CHAPTER SECTION 2 Magnetism From Electricity

NameClassCHAPTER 2DateElectromagnetism2 Magnetism from ElectricitySECTIONNational ScienceEducation StandardsBEFORE YOU READAfter you read this section, you should be able to answerthese questions:PS 3a How are an electric current and a magnetic field related? What are solenoids and electromagnets? How does electromagnetism run doorbells, electricmotors, and galvanometers?How Was Electromagnetism Discovered?Many things around you—doorbells and motors, forexample—use electricity to make magnetism. In thissection, you will learn how electricity and magnetism arerelated and how we make electromagnets.Danish physicist Hans Christian Oersted discoveredhow electricity and magnetism are related in 1820. Whileteaching a class, he held a compass near a wire carrying anelectric current. When the compass was brought close tothe wire, the compass needle moved. It no longer pointedto the north. He had accidentally discovered that electricity and magnetism are related.A compass needle is a magnet. It moves from its northsouth position only when it is in a magnetic field differentfrom Earth’s. Oersted tried a few experiments with thecompass and the wire. He learned that electric currentproduces a magnetic field. That made the needle of thecompass line up with the direction of the magnetic field.The picture below shows how his experiments worked.STUDY TIPCompare As you read thissection, make a table tocompare solenoids andelectromagnets.TAKE A LOOK1. Describe Why does acompass needle move whenit is near a wire with an electric current running though it?a If no electric currentc Electric current in theb Electric current in oneis in the wire, theopposite direction causesdirection causes twocompass needlesthe two compass needlescompass needles toEMG-P01-015b-AEMG-P01-015a-Apoint in the sameto deflect in a EMG-P01-015c-Acounterdeflect in a clockwisedirection.clockwise direction.direction.Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook35Electromagnetism

NameSECTION 2ClassDateMagnetism from Electricity continuedMORE RESEARCHREADING CHECK2. Define What iselectromagnetism?Oersted also found that the direction of the magneticfield depends on the direction of the current. The Frenchscientist André-Marie Ampère heard about Oersted’sfindings. Ampère did more research with electricity andmagnetism. Their work was the first research into electromagnetism. Electromagnetism is the interactionbetween electricity and magnetism.How Do We Use Electromagetism?The magnetic field produced by an electric current in awire can move a compass needle. But the magnetic field isnot strong enough to be very useful. However, two devices,the solenoid and the electromagnet, make the magneticfield stronger. They make electromagnetism more useful.SOLENOIDSREADING CHECKA single loop of wire that carries a current does nothave a very strong magnetic field. But if you form manyloops into a coil, it makes the magnetic field stronger.The magnetic fields of the loops work together.The picture below shows a solenoid. A solenoid is acoil of wire that produces a magnetic field when anelectric current runs through it. The magnetic fieldaround a solenoid is very similar to the magnetic fieldof a bar magnet. The magnetic field of a solenoid getsstronger if it has more loops. The magnetic field alsobecomes stronger as the current in the wire increases.3. Identify What are twothings you can do to makethe magnetic field of asolenoid stronger?The ends of the solenoid are like the poles of a bar magnetic.Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook36Electromagnetism

NameClassSECTION 2DateMagnetism from Electricity continuedELECTROMAGNETSWe can make magnets with an even stronger magneticfield than one made by a solenoid. Electromagnets canmake a train float! An electromagnet is made up of asolenoid wrapped around an iron core. It acts as amagnet when an electric current runs through the coil.The magnetic field of the solenoid makes the domainsinside the iron core line up.The magnetic field of the electromagnet is the fieldof the solenoid plus the field of the magnetized core. Asa result, the magnetic field of an electromagnet may behundreds of times stronger than the magnetic field of justthe solenoid. You can make an electromagnet stronger byincreasing the number of loops per meter in the solenoid.You can also increase the electric current in the wire.Engineers have developed trains called maglev trainsthat use electromagnetism. Maglev is a short name formagnetic levitation. Instead of rolling on wheels overthe tracks, these trains levitate, or float, above the track.How? Remember what happens when you bring two magnets close together—the magnets exert a magnetic forceon each other. In maglev trains, powerful electromagnetsin the rails repel strong magnets on the train cars.READING CHECK4. Explain Why is themagnetic field of anelectromagnet strongerthan a solenoid?Critical ThinkingElectromagnets used in salvage yards are turned on to pickup iron objects and turned off to put them down.TURNING ELECTROMAGNETS ON AND OFF5. Infer Why could anelectromagnet be more usefulthan a permanent magnet?Electromagnets are very useful because we can turnthem on and off. The solenoid has a field only when electriccurrent runs through it. So electromagnets attract thingsonly when a current exists in the wire. When no currentruns through the wire, the electromagnet turns off.Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook37Electromagnetism

