Download 21.2 Electromagnetism

Section 21.2
21.2 Electromagnetism
1 FOCUS
Objectives
Key Concepts
Vocabulary
How can an electric charge
create a magnetic field?
◆
How is an electromagnet
controlled?
◆
How do galvanometers,
electric motors, and
loudspeakers work?
◆
◆
◆
electromagnetic
force
solenoid
electromagnet
galvanometer
electric motor
Reading Strategy
Identifying Main Idea Copy the table
below. As you read, write the main idea of
the text that follows each topic.
Topic
Main Idea
Electricity and magnetism
a.
?
Direction of
magnetic fields
b.
?
Direction of electric currents
c.
?
Solenoids and
electromagnets
d.
?
Electromagnetic devices
e.
?
Y
ou know that unlike electric charges attract one another and that
like electric charges repel one another. It is easy to discover a similar
effect with the north and south poles of two magnets. However, it’s
much more difficult to figure out the relationship between electricity and magnetism. In fact, the connection was discovered
accidentally by the Danish scientist Hans Christian Oersted in 1820.
One evening Oersted, pictured in Figure 6, was conducting scientific demonstrations for his friends and students in his home. One
demonstration used electric current in a wire, and another used a compass needle attached to a wooden stand. As Oersted turned on the
current for the electricity demonstration, he saw the compass needle
move. When he turned off the current, the needle moved back to its
original position. Further investigation showed that the current in the
wire produced a magnetic field. Oersted had discovered a relationship
between electricity and magnetism.
Figure 6 In 1820 Hans Oersted
discovered how magnetism and
electricity are connected. A unit
of measure of magnetic field
strength, the oersted, is named
after him.
Reading Focus
Build Vocabulary
L2
Concept Map Have students make a
concept map comparing the devices in
the vocabulary list.
Reading Strategy
L2
a. Electricity and magnetism are different
aspects of electromagnetic force.
b. Magnetic fields are produced at right
angles to an electric current. c. Electric
currents are deflected perpendicular to a
magnetic field. d. Changing the current
in an electromagnet controls the strength
and direction of its magnetic field.
e. Electromagnetic devices change
electrical energy into mechanical energy.
Electricity and Magnetism
Electricity and magnetism are different aspects of a single force
known as the electromagnetic force. The electric force results from
charged particles. The magnetic force usually results from the
movement of electrons in an atom. Both aspects of the electromagnetic force are caused by electric charges.
Magnetism
635
2 INSTRUCT
Electricity and
Magnetism
Section Resources
Print
• Guided Reading and Study Workbook
With Math Support, Section 21.2
• Transparencies, Section 21.2
21.2.1 Describe how a moving electric
charge creates a magnetic field
and determine the direction
of the magnetic field based on
the type of charge and the
direction of its motion.
21.2.2 Relate the force a magnetic
field exerts on a moving electric
charge to the type of charge
and the direction of its motion.
21.2.3 Explain how solenoids and
electromagnets are constructed
and describe factors that affect
the field strength of both.
21.2.4 Describe how electromagnetic
devices use the interaction
between electric currents and
magnetic fields.
Build Reading Literacy
Technology
• iText, Section 21.2
• Presentation Pro CD-ROM, Section 21.2
• Go Online, NSTA SciLinks, Electromagnets
L1
Predict Refer to page 66D in
Chapter 3, which provides the
guidelines for predicting.
Have students read the first two
paragraphs on p. 635. Ask them to
predict what Oersted discovered about
the relationship between electricity and
magnetism. (Predictions should indicate
that an electric current produces a
magnetic field.) Logical
Magnetism 635
Section 21.2 (continued)
Magnetic Field from
Electric Current
Figure 7 If you point the thumb of
your right hand in the direction of
the current, your fingers curve in the
direction of the magnetic field.
Inferring How can you determine
the magnetic field direction from
the direction of electron flow?
Direction
of current
L2
Direction
of electron
flow
Purpose Students observe how an
electric current produces a magnetic field.
Current-carrying wire
Materials insulated wire, cardboard
(10 cm ⫻ 10 cm), a burner tripod,
a variable DC power supply,
4–6 compasses
Direction
of magnetic
field
Procedure Punch a small hole in the
center of the cardboard and thread the
wire through the hole. Lay the cardboard
flat on the burner tripod’s ring support so
that the wire passes through the tripod
center, perpendicular to the cardboard
and extending in a straight line 10 cm on
either side. (A ring stand and clamp may
be needed to support the upper end
of the wire.) Connect both ends of the
wire to the terminals of the power supply.
