21.2 Electromagnetism

Reading StrategyIdentifying Main Idea Copy the tablebelow. As you read, write the main idea of the text that follows each topic.

Key ConceptsHow can an electric chargecreate a magnetic field?

How is an electromagnetcontrolled?

How do galvanometers,electric motors, andloudspeakers work?

Vocabulary◆ electromagnetic

force◆ solenoid◆ electromagnet◆ galvanometer◆ electric motor

Topic Main Idea

Electricity and magnetism

Direction ofmagnetic fields

Direction of electric currents

Solenoids and electromagnets

Electromagnetic devices

a. ?

b. ?

c. ?

d. ?

e. ?

You know that unlike electric charges attract one another and thatlike electric charges repel one another. It is easy to discover a similareffect with the north and south poles of two magnets. However, it’smuch more difficult to figure out the relationship between electric-ity and magnetism. In fact, the connection was discoveredaccidentally by the Danish scientist Hans Christian Oersted in 1820.

One evening Oersted, pictured in Figure 6, was conducting scien-tific demonstrations for his friends and students in his home. Onedemonstration used electric current in a wire, and another used a com-pass needle attached to a wooden stand. As Oersted turned on thecurrent for the electricity demonstration, he saw the compass needlemove. When he turned off the current, the needle moved back to itsoriginal position. Further investigation showed that the current in thewire produced a magnetic field. Oersted had discovered a relationshipbetween electricity and magnetism.

Electricity and MagnetismElectricity and magnetism are different aspects of a single forceknown as the electromagnetic force. The electric force results fromcharged particles. The magnetic force usually results from themovement of electrons in an atom. Both aspects of the electro-magnetic force are caused by electric charges.

Figure 6 In 1820 Hans Oersteddiscovered how magnetism andelectricity are connected. A unit of measure of magnetic fieldstrength, the oersted, is namedafter him.

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FOCUS

Objectives21.2.1 Describe how a moving electric

charge creates a magnetic fieldand determine the direction of the magnetic field based onthe type of charge and thedirection of its motion.

21.2.2 Relate the force a magneticfield exerts on a moving electriccharge to the type of chargeand the direction of its motion.

21.2.3 Explain how solenoids andelectromagnets are constructedand describe factors that affectthe field strength of both.

21.2.4 Describe how electromagneticdevices use the interactionbetween electric currents andmagnetic fields.

Build VocabularyConcept Map Have students make aconcept map comparing the devices inthe vocabulary list.

Reading Strategya. Electricity and magnetism are differentaspects of electromagnetic force.b. Magnetic fields are produced at rightangles to an electric current. c. Electriccurrents are deflected perpendicular to amagnetic field. d. Changing the currentin an electromagnet controls the strengthand direction of its magnetic field.e. Electromagnetic devices changeelectrical energy into mechanical energy.

INSTRUCT

Electricity andMagnetismBuild Reading Literacy

Predict Refer to page 66D inChapter 3, which provides theguidelines for predicting.

Have students read the first twoparagraphs on p. 635. Ask them topredict what Oersted discovered aboutthe relationship between electricity andmagnetism. (Predictions should indicatethat an electric current produces amagnetic field.) Logical

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Print• Guided Reading and Study Workbook

With Math Support, Section 21.2• Transparencies, Section 21.2

Technology• iText, Section 21.2• Presentation Pro CD-ROM, Section 21.2• Go Online, NSTA SciLinks, Electromagnets

Section Resources

Section 21.2

Force deflecting the charge

Velocity of charge+

Magnetic Fields Around Moving Charges Oersted’sdiscovery about the relationship between a current-carrying wire anda magnet established an important physics principle. Movingelectric charges create a magnetic field. These moving charges maybe the vibrating charges that produce an electromagnetic wave. Theymay also be, as in Oersted’s experiment, the moving charges in a wire.Figure 7 shows how to remember the direction of the magnetic fieldthat is produced. The magnetic field lines form circles around astraight wire carrying a current.

Forces Acting on Moving Charges Recall that an electricfield exerts a force on an electric charge. The force is either in the samedirection as the electric field or in the opposite direction, depending onwhether it is a positive or negative charge.

