about 81° N latitude. Because of this, a
compass may point east or west of north. The angle between the direction to true north and to magnetic north is called magnetic
declination. Magnetic declination varies with your location on Earth.
3. Place the paper over the
magnets to form a bridge.
4. Sprinkle iron filings on the
paper until you can see the
magnetic field lines. Sketch
your observations.
5. Carefully return the filings
to their container.
6. Repeat Steps 2 through 5
with opposite poles facing.
Analyze and Conclude
1. Inferring Where was the
magnetic field the
strongest? The weakest?
2. Analyzing Data How did
the fields of like poles
facing differ from those of
unlike poles facing?
3. Predicting What result
would you expect if you
used sawdust instead of
iron filings?
Geographic
South Pole
Why does a compass point toward north?
Magnetic Materials
Within an atom, electrons move around the nucleus. This movement,
along with a property of electrons called “spin,” causes electrons to act
like tiny magnets. In many materials, each electron is paired with another
having an opposite spin. Magnetic effects mostly cancel each other. As a
result, these materials have extremely weak magnetic fields.
Many other materials have one or more unpaired electrons. The
unpaired electrons produce magnetic fields. But the fields usually
don’t combine because the arrangement of the atoms isn’t quite
right. These materials have weak magnetic fields. In a few materials,
such as iron, nickel, and cobalt, the unpaired electrons make a strong
magnetic field. Then the fields combine to form magnetic domains.
A magnetic domain is a region that has a very large number of atoms
with aligned magnetic fields. A ferromagnetic material (fehr oh mag
NET ik), such as iron, can be magnetized because it contains magnetic
domains.
When a material is magnetized, most of its magnetic
domains are aligned.
632 Chapter 21
Facts and Figures
Strong and Weak Magnetic Fields Earth
has the strongest magnetic field of the rocky
inner planets. Earth has an average field
strength at its surface of between 30 ␮T
and 60 ␮T. (A tesla, T, is the unit by which
magnetic fields are measured.) This is some
100 times stronger than the magnetic field of
Mercury, roughly 1000 to 5000 times stronger
than the field of Mars, and about 100,000
times stronger than the field of Venus.
Nonmagnetized Materials The fact that a material is
ferromagnetic does not mean it is a magnet. If the domains of a
ferromagnetic material are aligned randomly, the magnetization of the
domains is cancelled, and it is not a magnet. An iron nail is an example of a nonmagnetized material. It is ferromagnetic, so the domains
have the potential to be aligned, but normally they are not. Figure 5A
shows the random orientation of domains in nonmagnetized iron.
A
L2
Students may be visualizing the motion
of the electron as if it were a planet
rotating on its axis. Emphasize that the
“spin” of an electron is not like the spin
of a ball, any more than the orbital
motion of an electron around an atom’s
nucleus is like the motion of a planet
around the sun. The term spin is applied
to electron behavior that mathematically
resembles that of a spinning object.
Remind students of how electrons in
atoms are modeled as “clouds” where
they are most likely to be located.
Logical
Magnetized Materials You can easily magnetize a nonmagnetized ferromagnetic material by placing it in a magnetic field. For
example, if you put a nonmagnetized iron nail near a magnet, you
will turn the nail into a magnet. Figure 5B shows the alignment of
magnetic domains in magnetized iron. The applied magnetic field
causes magnetic domains aligned with the field to grow larger. This
magnetization can be temporary. If the magnet is moved away from
the nail, the motion of the atoms in the nail causes the magnetic
domains to become randomly oriented again. In some ferromagnetic materials, the domains stay aligned for a long time. These
materials are called permanent magnets. They are not truly permanant, because heat or a jarring impact can realign the domains.
If you cut a magnet in half, each half will have its own north pole
and south pole because the domains will still be aligned. If you cut
the pieces in half again, each half will again have a north pole and a
south pole. No matter how many times you cut the magnets, each
piece will have two different poles. A magnet can never have just a
north pole or just a south pole.
B
3 ASSESS
Evaluate
Understanding
Figure 5 A magnetic field can
magnetize ferromagnetic materials.
A Before magnetization, domains are
random. B Domains aligned with the
field grow during magnetization.
Unaligned domains can shrink.
Section 21.1 Assessment
Reviewing Concepts
1.
2.
Describe the interaction of magnetic poles.
What two things can happen to a magnet
entering a magnetic field?
3.
What makes a material magnetic?
4. Describe what happens to the fields of two bar
magnets when you bring their north poles
together.
Critical Thinking
5. Predicting What happens if you suspend a
bar magnet so that it can swing freely?
6. Relating Cause and Effect How are
electrons responsible for magnetism?
7. Predicting What will happen if you hit a
magnet with a hammer? Explain.
8. Designing Experiments How could you
test the effects of heating and cooling on the
magnetization of a bar magnet?
Section 21.1
Assessment
1. Magnetic poles that are alike repel one
another, and magnetic poles that are different
attract one another.