Place the compasses on the cardboard
at a distance of 3–4 cm from the wire.
Turn on the power supply and increase
the current until the compass needles
begin to deflect. Have students notice
how the needles deflect with respect to
the wire. Remove the compasses, turn
off the power supply, reverse the wire
connections, and repeat the
demonstration.
Safety Use insulated wire. Follow
procedures for electrical safety.
Expected Outcome When the top
end of the wire is connected to the
positive terminal of the power supply,
the magnetic field will be in a counterclockwise pattern around the wire,
according to the right-hand rule. This
will cause the poles of the compasses to
align themselves along the edge of a
circle around the wire. The south poles
will form a clockwise pattern. When the
connections are reversed, the direction in
which the compasses point will reverse.
Visual, Group
Use Visuals
L1
Figure 8 Explain that the right-hand
rule also applies to Figure 8. Ask, How
could you use your hand to determine
the deflection of an electron moving
through the magnetic poles? (Use your
right hand with your thumb in the direction
of the current, which will be opposite the
direction of the electron’s travel.)
Visual
636 Chapter 21
Magnetic Fields Around Moving Charges Oersted’s
discovery about the relationship between a current-carrying wire and
a magnet established an important physics principle.
Moving
electric charges create a magnetic field. These moving charges may
be the vibrating charges that produce an electromagnetic wave. They
may also be, as in Oersted’s experiment, the moving charges in a wire.
Figure 7 shows how to remember the direction of the magnetic field
that is produced. The magnetic field lines form circles around a
straight wire carrying a current.
Forces Acting on Moving Charges Recall that an electric
field exerts a force on an electric charge. The force is either in the same
direction as the electric field or in the opposite direction, depending on
whether it is a positive or negative charge.
The effect of a magnetic field on a moving charge is different, as
shown in Figure 8. A charge moving in a magnetic field will be
deflected in a direction perpendicular to both the magnetic field and
to the velocity of the charge. If a current-carrying wire is in a magnetic
field, the wire will be pushed in a direction perpendicular to both the
field and the direction of the current. Reversing the direction of the
current will still cause the wire to be deflected, but in the opposite
direction. If the current is parallel to the magnetic field, the force is
zero and there is no deflection.
Force deflecting
the charge
What are two kinds of moving charges that
can create a magnetic field?
Velocity
+ of charge
Figure 8 A moving positive charge is deflected at
a right angle to its motion by a magnetic field.
Inferring In what direction would the particle
be deflected if it had a negative charge instead of
a positive charge?
636 Chapter 21
Customize for Inclusion Students
Visually Impaired
The right-hand rule can be used by students
with visual impairments to understand
magnetic fields and forces. Explain how
students can use their right hand to predict
the directions of magnetic fields for an electric
current in a straight wire and a solenoid. As
students may have difficulty using Figure 7,
instruct them using a wire, so that they can
understand how the right-hand thumb and
fingers are oriented for a positive current and
its magnetic field. Then, have students adapt
the rule for positive charges moving in a
magnetic field, as shown in Figure 8 (that is,
the thumb points in the direction of the
moving charge, the fingers extend in the
direction of the magnetic field, and the force
on the charge points outward from the palm).
Encourage those students who successfully
master the rule to explain it to the class.
Solenoids and
Electromagnets
Making an Electromagnet
Materials
3. Open the switch. CAUTION If the switch is left
closed, the wire will become very warm. Wrap
the longer wire 40 more times around the nail
in the same direction as before.
iron nail, 20 small metal paper clips, 20-cm length
and 1-m length of insulated wire with stripped
ends, 6-volt battery, switch
4. Close the switch. Record how many paper
clips the nail can pick up now.
Procedure
1. Make a circuit using the nail, wire, battery,
and switch. Use the shorter wire to connect
one terminal of the battery to the switch.
Connect the longer wire to the other
terminal of the battery. Wrap this wire
around the nail 10 times. Then connect
the longer wire to the switch.
Making an
Electromagnet
Objective
After completing this activity, students
will be able to
• predict how the number of turns of
wire affects the strength of the
electromagnet.
5. Open the switch and disconnect the circuit.
Analyze and Conclude
2. Hold the head of the nail over the pile of
paper clips. Close the switch. Record how
many paper clips the nail can pick up.
