420 Unit 3: Electricity and Magnetism

VOCABULARYelectromagnetism p. 421electromagnet p. 422

BEFORE, you learned

• Electric current is the flow of charge

• Magnetism is a force exertedby magnets

• Magnets attract or repel other magnets

NOW, you will learn

• How an electric current canproduce a magnetic field

• How electromagnets are used• How motors use electro-

magnets

KEY CONCEPT

Current can producemagnetism.

An electric current produces a magnetic field.Like many discoveries, the discovery that electric current is related tomagnetism was unexpected. In the 1800s, a Danish physicist namedHans Christian Oersted (UR-stehd) was teaching a physics class.Oersted used a battery and wire to demonstrate some properties ofelectricity. He noticed that as an electric charge passed through thewire, the needle of a nearby compass moved.

When he turned the current off, the needle returned to its originaldirection. After more experiments, Oersted confirmed that there is arelationship between magnetism and electricity. He discovered that anelectric current produces a magnetic field.

EXPLORE Magnetism from Electric Current

What is the source of magnetism?

PROCEDURE

Tape one end of the wire to the battery.

Place the compass on the table. Place the wire so that it is lying beside the compass,parallel to the needle of the compass. Record your observations.

Briefly touch the free end of the wire to the other end of the battery. Record your observations.

Turn the battery around and tape the other end to the wire. Repeat steps 2 and 3.

WHAT DO YOU THINK?• What did you observe?• What is the relationship between the direction of the

battery and the direction of the compass needle?

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reminder

Current is the flow of elec-trons through a conductor.

MATERIALS• electrical tape• copper wire• AA cell (battery)• compass

Sunshine StateSTANDARDSSC.C.2.3.1: The studentknows that manyforces (e.g., gravita-tional, electrical, andmagnetic) act at a dis-tance (e.g., withoutcontact).SC.H.3.3.4: The studentknows that technologi-cal design shouldrequire taking intoaccount constraintssuch as natural laws,the properties of thematerials used, andeconomic, political,social, ethical, and aesthetic values.

Chapter 12: Magnetism 421

ElectromagnetismThe relationship between electric current and magnetism plays animportant role in many modern technologies.is magnetism that results from an electric current. When a charged particle such as an electron moves, it produces a magnetic field.Because an electric current generally consists of moving electrons,a current in a wire produces a magnetic field. In fact, the wire acts as a magnet. Increasing the amount of current in the wire increases thestrength of the magnetic field.

You have seen how magneticfield lines can be drawn arounda magnet. The magnetic fieldlines around a wire are usuallyillustrated as a series of circles.The magnetic field of a wireactually forms the shape ofa tube around the wire. Thedirection of the current determines the direction of the magnetic field.If the direction of the electric current is reversed, the magnetic fieldstill exists in circles around the wire, but is reversed.

If the wire is shaped into a loop, the magnetism becomes concen-trated inside the loop. The field is much stronger in the middle of theloop than it is around a straight wire. If you wind the wire into a coil,the magnetic force becomes stronger with each additional turn of wireas the magnetic field becomes more concentrated.

A coil of wire with charge flowing through it has a magnetic fieldthat is similar to the magnetic field of a bar magnet. Inside the coil,the field flows in one direction, forming a north pole at one end.The flow outside the coil returns to the south pole. The direction ofthe electric current in the wire determines which end of the coilbecomes the north pole.

Check Your Reading How is a coil of wire that carries a current similar to a bar magnet?

Electromagnetism VOCABULARYRemember to record electromagnetism in yournotebook.

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current-carrying wire

magneticfield

current-carrying wire

coil

magneticfield

Making an ElectromagnetRecall that a piece of iron in a strong magnetic field becomes a magnetitself. An is a magnet made by placing a piece of ironor steel inside a coil of wire. As long as the coil carries a current, themetal acts as a magnet andincreases the magnetic field ofthe coil. But when the currentis turned off, the magneticdomains in the metal becomerandom again and the mag-netic field disappears.

By increasing the numberof loops in the coil, you canincrease the strength of the electromagnet. Electromagnets exert a muchmore powerful magnetic field than a coil of wire without a metal core.They can also be much stronger than the strongest permanent magnetsmade of metal alone. You can increase the field strength of an electro-magnet by adding more coils or a stronger current. Some of the mostpowerful magnets in the world are huge electromagnets that are usedin scientific instruments.

check your reading How can you increase the strength of an electromagnet?

electromagnet

How can you make an electromagnet?PROCEDURE

Starting about 25 cm from one end of the wire, wrap the wire in tight coilsaround the nail. The coils should cover the nail from the head almost to the point.

Tape the two batteries together as shown. Tape one end of the wire to a freebattery terminal.

Touch the point of the nail to a paper clip and record your observations.

Connect the other end of the wire to the other battery terminal. Again touchthe point of the nail to a paper clip. Disconnect the wire from the battery.Record your observations.

WHAT DO YOU THINK?• What did you observe?

• Did you make an electromagnet? How do you know?

CHALLENGE Do you think the result would be different if you used an aluminum nail instead of an iron nail? Why?

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ElectromagnetsElectromagnetsSKILL FOCUSObserving

MATERIALS• insulated wire• large iron nail• 2 D cells• electrical tape• paper clip

TIME20 minutes

ironcore

coil

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422 Unit 3: Electricity and Magnetism

Uses of ElectromagnetsBecause electromagnets can be turned on and off, they have more usesthan permanent magnets. The photograph below shows a powerfulelectromagnet on a crane. While the electric charge flows through thecoils of the magnet, it lifts hundreds of cans at a recycling plant. Whenthe crane operator turns off the current, the magnetic field disappearsand the cans drop from the crane.

