Chapter 9

Magnetism

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• Objectives– After completing this chapter, the student should be

able to:• Identify three types of magnets.

• Describe the basic shapes of magnets.

• Describe the difference between permanent magnets and temporary magnets.

• Describe how the earth functions as a magnet.

• State the laws of magnetism.

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• Explain magnetism based on the theory of atoms and electron spin.

• Explain magnetism based on the domain theory.• Identify flux lines and their significance.• Define permeability.• Describe the magnetic effects of current flowing

through a conductor.• Describe the principle of an electromagnet.• Describe how to determine the polarity of an

electromagnet using the left-hand rule.

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• Define magnetic induction.

• Define retentivity and residual magnetism.

• Define a magnetic shield.

• Describe how magnetism is used to generate electricity.

• State the basic law of electromagnetism.

• Describe how the left-hand rule for generators can be used to determine the polarity of induced voltage.

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• Describe how AC and DC generators convert mechanical energy into electrical energy.

• Describe how a relay operates as an electromechanical switch.

• Discuss the similarities between a doorbell and a relay.

• Discuss the similarities between a solenoid and a relay.

• Describe how a magnetic phonograph cartridge works.

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• Describe how a loudspeaker operates.

• Describe how information can be stored and retrieved using magnetic recording.

• Describe how a DC motor operates.

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• Magnets– Natural magnet

• Derived from magnetite.

– Artificial magnet• Created by rubbing a piece of soft iron with a piece

of magnetite.

– Electromagnet• Created by current flowing through a coil of wire.

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• Permanent magnets– Retain their magnetic properties.

• Temporary magnets– Retain only a small portion of their magnetic

properties.

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• The earth is a huge magnet.

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• The laws of magnetism– Unlike magnetic poles attract each other.– Like magnetic poles repel each other.

• The color code for magnets– Red for the North Pole.– Blue for the South Pole.

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• Magnetism– Can be traced to the atom.– As electrons orbit the nucleus, they also spin on their

axis.– This electrostatic charge produces a magnetic field.– The direction of the magnetic field is the same as the

electron’s direction of spin.

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• Ferromagnetic materials– Materials that respond to magnetic fields.– Atoms combine into domains or groups.– When unmagnetized, the domains are random.

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– When magnetized, the domains align in a common direction and the material becomes a magnet.

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• Magnetic field– The invisible lines of force that surround a

magnet. These are called flux lines.

• Flux lines– Have polarity from North to South.– Always form a complete loop.– Do not cross each other.– Tend to form the smallest possible loop.

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• Permeability– The ability of a material to accept magnetic

lines of force.

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• Electricity and Magnetism– A magnetic field is generated when current

flows through a wire.

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• Electromagnets– Composed of many turns of wire close together.

• The principle of the electromagnet.– When wire is twisted into a loop:

• The flux lines are brought together.

• The flux lines are concentrated at the center of the loop.

• A North and South Pole are established.

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– The strength of the magnetic field can be increased three ways.

• The more turns of wire, the more flux lines are added together.

• The greater the current, the greater the number of flux lines generated.

• A ferromagnetic core is inserted into the center of the coil, usually iron.

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• Magnetic induction– The effect a magnet has on an object without

physical contact.

• Residual magnetism– The magnetic field that remains when an object

is separated from a magnet.

• Retentivity– The ability of a material to retain its magnetic

field after the magnetizing force is removed.

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• Magnetic shields– Low-reluctance materials– Used to protect electronic equipment from

magnetic flux lines.

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• Electromagnetic induction– The principle behind the generation of

electricity.• A current is produced when a conductor passes or is

passed by a magnetic field.

• As the conductor passes through the magnetic field, a deficiency of electrons is created.

• This results in a difference of potential between the ends of the conductor.

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• When the conductor is removed from the magnetic field, the free electrons return to their parent atoms.

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• Faraday’s law– The induced voltage in a conductor is directly

proportional to the rate at which the conductor cuts the magnetic lines of force.

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• The left-hand rule

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• Magnetic and Electromagnetic Applications– AC generator

• Converts mechanical energy to electrical energy by utilizing the principle of electromagnetic induction.

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– DC generator• Functions like an AC generator with the exception

that it converts the AC voltage to DC voltage.

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• Relay– An electromagnetic switch that opens and

closes with an electromagnetic coil.• Used where it is desirable to have one circuit control

another circuit.

• It electrically isolates the two circuits.

• Also used to control several circuits some distance away.

– Doorbell

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• Solenoid– A coil, when energized, pulls a plunger that

does some mechanical work.• Door chimes

• Automotive starters

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• Phonograph pickups– Use the electromagnetic principle.

• A magnetic field is produced by a permanent magnet attached to the stylus.

• The stylus tracks through the groove of a record in response to the audio signal recorded.

• The movement induces a small voltage that varies at the audio signal response.

• The induced voltage is amplified and used to drive a loudspeaker, producing the audio signal.

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• Loudspeakers– Constructed of a moving coil around a

permanent magnet.• The magnet produces a stationary magnetic field.• The current passes through the coil, producing a

magnetic field that varies at the rate of the audio signal.

• The magnetic field of the coil is attracted and repelled by the field of the magnet.

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• The coil is attached to a cone that moves in response to the audio signal.

• The cone reproduces the audio signal.

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• Magnetic recording– Uses the electromagnetic principle to store

information.• A signal is stored on tape or disk with a record head,

to be read back later with a playback head.– Some are combined in one package.– They may be on the same head.

• The record and playback heads are a coil of wire with a ferromagnetic core.

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• A tiny gap between the ends of the core is a magnetic field.

• A piece of material covered with iron oxide, is pulled across the record head, magnetizing it.

– Information is written in a magnetized pattern.

• To play back or read the information, the material is moved past the gap in the playback head.

• The magnetic field induces a small voltage into the coil winding.

• When amplified, the information is reproduced.

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• Examples-– Cassette recorders– Video recorders– Reel-to-reel recorders– Floppy disk drives– Hard disk drives

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• DC motor– Operation depends on the principle that a

current-carrying conductor, placed in and at right angles to a magnetic field, tends to move at right angles to the direction of the field.

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( A ) The magnetic field extending between a north and south pole.

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( B ) The magnetic field that exists around a current-carrying conductor.

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( C ) The conductor placed in the magnetic field.

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( D ) If the current through the conductor is reversed, the direction of the magnetic flux around the conductor is reversed.

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• TV, radar, computer terminals– Uses the current-carrying principle.

• The conductor carrying current is deflected by a magnetic field.

• The electrons travel through a vacuum to strike a phosphor screen where they emit light.

• By varying the electron beam over the surface of the picture screen, a picture can be created.

• Two magnetic fields deflect the beam.

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• One field moves the beam from side to side.

• One beam moves the beam up and down.

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• In Summary– Magnet

• Three kinds

• Many shapes

• Unlike poles attract, like poles repel

– Two theories of magnetism• Electron spin

• Domains

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– Flux lines– Permeability– Magnetic field– Determining direction of flux lines.– Electromagnets

• Strength

• Polarity

– Retentivity

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– Electromagnetic induction– Faraday’s law

• Induced voltage is directly proportional to the rate at which the conductor cuts the magnetic lines of force.

– The left-hand rule for generators– AC and DC generators– Relays

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– Electromagnetic principles are applied in• Doorbells

• Solenoids

• Phonograph pickups

• Loudspeakers

• Magnetic recordings

– DC motors and meters • Use the same principles

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– Electron beams• Television

• Radar

• Oscilloscopes

• Computer terminals