1. Observing How did your ability to pick up
paper clips with the nail change when you
increased the number of turns in the coil?
Skill Focus Observing, Drawing
Conclusions
2. Drawing Conclusions Why did the nail
become a magnet when a current-carrying
wire was wrapped around it?
Solenoids and Electromagnets
Prep Time 20 minutes
Advance Prep Cut the wires in
advance and use a wire stripper or wirecutting pliers to remove 2 cm of
insulation from each end of the wires.
A
Before you can use electromagnetic force, you need to be able
to control it. Using electromagnetic force requires some simple
tools. Figure 9A shows a current-carrying wire with a loop in it.
The magnetic field in the center of the loop points right to left
through the loop, as shown in Figure 9A.
Suppose you loop the wire many times to make a coil, as
shown in Figure 9B. Then the magnetic fields of the loops
combine so that the coiled wire acts like a bar magnet. The
field through the center of the coil is the sum of the fields
from each turn of the wire. A coil of current-carrying wire
that produces a magnetic field is called a solenoid.
If you place a ferromagnetic material, such as an iron rod,
inside the coil of a solenoid, the strength of the magnetic
field increases. The magnetic field produced by the current
causes the iron rod inside the coil of the solenoid to become
a magnet. An electromagnet is a solenoid with a ferromagnetic core.
Changing the current in an electromagnet
controls the strength and direction of its magnetic field.
You can also use the current to turn the magnetic field on
and off. People use many devices every day, such as hair
dryers, telephones, and doorbells, that utilize electromagnets.
L2
Class Time 25 minutes
Loop of wire
Safety Students should wear safety
goggles and be careful handling the coil
of wire, as the wire may become hot.
Students should open the switch when
the electromagnet is not in use.
Current
B
Pole
Pole
Analyze and Conclude
1. The electromagnet became stronger
with more turns in the coil.
2. The current in the coil produced a
magnetic field around and through the
nail. This caused the magnetic domains
in the nail to align, temporarily strengthening the magnetic field of the nail.
Logical, Group
Solenoid
Current
Figure 9 The magnetic field lines around
a solenoid are like those of a bar magnet.
Applying Concepts Which of the poles
is north?
Magnetism
Expected Outcome Students will
learn that the strength of an
electromagnet, as indicated by the
number of paper clips picked up, is
directly related to the number of turns
in the coil of wire. More turns make
the magnet stronger.
637
Answer to . . .
Facts and Figures
Big Magnets Because the magnetic fields
produced by electromagnets can be made
stronger by properly designing the
electromagnet, it is not surprising that the
strongest magnetic fields on Earth are produced
by specially designed electromagnets. At the
National High Magnetic Field Laboratory
(NHMFL) at Florida State University in
Tallahassee, electromagnets have been
designed that produce continuous magnetic
fields with strengths up to 50 teslas. These
fields are about a million times stronger than
Earth’s magnetic field at Earth’s surface.
Figure 7 Use the right-hand rule,
but point your thumb in the opposite
direction of the electron flow (which
will be the direction of the current).
Figure 8 It would be deflected down.
Figure 9 The one on the left because
magnetic field lines start at the north
pole and end at the south pole.
Vibrating charges,
flowing charges
in a current
Magnetism 637
Section 21.2 (continued)
L2
Students may wonder how the magnetic
field of a solenoid can be fairly simple
when there are magnetic fields around
each segment of wire in the coil. Explain
that such fields are present, but that they
combine in such a way that the field
outside the solenoid is much weaker than
inside. The fields combine to effectively
form a magnetic field that is similar to
that of a bar magnet.
Logical
The strength of an electromagnet depends on the current in the
solenoid, the number of loops in the coil in the solenoid, and the type
of ferromagnetic core. To increase the strength of an electromagnet,
increase the current flowing through the solenoid. A greater current
produces a stronger magnetic field. Increasing the number of turns,
while keeping the same current, will also increase the field strength.
Cores that are easily magnetized, such as “soft” iron, make stronger
electromagnets.
For: Links on electromagnets
Visit: www.SciLinks.org
Web Code: ccn-2212
What does the strength of an electromagnet
depend on?
Electromagnetic Devices
Electromagnets can convert electrical energy into motion that can do
work.