The effect of a magnetic field on a moving charge is different, asshown in Figure 8. A charge moving in a magnetic field will bedeflected in a direction perpendicular to both the magnetic field andto the velocity of the charge. If a current-carrying wire is in a magneticfield, the wire will be pushed in a direction perpendicular to both thefield and the direction of the current. Reversing the direction of thecurrent will still cause the wire to be deflected, but in the oppositedirection. If the current is parallel to the magnetic field, the force iszero and there is no deflection.

What are two kinds of moving charges thatcan create a magnetic field?

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Figure 8 A moving positive charge is deflected ata right angle to its motion by a magnetic field. Inferring In what direction would the particlebe deflected if it had a negative charge instead ofa positive charge?

Directionof current

Directionof electron

flow

Directionof magnetic

field

Current-carrying wire

Figure 7 If you point the thumb ofyour right hand in the direction of the current, your fingers curve in thedirection of the magnetic field.Inferring How can you determinethe magnetic field direction from the direction of electron flow?

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Magnetic Field from Electric CurrentPurpose Students observe how anelectric current produces a magnetic field.

Materials insulated wire, cardboard (10 cm � 10 cm), a burner tripod, a variable DC power supply, 4–6 compasses

Procedure Punch a small hole in thecenter of the cardboard and thread thewire through the hole. Lay the cardboardflat on the burner tripod’s ring support sothat the wire passes through the tripodcenter, perpendicular to the cardboardand extending in a straight line 10 cm oneither side. (A ring stand and clamp maybe needed to support the upper end of the wire.) Connect both ends of thewire 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 increasethe current until the compass needlesbegin to deflect. Have students noticehow the needles deflect with respect tothe wire. Remove the compasses, turn off the power supply, reverse the wireconnections, and repeat thedemonstration.

Safety Use insulated wire. Followprocedures for electrical safety.

Expected Outcome When the topend of the wire is connected to thepositive terminal of the power supply,the magnetic field will be in a counter-clockwise pattern around the wire,according to the right-hand rule. Thiswill cause the poles of the compasses toalign themselves along the edge of acircle around the wire. The south poleswill form a clockwise pattern. When theconnections are reversed, the direction inwhich the compasses point will reverse. Visual, Group

Use VisualsFigure 8 Explain that the right-handrule also applies to Figure 8. Ask, Howcould you use your hand to determinethe deflection of an electron movingthrough the magnetic poles? (Use yourright hand with your thumb in the directionof the current, which will be opposite thedirection of the electron’s travel.)Visual

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Section 21.2 (continued)

Customize for Inclusion Students

Visually ImpairedThe right-hand rule can be used by studentswith visual impairments to understandmagnetic fields and forces. Explain howstudents can use their right hand to predictthe directions of magnetic fields for an electriccurrent in a straight wire and a solenoid. Asstudents may have difficulty using Figure 7,instruct them using a wire, so that they canunderstand how the right-hand thumb and

fingers are oriented for a positive current andits magnetic field. Then, have students adaptthe rule for positive charges moving in amagnetic field, as shown in Figure 8 (that is,the thumb points in the direction of themoving charge, the fingers extend in thedirection of the magnetic field, and the forceon the charge points outward from the palm).Encourage those students who successfullymaster the rule to explain it to the class.

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Making an Electromagnet

Materialsiron nail, 20 small metal paper clips, 20-cm lengthand 1-m length of insulated wire with strippedends, 6-volt battery, switch

Procedure1. Make a circuit using the nail, wire, battery,

and switch. Use the shorter wire to connectone terminal of the battery to the switch.Connect the longer wire to the otherterminal of the battery. Wrap this wirearound the nail 10 times. Then connect the longer wire to the switch.

2. Hold the head of the nail over the pile ofpaper clips. Close the switch. Record howmany paper clips the nail can pick up.

3. Open the switch. CAUTION If the switch is leftclosed, the wire will become very warm. Wrapthe longer wire 40 more times around the nailin the same direction as before.