NameSECTION 2ClassDateMagnetism from Electricity continuedApplications of ElectromagnetismREADING CHECK6. Name What are twothings that operate usingelectromagnetism?You may not ride on a maglev train, but you use electromagnetism in simple ways every day. For example, you usea solenoid when you ring a doorbell. Electromagnetismmakes vending machines give you the right change. It’swhat makes metal detectors and motors work. An electromagnet makes the fuel gauge in a car work.DOORBELLSHave you ever noticed a doorbell button that has a lightinside? When you pushed the button, did the light go out?Two solenoids in the doorbell allow the doorbell to work.When you push the button, it opens the circuit of thefirst solenoid. The current stops. That makes the magnetic field drop and the light go out. The change in thefield causes a current in the second solenoid. This current induces a magnetic field that pushes an iron rod andthat makes the bell ring.MAGNETIC FORCE AND ELECTRIC CURRENTREADING CHECK7. Explain Why does a wirehaving an electric currentrunning through it cause acompass needle to move?An electric current in a wire can cause a compassneedle to move. The needle is a small magnet. The needlemoves because the electric current in a wire creates amagnetic field that exerts a force on the needle.Can a magnet cause a wire that has an electric currentrunning through it to move? The picture below showsthat the answer is yes. The wire is pushed up or downdepending on the direction of the current in the wire.This force makes electric motors work.a A wire is connected to a batteryb Switching the wires at the batteryreverses the direction of the elecas shown above. When the wiretric current. Now the wire will beis placed between two poles ofpushed down.a strong magnet, the wire willEMG-P01-018b-AEMG-P01-018a-Amove up.Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook38Electromagnetism

NameClassSECTION 2DateMagnetism from Electricity continuedELECTRIC MOTORSAn electric motor is a device that changes electricalenergy into mechanical energy. All electric motors havean armature—a loop or coil of wire that can rotate. Thearmature is mounted between the poles of a permanentmagnet or electromagnet.An electric motor that uses direct current, like theone below, contains a device called a commutator. It isattached to the armature to reverse the direction of theelectric current in the wire.A commutator is a ring that is split in half and connectedto the ends of the armature. Electric current enters thearmature through brushes that touch the commutator. Everytime the armature and the commutator make a half turn, thedirection of the current in the armature reverses.Getting Started Theelectric current in thearmature causes themagnet to push on thearmature. The currentis moving in a differentdirection in either sideof the armature. So oneside is pushed up andthe other side down.This causes the armatureto rotate.CommutatorPS 3a Energy is a property ofmany substances and is associated with heat, light, electricity,mechanical motion, sound,nuclei, and the nature of achemical. Energy is transferredin many ways.8. Identify An electricmotor changes electricalenergy into what kind ofenergy?Permanent magnetBrushesDirectionof rotationNSource ofelectricalenergySArmaturePermanent magnetNSource ofelectricalenergySTANDARDS CHECKRunning the Motor Thecommutator causes theelectric current in the rotatingarmature to change directions. This causes the sidethat was pushed down to bepushed up and the other sideto be pulled down. So thearmature keeps rotating in thesame direction.SArmatureGALVANOMETERSA galvanometer measures current or voltage.Electricians use galvanometers when they use ammetersand voltmeters.A galvanometer has an electromagnet placed betweenthe poles of a permanent magnet. The electromagnetis free to rotate and is attached to a pointer. A currentin the galvanometer causes the permanent magnet topush the electromagnet, rotating the pointer. The pointermoves along a scale that shows the size and direction ofthe current, or the voltage.READING CHECK9. Identify What does agalvanometer measure?Copyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook39Electromagnetism

NameClassDateSection 2 ReviewNSESPS 3aSECTION VOCABULARYelectric motor a device that converts electricalenergy into mechanical energyelectromagnet a coil that has a soft iron coreand that acts as a magnet when an electriccurrent is in the coilelectromagnetism the interaction betweenelectricity and magnetismsolenoid a coil of wire with an electric currentin it1. Describe What does an electric current through a wire produce? What affects thestrength of what is produced?2. Interpret Graphics Which of the magnets below is a solenoid? Which one is anelectromagnet? Label each one. If the current and the number of coils are thesame, which of the magnets has a stronger magnetic field?3. Explain Why does a current in a galvanometer move a pointer?4. Name What are four devices that use electromagnetism?What makes the armature in an electric motor rotate?5. ExplainST Physical InteractiveTextbook,ational Editionh07ni emg000204ast pass 02/12/06eve TooleCopyright by Holt, Rinehart and Winston. All rights reserved.Interactive Textbook40Electromagnetism