A permanent magnet would not be nearly as useful for this purpose.Although you could use a large permanent magnet to lift the cans, itwould be hard to remove them from the magnet.

You use an electromagnet every time you store information on acomputer. The computer hard drive contains disks that have billionsof tiny magnetic domains in them. When you save a file, a tiny electromagnet in the computer is activated. The magnetic field of theelectromagnet changes the orientation of the small magnetic domains.The small magnets store your file in a form that can be read later bythe computer. A similar system is used to store information on mag-netic tape of an audiocassette or videocassette. Sound and pictures arestored on the tape by the arrangement of magnets embedded in theplastic film.

Magnetic information is often stored on credit cardsand cash cards. A black strip on the back of the card contains information about the account number andpasswords. The cards can be damaged if they are fre-quently exposed to magnetic fields. For example, cardsshould not be stored with their strips facing each other, ornear a magnetic clasp on a purse or wallet. These magneticfields can change the arrangement of the tiny magneticdomains on the card and erase the stored information.

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wire supplying

electric current

electromagnet

This powerful electromag-net can be turned on andoff to collect and movecans at a recycling plant.

MAIN IDEA WEBMake a main idea web for the uses of electromagnets.

424 Unit 3: Electricity and Magnetism

Motors use electromagnets.Because magnetism is a force, magnets can be used to move things.Electric motors convert the energy of an electric current into motion bytaking advantage of the interaction between current and magnetism.

There are hundreds of devices that contain electric motors. Examplesinclude power tools, electrical kitchen appliances, and the small fans ina computer. Almost anything with moving parts that uses current hasan electric motor.

MotorsPage 425 shows how a simple motor works. The photograph at the topof the page shows a motor that turns the blades of a fan. The illustra-tion in the middle of the page shows the main parts of a simplemotor. Although they may look different from each other, all motorshave similar parts and work in a similar way. The main parts of anelectrical motor include a voltage source, a shaft, an electromagnet,and at least one additional magnet. The shaft of the motor turns otherparts of the device.

Recall that an electromagnet consists of a coil of wire with currentflowing through it. Find the electromagnet in the illustration on page425. The electromagnet is placed between the poles of another magnet.

When current from the voltage source flows through the coil, amagnetic field is produced around the electromagnet. The poles ofthe magnet interact with the poles of the electromagnet, causing themotor to turn.

The poles of the magnet push on the like poles of the electromagnet,causing the electromagnet to turn.

As the motor turns, the opposite poles pull on each other.

When the poles of the electromagnet line up withthe opposite poles of the magnet, a part of themotor called the commutator reverses the polarityof the electromagnet. Now, the poles push on each

other again and the motor continues to turn.

The illustration of the motor on page 425 issimplified so that you can see all of the parts. Ifyou saw the inside of an actual motor, it mightlook like the illustration on the left. Notice thatthe wire is coiled many times. The electromag-net in a strong motor may coil hundreds oftimes. The more coils, the stronger the motor.

What causes the electromagnet in a motor to turn?

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VISUALIZATIONCLASSZONE.COM

See a motor in motion.

coil ofwire

magnet

shaft

reminder

Notice how simplemachines, which you readabout in grade 6, are com-bined to create complicatedmachines.

Content ReviewFLORIDA

Would a motor work without an electromagnet? Why or why not?

As the motor turns, oppositepoles attract.

The electromagnet’s poles areswitched, and like poles again repel.

voltage source

shaft

electromagnet

magnet

commutator

The commutator rotates alongwith the electromagnet, causingthe electromagnet’s poles toswitch with every half-rotation.

How a Motor Works

Although motors may look different from each other,they all have similar parts and work in a similar way.

electromagnet shaft

motor in fan

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Like poles of the magnets pushon each other.

Chapter 12: Magnetism 425

426 Unit 3: Electricity and Magnetism

Uses of MotorsMany machines and devices contain electric motors that may not beas obvious as the motor that turns the blades of a fan, for example.Even though the motion produced by the motor is circular, motorscan move objects in any direction. For example, electric motors movepower windows in a car up and down.

Motors can be very large, such as the motors thatpower an object as large as a subway train. They drawelectric current from a third rail on the track or wiresoverhead that carry electric current. A car uses anelectric current to start the engine. When the key isturned, a circuit is closed, producing a current from thebattery to the motor. Other motors are very small, likethe battery-operated motors that move the hands ofa wristwatch.

The illustration on the left shows thetwo small motors in a portable CD player.Motor A causes the CD to spin. Motor Bis connected to a set of gears. The gearsconvert the rotational motion of themotor into a straight-line motion, or linear motion. As the CD spins, a lasermoves straight across the CD from thecenter outward. The laser reads the information on the CD. The motionfrom Motor B moves the laser across the CD.

Check Your Reading Explain the function served by each motor in a CD player.

KEY CONCEPTS1. Explain how electric current and

magnetism are related.

2. Describe three uses ofelectromagnets.

3. Explain how electrical energy is converted to motion in a motor.

CRITICAL THINKING4. Contrast How does an

electromagnet differ from a permanent magnet?

5. Apply Provide examples oftwo things in your home thatuse electric motors, andexplain why they are easier touse because of the motors.

CHALLENGE6. Infer Why is it necessary

to change the direction of the current in the coil of anelectric motor as it turns?

Motor B movesa laser acrossthe CD.

These gears changethe rotational motionof the motor into astraight motion.

Motor Aturns the CD.

laser