4. Close the switch. Record how many paperclips the nail can pick up now.

5. Open the switch and disconnect the circuit.

Analyze and Conclude1. Observing How did your ability to pick up

paper clips with the nail change when youincreased the number of turns in the coil?

2. Drawing Conclusions Why did the nailbecome a magnet when a current-carryingwire was wrapped around it?

Solenoids and ElectromagnetsBefore you can use electromagnetic force, you need to be ableto control it. Using electromagnetic force requires some simpletools. Figure 9A shows a current-carrying wire with a loop in it.The magnetic field in the center of the loop points right to leftthrough the loop, as shown in Figure 9A.

Suppose you loop the wire many times to make a coil, asshown in Figure 9B. Then the magnetic fields of the loopscombine so that the coiled wire acts like a bar magnet. Thefield through the center of the coil is the sum of the fieldsfrom each turn of the wire. A coil of current-carrying wirethat 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 currentcauses the iron rod inside the coil of the solenoid to become a magnet. An electromagnet is a solenoid with a ferromag-netic core. Changing the current in an electromagnetcontrols the strength and direction of its magnetic field.You can also use the current to turn the magnetic field onand off. People use many devices every day, such as hairdryers, telephones, and doorbells, that utilize electromagnets.

Pole Pole

Current

Loop of wire

Solenoid

Current

A

B

Figure 9 The magnetic field lines around a solenoid are like those of a bar magnet.Applying Concepts Which of the poles is north?

Solenoids andElectromagnets

Making an Electromagnet

ObjectiveAfter completing this activity, studentswill be able to• predict how the number of turns of

wire affects the strength of theelectromagnet.

Skill Focus Observing, DrawingConclusions

Prep Time 20 minutes

Advance Prep Cut the wires inadvance and use a wire stripper or wire-cutting pliers to remove 2 cm ofinsulation from each end of the wires.

Class Time 25 minutes

Safety Students should wear safetygoggles and be careful handling the coilof wire, as the wire may become hot.Students should open the switch whenthe electromagnet is not in use.

Expected Outcome Students willlearn that the strength of anelectromagnet, as indicated by thenumber of paper clips picked up, isdirectly related to the number of turnsin the coil of wire. More turns make the magnet stronger.

Analyze and Conclude1. The electromagnet became strongerwith more turns in the coil.2. The current in the coil produced amagnetic field around and through thenail. This caused the magnetic domainsin the nail to align, temporarily strength-ening the magnetic field of the nail.Logical, Group

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Answer to . . .

Figure 7 Use the right-hand rule, but point your thumb in the oppositedirection of the electron flow (whichwill be the direction of the current).

Figure 8 It would be deflected down.

Figure 9 The one on the left becausemagnetic field lines start at the northpole and end at the south pole.

Vibrating charges,flowing charges

in a current

Big Magnets Because the magnetic fieldsproduced by electromagnets can be madestronger by properly designing theelectromagnet, it is not surprising that thestrongest magnetic fields on Earth are producedby specially designed electromagnets. At theNational High Magnetic Field Laboratory

(NHMFL) at Florida State University inTallahassee, electromagnets have beendesigned that produce continuous magneticfields with strengths up to 50 teslas. These fields are about a million times stronger thanEarth’s magnetic field at Earth’s surface.

Facts and Figures

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The strength of an electromagnet depends on the current in thesolenoid, the number of loops in the coil in the solenoid, and the typeof ferromagnetic core. To increase the strength of an electromagnet,increase the current flowing through the solenoid. A greater currentproduces 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 strongerelectromagnets.

Electromagnetic DevicesElectromagnets can convert electrical energy into motion that can dowork. Electromagnetic devices such as galvanometers, electricmotors, and loudspeakers change electrical energy into mechanicalenergy. A galvanometer measures current in a wire through the deflec-tion of an electromagnet in an external magnetic field. An electric motoruses a rotating electromagnet to turn an axle. A loudspeaker uses elec-tromagnets to convert electrical signals into sound waves you can hear.

Galvanometers Figure 10 shows a galvanometer, a device thatuses an electromagnet to measure small amounts of current. A gal-vanometer has a small eletromagnet attached to a spring. Theelectromagnet is placed between the poles of two permanent magnets.