Electromagnetic devices such as galvanometers, electric
motors, and loudspeakers change electrical energy into mechanical
energy. A galvanometer measures current in a wire through the deflection of an electromagnet in an external magnetic field. An electric motor
uses a rotating electromagnet to turn an axle. A loudspeaker uses electromagnets to convert electrical signals into sound waves you can hear.
Electromagnetic
Devices
Galvanometers Figure 10 shows a galvanometer, a device that
Electromagnetic Force
L2
Purpose Students observe the
magnetic force exerted on a wire
carrying an electric current.
Procedure Pass the wire through the
rings of the ring stands, so that it
extends horizontally about 5–10 cm
above the table surface. Position the
magnet on its side, so that the wire
passes between the magnet’s poles.
Connect the wires to the power supply
and turn it on, increasing the current
until the wire is deflected. Turn off the
power, reverse the connections, and
repeat the demonstration.
Safety Use insulated wire. Follow
procedures for electrical safety.
Expected Outcome Depending on
the orientation of the magnet, the wire
will be deflected either in toward the
magnet’s center or away from it. The
deflecting force is proportional to the
current in the wire and the strength of
the magnetic field.
Visual, Group
Download a worksheet on
electromagnets for students to
complete, and find additional
teacher support from NSTA SciLinks.
638 Chapter 21
4
3
0 1 2
2 1
3
5
Materials insulated wire, a large
horseshoe magnet, a variable DC power
supply, 2 ring stands with clamps
Magnet
Pointer
Wire
Figure 10 A galvanometer uses an
electromagnet to move a pointer.
One common application is in an
automobile gas gauge. The pointer
indicates the amount of current in
the wire. The wire is connected to
a sensor in the gas tank.
638 Chapter 21
4
5
uses an electromagnet to measure small amounts of current. A galvanometer has a small eletromagnet attached to a spring. The
electromagnet is placed between the poles of two permanent magnets.
When there is a current in the electromagnet’s coils, the resulting
magnetic field attempts to align with the field of the permanent
magnets. The greater the current, the more the electromagnet
rotates, as shown by the pointer on the scale. In an automobile
fuel gauge, for example, a sensor in the gas tank reduces the
Scale
current as the gas level decreases. This causes the needle to
rotate towards the “empty” mark.
Loop of
wire
Electric Motors An electric motor is a
Build Science Skills
device that uses an electromagnet to turn an
Current
axle. Figure 11 shows how an electric motor
Commutator
works. In this figure, the wire is connected to a
battery. An actual motor has many loops of
wire around a central iron core to make the
Brush
motor stronger. In the motor of an electric
Direction of
rotation
appliance, the wire would be connected to an
electrical circuit in a building.
What makes a motor turn? When current flows through a loop of
Figure 11 A battery supplies
current to a loop of wire through
wire, one side of the loop is pushed by the field of the permanent
the commutator. As the
magnet. The other side of the loop is pulled. These forces rotate the
commutator turns, the direction
loop. If there were no commutator ring, the coil would come to rest.
of current switches back and
forth. As a result, the coil’s
But as the loop turns, each C-shaped half of the commutator connects
magnetic field keeps switching
with a different brush, reversing the current. The forces now change
direction, and this turns the coil
about an axle.
direction, so the coil continues to rotate. As long as current flows,
Predicting What would happen
rotation continues.
Loudspeakers A loudspeaker contains an electromagnet and a
permanent magnet, much like a motor. However, the current in the
wires entering the loudspeaker changes direction and increases or
decreases to reproduce music, voices, or other sounds. The changing
current produces a changing magnetic field in the electromagnet’s
coil. The magnetic force exerted by the permanent magnet moves the
coil back and forth. As the coil moves, it causes a thin membrane to
vibrate, producing sound waves that match the original sound.
if you reversed the positive
and negative connections on
the battery?
1.
Besides a magnet, what can create
a magnetic field?
2.
How is the magnetic field of an
electromagnet controlled?
3.
How are electromagnets used
in galvanometers, electric motors,
and loudspeakers?
4. How does a ferromagnetic rod inside
a solenoid affect the strength of
an electromagnet?
Critical Thinking
5. Comparing and Contrasting What is the
effect of a magnetic field on a stationary
electric charge? On a moving electric charge?
Evaluate
Understanding
Assessment
1. A moving electric charge can create a
magnetic field.
2. It can be turned on and off. Its strength
and direction can be controlled by controlling
the current.
3. They change electrical energy into
mechanical energy.