2. A magnetic field will either attract or repel
another magnet that enters the field.
3. For a material to be magnetized, most of its
magnetic domains must be aligned.
4. The fields interact, and the field between
the magnets becomes very weak.
Ask students why a refrigerator magnet
sticks to the door of a refrigerator. Be
sure they explain which material is a
permanent magnet, and what happens
at the atomic level in the magnetized
material. (The atoms in the refrigerator
magnet, which is made of ferromagnetic
material, are aligned in the various
magnetic domains, and so give the
magnet a permanent field. When the
magnet is attached to the unmagnetized
door of the refrigerator, the atoms of
the door are aligned, and so become
magnetized temporarily.)
Reteach
L1
Use Figure 3 to explain the shape and
direction of a magnetic field around a
bar magnet.
Electric Charge Review electric charge in
Section 20.1. Compare the attraction and
repulsion of positive and negative charges
with the behavior of two bar magnets
placed near one another.
Magnetism
L2
633
5. The north end of the magnet will swing
toward north, aligning with Earth’s magnetic
field just as a compass does.
6. The spin and orbital motion of electrons in
an atom give the atom a magnetic field.
7. The motion of the atoms can cause the
magnetic domains to become randomly
aligned. The material loses its magnetization.
8. Students’ suggestions for experiments
should include some method for testing the
magnetization of a bar magnet before and after
it is heated and before and after it is cooled.
Like charges and like poles repel, while
opposite charges and opposite poles
attract. In contrast to electric charges,
magnetic poles can’t be separated.
If your class subscribes
to iText, use it to review key concepts in
Section 21.1.
Answer to . . .
The north end of a
compass points north
because a freely suspended bar magnet
aligns with Earth’s magnetic field.
Magnetism 633
Section 21.1 (continued)
Anti-Theft
Security Devices
Anti-Theft
Security Devices
L2
Electromagnetic tag systems were first
developed in the 1960s, along with
other similar RFID (Radio Frequency
Identification) technology. This type
of system uses electromagnetic waves
to identify objects that have been
tagged with magnetic material.
Electromagnetic waves consist of
changing electric fields and changing
magnetic fields that are at right angles
to each other and to the direction of
the wave. The EM tag system uses the
magnetic component of an electromagnetic wave to temporarily magnetize
an activated tag as it passes between
the pedestals. This change in the tag’s
magnetic domains produces a small
electromagnetic wave with a particular
frequency. The wave is detected by a
receiver, causing an alarm to sound.
A deactivated tag, however, is fully
magnetized, so no change occurs in
the magnetic domains when the tag
passes between the pedestals. Thus,
no electromagnetic wave is produced,
and the tag passes through the
electromagnetic field undetected.
The magnetic properties of the tag
cause it to become temporarily
magnetized more easily than ordinary
steel objects. This is why a screwdriver
or box of paper clips can pass through
the system without setting off the alarm.
Other systems make use of thin wire
coils in the tags that act as antennas,
as well as small circuit elements.
Electromagnetic waves emitted from
the pedestal at a particular frequency
induce a current in the tag’s antenna,
and this induced current produces
an electromagnetic wave with a
characteristic frequency. This wave is
then detected by the receiver pedestal.
Applying Concepts The deactivated
tag is more highly magnetized than the
activated tag.
Logical
For Enrichment
L3
Students can make a multimedia presentation about the EM tag system, as well
as other RFID systems. Articles on the
subject can be found on the Internet and
in science and engineering periodicals.
Verbal, Portfolio
634 Chapter 21
Anti-theft security devices are found in stores across the
world. One of the best of these devices is the electromagnetic (EM) tag system. This system is based on the
interaction between a small piece of magnetic material
(a tag) and an EM field created between two pedestals
at the store exit. Applying concepts Which is more
highly magnetized, an activated or a deactivated tag?
Activated tag An activated tag
is slightly demagnetized. When it
passes through the pedestal’s EM
field, the tag’s magnetic domains line
up with the field. This change in
magnetic domain emits a signal that is
picked up by the receiver, which sets
off the alarm.
Library security
Powerful magnets are used to
deactivate tags in library books
before borrowing. If the tag is
not deactivated, the alarm will
go off at the library exit.
Receiver pedestal
Flashing
alarm light
Transmitter
pedestal
Activated tag
(demagnetized)
Magnetic
domain
Changing
electromagnetic
field
Deactivated tag
A deactivated tag is fully
magnetized. When it passes through
the exit, the tag’s domains do not
change. Because no signal is
emitted, the alarm is not set off.
Wire loop carrying
alternating current
Deactivated
tag (fully
magnetized)
634 Chapter 21
Tag signal
Activated tag
attached to item
Changing
electromagnetic
field
The pedestals The transmitter
pedestal contains a wire loop that
produces a changing EM field in the region
between the pedestals. The receiver pedestal
picks up any signal produced by the tag.