When there is a current in the electromagnet’s coils, the resultingmagnetic field attempts to align with the field of the permanent

magnets. The greater the current, the more the electromagnetrotates, as shown by the pointer on the scale. In an automobilefuel gauge, for example, a sensor in the gas tank reduces thecurrent as the gas level decreases. This causes the needle torotate towards the “empty” mark.

What does the strength of an electromagnetdepend on?

For: Links on electromagnets

Visit: www.SciLinks.org

Web Code: ccn-2212

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ScalePointer

Wire

Magnet

Figure 10 A galvanometer uses anelectromagnet to move a pointer.One common application is in anautomobile gas gauge. The pointerindicates the amount of current inthe wire. The wire is connected toa sensor in the gas tank.

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Students may wonder how the magneticfield of a solenoid can be fairly simplewhen there are magnetic fields aroundeach segment of wire in the coil. Explainthat such fields are present, but that theycombine in such a way that the fieldoutside the solenoid is much weaker thaninside. The fields combine to effectivelyform a magnetic field that is similar tothat of a bar magnet.Logical

ElectromagneticDevices

Electromagnetic ForcePurpose Students observe themagnetic force exerted on a wirecarrying an electric current.

Materials insulated wire, a largehorseshoe magnet, a variable DC powersupply, 2 ring stands with clamps

Procedure Pass the wire through therings of the ring stands, so that itextends horizontally about 5–10 cmabove the table surface. Position themagnet on its side, so that the wirepasses between the magnet’s poles.Connect the wires to the power supplyand turn it on, increasing the currentuntil the wire is deflected. Turn off thepower, reverse the connections, andrepeat the demonstration.

Safety Use insulated wire. Followprocedures for electrical safety.

Expected Outcome Depending onthe orientation of the magnet, the wirewill be deflected either in toward themagnet’s center or away from it. Thedeflecting force is proportional to thecurrent in the wire and the strength ofthe magnetic field.Visual, Group

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Section 21.2 (continued)

Download a worksheet onelectromagnets for students tocomplete, and find additionalteacher support from NSTA SciLinks.

Section 21.2 Assessment

Reviewing Concepts1. Besides a magnet, what can create

a magnetic field?

2. How is the magnetic field of anelectromagnet 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 stationaryelectric charge? On a moving electric charge?

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Insulators In Section 20.2 you learnedthat electric charge doesn’t flow easilythrough electrical insulators. Use this toexplain why a solenoid has insulated wires.

Electric Motors An electric motor is adevice that uses an electromagnet to turn anaxle. Figure 11 shows how an electric motorworks. In this figure, the wire is connected to abattery. An actual motor has many loops ofwire around a central iron core to make themotor stronger. In the motor of an electricappliance, the wire would be connected to anelectrical circuit in a building.

What makes a motor turn? When current flows through a loop ofwire, one side of the loop is pushed by the field of the permanentmagnet. The other side of the loop is pulled. These forces rotate theloop. If there were no commutator ring, the coil would come to rest.But as the loop turns, each C-shaped half of the commutator connectswith a different brush, reversing the current. The forces now changedirection, so the coil continues to rotate. As long as current flows,rotation continues.

Loudspeakers A loudspeaker contains an electromagnet and apermanent magnet, much like a motor. However, the current in thewires entering the loudspeaker changes direction and increases ordecreases to reproduce music, voices, or other sounds. The changingcurrent produces a changing magnetic field in the electromagnet’scoil. The magnetic force exerted by the permanent magnet moves thecoil back and forth. As the coil moves, it causes a thin membrane tovibrate, producing sound waves that match the original sound.

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 thecommutator in an electric motor?

8. Relating Cause and Effect What causesthe membrane in a loudspeaker to vibrate?

Commutator

Brush

Loop ofwire

Direction ofrotation

Current

Figure 11 A battery suppliescurrent to a loop of wire throughthe commutator. As thecommutator turns, the directionof current switches back andforth. As a result, the coil’smagnetic field keeps switchingdirection, and this turns the coilabout an axle. Predicting What would happenif you reversed the positive and negative connections on the battery?