4. It makes the magnetic field much stronger.
When current flows through the coil, it creates
a magnetic field that magnetizes the
ferromagnetic rod.
L2
Ask students to list three examples of
devices that use electromagnetic forces
(at least one of which is not given in the
section). Have students explain what
each device does and how electricity
and magnetism interact in the device.
(Could be an electric bell, relay switch,
or microphone)
6. Applying Concepts Why is it a good idea
to have the coil of a solenoid wound closely
with many turns of wire?
7. Inferring What is the purpose of the
commutator in an electric motor?
8. Relating Cause and Effect What causes
the membrane in a loudspeaker to vibrate?
L1
Reteach
Use Figures 7 and 9 to review the
direction of magnetic fields produced
by electric currents.
Insulators In Section 20.2 you learned
that electric charge doesn’t flow easily
through electrical insulators. Use this to
explain why a solenoid has insulated wires.
Magnetism
Section 21.2
Applying Concepts Have students
read the paragraphs on electric motors
and help them apply what they already
know about work, different forms of
energy, and energy conservation. Ask,
What forms of energy are shown for
the electric motor in Figure 11?
(Chemical energy, electrical energy, and
mechanical energy) Ask, What energy
transformations take place when
operating the motor? (Chemical energy
in the battery is converted to electrical
energy. Electrical energy interacts with
the magnetic field to do work, and so is
transformed into the kinetic energy of
the rotating wire loop and into any work
the motor does.)
Logical
3 ASSESS
Section 21.2 Assessment
Reviewing Concepts
L2
639
5. A magnetic field doesn’t affect a stationary
charge. A magnetic field deflects a moving
charge in a direction perpendicular to both
the field and the velocity.
6. It produces a more uniform field and
increases its strength.
7. The commutator reverses the current in the
electromagnet, reversing the magnetic field of
the electromagnet, and enabling the axle to
turn continuously in one direction.
8. The interaction of the magnetic field of the
permanent magnet with the changing field
of the electromagnet
The strength of the electromagnet
depends upon the current in the
solenoid. Insulated wires make it
possible to direct the current through
several tightly wound loops, enhancing
the strength of the magnetic field. The
insulated wire prevents a short circuit
between adjacent coils.
If your class subscribes
to iText, use it to review key concepts in
Section 21.2.
Answer to . . .
Figure 11 The motor’s axle would
spin in the opposite direction.
It depends on the
strength of the current,
the number of coils of wire, and the
type of ferromagnetic core.
Magnetism 639
Peeking Inside
the Human Body
Peeking Inside
the Human Body
L2
Background
MRI is an example of a procedure called
tomography, where many images of
the body are combined to give a composite view. MRI uses nuclear magnetic
resonance, or NMR, to obtain information
from hydrogen atoms in the body.
NMR was discovered in 1946, and was
originally used to identify hydrocarbon
molecules. In the 1970s, the technique
was combined with computers to
produce images of tissues in the body.
Build Science Skills
Magnetic Resonance Imaging (MRI) is used by
doctors to create more detailed images of the human
body than are possible with X-rays.
Purpose Students
will simulate how
applied magnetic fields
can disrupt the magnetic fields of atoms.
Main magnet This powerful
magnet immerses the patient
in a stable, intense magnetic
field—the other three magnets
create a variable field.
Materials a short pencil (about
5 cm long), a cardboard disk
(7 cm wide), a steel thumbtack,
a bar magnet, paper
Radio-frequency
source
Class Time 15 minutes
640 Chapter 21
Head-to-toe
variation
Creating an MRI image
Using Analogies
Expected Outcome Because the top is
fairly stable while spinning, it is analogous
to the spinning hydrogen atoms in the
body. The alignment and deflection of
these atoms by the magnetic fields is
analogous to the deflection of the top by
the magnet. By observing a large-scale
model of an atomic process, students can
visualize the atomic process more clearly.
Visual
Left-to-right
variation
Body tissues vary in their concentration of hydrogen atoms. Fat
The scanner uses three magnetic
fields to read data up and down
has a high concentration, as do tissues containing water, because
and along slices of the body.
of the hydrogen in H2O. The concentration of hydrogen atoms
This produces an image that is
in bone is very low. MRI reveals these differences in great detail,
viewed and interpreted by
doctors and radiographers.
with fat and fluids (including blood) showing up as bright areas
and bone as dark areas. MRI scans can even depict the
brain. It produces images of such
Top-to-bottom
detail that they are used by
Head-to-toe
field magnets
researchers studying how the brain
field magnets
works, as well as by doctors
investigating diseases.