Build Science SkillsApplying Concepts Have studentsread the paragraphs on electric motorsand help them apply what they alreadyknow about work, different forms ofenergy, and energy conservation. Ask,What forms of energy are shown forthe electric motor in Figure 11?(Chemical energy, electrical energy, andmechanical energy) Ask, What energytransformations take place whenoperating the motor? (Chemical energyin the battery is converted to electricalenergy. Electrical energy interacts with the magnetic field to do work, and so istransformed into the kinetic energy of the rotating wire loop and into any workthe motor does.)Logical

ASSESSEvaluateUnderstandingAsk students to list three examples ofdevices that use electromagnetic forces(at least one of which is not given in thesection). Have students explain whateach device does and how electricityand magnetism interact in the device.(Could be an electric bell, relay switch, or microphone)

ReteachUse Figures 7 and 9 to review thedirection of magnetic fields produced by electric currents.

The strength of the electromagnetdepends upon the current in thesolenoid. Insulated wires make itpossible to direct the current throughseveral tightly wound loops, enhancingthe strength of the magnetic field. Theinsulated wire prevents a short circuitbetween adjacent coils.

If your class subscribesto iText, use it to review key concepts inSection 21.2.

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5. A magnetic field doesn’t affect a stationarycharge. A magnetic field deflects a movingcharge in a direction perpendicular to boththe field and the velocity.6. It produces a more uniform field andincreases its strength.7. The commutator reverses the current in theelectromagnet, reversing the magnetic field ofthe electromagnet, and enabling the axle toturn continuously in one direction.8. The interaction of the magnetic field of thepermanent magnet with the changing field of the electromagnet

Section 21.2 Assessment

1. A moving electric charge can create amagnetic field.2. It can be turned on and off. Its strength and direction can be controlled by controllingthe current.3. They change electrical energy intomechanical energy.4. It makes the magnetic field much stronger.When current flows through the coil, it createsa magnetic field that magnetizes theferromagnetic rod.

Answer to . . .

Figure 11 The motor’s axle wouldspin in the opposite direction.

It depends on thestrength of the current,

the number of coils of wire, and thetype of ferromagnetic core.

Peeking Inside the Human BodyMagnetic Resonance Imaging (MRI) is used bydoctors to create more detailed images of the humanbody than are possible with X-rays.

Body tissues vary in their concentration of hydrogen atoms. Fathas a high concentration, as do tissues containing water, becauseof the hydrogen in H2O. The concentration of hydrogen atomsin bone is very low. MRI reveals these differences in great detail,with fat and fluids (including blood) showing up as bright areasand bone as dark areas. MRI scans can even depict thebrain. It produces images of suchdetail that they are used byresearchers studying how the brainworks, as well as by doctorsinvestigating diseases.

Creating an MRI imageThe scanner uses three magneticfields to read data up and downand along slices of the body.This produces an image that isviewed and interpreted bydoctors and radiographers.

Inside the scannerThe varying magnetic fields canmake images of “slices” throughthe body in different planes. Themain magnet produces a magneticfield as much as 30,000 timesstronger than that of Earth.

Head-to-toevariation

Top-to-bottomvariation

Left-to-rightvariation

Main magnet This powerfulmagnet immerses the patientin a stable, intense magnetic

field—the other three magnetscreate a variable field.

Radio-frequencysource

Motorizedbed

Head-to-toefield magnets

Left-to-rightfield magnets

Top-to-bottomfield magnets

Each scan can takeseveral minutes, sothe patient mustlie very still.

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Peeking Inside the Human BodyBackgroundMRI is an example of a procedure calledtomography, where many images of the body are combined to give a com-posite view. MRI uses nuclear magneticresonance, or NMR, to obtain informationfrom hydrogen atoms in the body. NMR was discovered in 1946, and wasoriginally used to identify hydrocarbonmolecules. In the 1970s, the techniquewas combined with computers toproduce images of tissues in the body.

Build Science SkillsUsing Analogies

Purpose Studentswill simulate how applied magnetic fields can disrupt the magnetic fields of atoms.

Materials a short pencil (about 5 cm long), a cardboard disk (7 cm wide), a steel thumbtack, a bar magnet, paper

Class Time 15 minutes

Procedure Insert the thumbtack intothe eraser end of the pencil, and punchthe pencil through the center of thecardboard disk to make a “top” that canspin. Make sure the cardboard does notslip along the surface of the pencil. Placethe top on a piece of paper to preventmarking the table. Spin the top with thethumbtack side upward, making sure thatthe top is neither too stable or unstablewhile spinning. Spin the top again, andplace one end of the magnet about 2 cmto the side of the thumbtack. Repeat thetest, placing the magnet slightly closer,until the spinning top is deflected by themagnet. Make sure that the top is notsimply pulled into contact with themagnet. Remove the magnet while thetop is still spinning and note its behavior.

Expected Outcome Because the top isfairly stable while spinning, it is analogousto the spinning hydrogen atoms in thebody. The alignment and deflection ofthese atoms by the magnetic fields isanalogous to the deflection of the top bythe magnet. By observing a large-scalemodel of an atomic process, students canvisualize the atomic process more clearly.Visual

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� Items such as jewelry, watches, coins, keys,and credit cards must be removed beforebeginning an MRI. Research inthe library or on the Internetwhy these items interfere with the procedure or pose a risk to the patient.

� Take a Discovery Channel VideoField Trip by watching“Magnetic Viewpoints.”

Going Further

How MRI worksMRI affects the nuclei of hydrogen atoms in the body. Thenuclei are made to absorb and then re-emit energy by acombination of strong magnetic fields and radio wavepulses. The emitted signals are then used to mapconcentrations of hydrogen in the body.

Hydrogen nucleus Spin axis

MRI spinal cord scanThe bright red patch

here indicates a tumoron the dark green

spinal cord. While bone tissue

itself is not visible, the vertebrae can beseen because of the

marrow they contain.

1. Random axes The spins of hydrogennuclei point inrandom directions.Like tiny magnets,each nucleus has anorth pole and asouth pole.

2. Aligning axes When the main MRImagnet is switchedon, the magnetic fieldmakes the spins ofhydrogen nucleimostly point in thesame direction.

Pulse of radio wavesfrom scanner

Spin axes realignwith magnetic field.

Spin axes change direction. Radio waves

emitted by nuclei.

Spinal cord tumorhighlighted by MRI

Spin axes line up.

4. Realigning axesWhen the pulse stops,hydrogen nuclei emitradio waves as theyreturn to alignmentwith the main magneticfield. With the lessermagnets switched on asnecessary to alter themagnetic field at a locallevel, these waves arepicked up by thescanner, which buildsup an image ofdifferent tissues.

3. Wobbling axes A pulse of radiowaves from the MRIscanner knocks thehydrogen nuclei outof alignment.

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Video Field Trip

Going FurtherStudent research should indicate thatmost of these items can be attracted bythe powerful magnets in the MRIscanner, and this attraction could resultin injury to the patient or damage to the machine. Credit cards and otheridentification with magnetic strips are indanger of being erased by the magneticfield. Watches with mechanical workscan become permanently magnetized,and so cease to keep correct time. Theelectronics in digital watches may alsobe temporarily or permanently affectedby strong magnetic fields.Verbal, Logical

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After students have viewed the Video Field Trip,ask them the following questions: What is thepurpose of magnetic resonance imaging (MRI)?(Student answers may include recording images ofinternal body organs, detecting tumors, and obser-ving how the brain works.) How does MRI work?(The patient is bathed in a strong magnetic field thatcauses some nuclei in the body’s atoms to line up likespinning tops. A radio pulse knocks the nuclei out of

alignment, and when the pulse stops the nuclei emita signal as they line up again. A computer analyzesthe signal to form an image.) What advantagedoes MRI have over X-rays in the detection ofcancers? (It can detect some kinds of cancer earlierthan X-rays can, and MRI is safer to use than X-rays.) Give an example of how MRI is used tostudy how the brain works. (Student answers may include that MRI images show the area of thebrain 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.)

Video Field Trip

Magnetic Viewpoints