L2
Procedure Insert the thumbtack into
the eraser end of the pencil, and punch
the pencil through the center of the
cardboard disk to make a “top” that can
spin. Make sure the cardboard does not
slip along the surface of the pencil. Place
the top on a piece of paper to prevent
marking the table. Spin the top with the
thumbtack side upward, making sure that
the top is neither too stable or unstable
while spinning. Spin the top again, and
place one end of the magnet about 2 cm
to the side of the thumbtack. Repeat the
test, placing the magnet slightly closer,
until the spinning top is deflected by the
magnet. Make sure that the top is not
simply pulled into contact with the
magnet. Remove the magnet while the
top is still spinning and note its behavior.
Top-tobottom
variation
Motorized
bed
Inside the scanner
The varying magnetic fields can
make images of “slices” through
the body in different planes. The
main magnet produces a magnetic
field as much as 30,000 times
stronger than that of Earth.
640
Chapter 21
Left-to-right
field magnets
Each scan can take
several minutes, so
the patient must
lie very still.
Going Further
Student research should indicate that
most of these items can be attracted by
the powerful magnets in the MRI
scanner, and this attraction could result
in injury to the patient or damage to
the machine. Credit cards and other
identification with magnetic strips are in
danger of being erased by the magnetic
field. Watches with mechanical works
can become permanently magnetized,
and so cease to keep correct time. The
electronics in digital watches may also
be temporarily or permanently affected
by strong magnetic fields.
Verbal, Logical
How MRI works
MRI affects the nuclei of hydrogen atoms in the body. The
nuclei are made to absorb and then re-emit energy by a
combination of strong magnetic fields and radio wave
pulses. The emitted signals are then used to map
concentrations of hydrogen in the body.
1. Random axes
The spins of hydrogen
nuclei point in
random directions.
Like tiny magnets,
each nucleus has a
north pole and a
south pole.
Spin axes line up.
2. Aligning axes
When the main MRI
magnet is switched
on, the magnetic field
makes the spins of
hydrogen nuclei
mostly point in the
same direction.
Spin axis
Hydrogen nucleus
Spin axes realign
with magnetic field.
3. Wobbling axes
A pulse of radio
waves from the MRI
scanner knocks the
hydrogen nuclei out
of alignment.
Pulse of radio waves
from scanner
Spin axes
change direction.
MRI spinal cord scan
The bright red patch
here indicates a tumor
on the dark green
spinal cord.
While bone tissue
itself is not visible,
the vertebrae can be
seen because of the
marrow they contain.
Spinal cord tumor
highlighted by MRI
4. Realigning axes
When the pulse stops,
hydrogen nuclei emit
radio waves as they
return to alignment
with the main magnetic
field. With the lesser
magnets switched on as
necessary to alter the
magnetic field at a local
level, these waves are
picked up by the
scanner, which builds
Radio waves
up an image of
emitted by nuclei.
different tissues.
Going Further
Items such as jewelry, watches, coins, keys,
and credit cards must be removed before
beginning an MRI. Research in
the library or on the Internet
why these items interfere
with the procedure or pose
a risk to the patient.
Take a Discovery Channel Video
Field Trip by watching
“Magnetic Viewpoints.”
Video Field Trip
Magnetism
Video Field Trip
Magnetic Viewpoints
After students have viewed the Video Field Trip,
ask them the following questions: What is the
purpose of magnetic resonance imaging (MRI)?
(Student answers may include recording images of
internal body organs, detecting tumors, and observing how the brain works.) How does MRI work?
(The patient is bathed in a strong magnetic field that
causes some nuclei in the body’s atoms to line up like
spinning tops. A radio pulse knocks the nuclei out of
641
alignment, and when the pulse stops the nuclei emit
a signal as they line up again. A computer analyzes
the signal to form an image.) What advantage
does MRI have over X-rays in the detection of
cancers? (It can detect some kinds of cancer earlier
than X-rays can, and MRI is safer to use than Xrays.) Give an example of how MRI is used to
study how the brain works. (Student answers
may include that MRI images show the area of the
brain that responds to a sensation such as pain in
a particular part of the body. The images can be
used to study medical disorders such as epilepsy
and schizophrenia.)
Magnetism 641