ISU EE C.Y. Lee

Chapter 7Magnetism and

Electromagnetism

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Objectives

Explain the principles of the magnetic fieldExplain the principles of electromagnetismDescribe the principle of operation for several types of electromagnetic devicesDiscuss the principle of electromagnetic inductionDescribe some applications of electromagnetic induction

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The Magnetic FieldA permanent magnet has a magnetic field surrounding itA magnetic field is envisioned to consist of flux lines that radiate from the north pole to the south pole and back to the north pole through the magnetic material

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Attraction and RepulsionUnlike magnetic poles have an attractive force between them

Two like poles repel each other

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Altering a Magnetic FieldWhen nonmagnetic materials such as paper, glass, wood or plastic are placed in a magnetic field, the lines of force are unaltered

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Altering a Magnetic FieldWhen a magnetic material such as iron is placed in a magnetic field, the flux lines tend to be altered to pass through the magnetic material

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Magnetic FluxThe force lines going from the north pole to the south pole of a magnet are called magnetic flux (φ); units: weber (Wb)The magnetic flux density (B) is the amount of flux per unit area perpendicular to the magnetic field; units: tesla (T)

1 T = 10000 G (Gauss)

AB φ=

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Magnetic Flux

AB φ=

Example: Find the flux density in a magnetic field in which the flux in 0.1 m2 is 800 μWb

TmWb

AB μμφ 8000

10800

2 ===.

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Magnetizing Materials

Ferromagnetic materials such as iron, nickel and cobalt have randomly oriented magnetic domains, which become aligned when placed in a magnetic field, thus they effectively become magnets

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Application (Magnetic Switch)

Operation of a magnetic switch

Connection of a typical perimeter alarm system

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Electromagnetism

The right-hand rule is used to determine the direction of the lines of force

Electromagnetism is the production of a magnetic field by current in a conductor

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Electromagnetic FieldWhen current passes through a conductor, an electromagnetic field is created around the conductor

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Electromagnetic DevicesElectromagnets are used in devices such as magnetic disk, electric motors, speakers, solenoids, and relays

Read/write function on a magnetic surface

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Electromagnetic DevicesThe solenoid is used for applications such as opening and closing valves and automobile door locks

Basic solenoid operation:

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Electromagnetic DevicesRelays differ from solenoids in that the electromagnetic action is used to open or close electrical contacts rather than to provide mechanical movement

Basic structure of a relay:

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Electromagnetic DevicesPermanent-magnet speakers are commonly used in stereos, radios, and TVs, and their operation is based on the principle of electromagnetism

Basic speaker operation with movement:

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Electromagnetic InductionWhen a conductor is moved through a magnetic field, a voltage is induced across the conductorThis principle is known as electromagnetic induction

The faster the relative motion, the greater the induced voltage

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Relative MotionWhen a magnetic field is moved past a stationary wire, there is also relative motionThe induce voltage depends on the rate of relative motion between the wire and the magnetic field

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Polarity of Induced VoltageThe direction of movement of a wire through a magnetic field will affect the polarity of the induced voltage as indicated below:

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Induced CurrentThe voltage induced by the relative motion of a wire through a magnetic field will cause a current in a load connected to the wireThis current is called an induced current

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Motor ActionWhen a current flows through a wire that is in a magnetic field, the direction of current flow will result in the magnetic field being weakened on one side, and strengthened on the otherThe result of the weakened and strengthened field is a force on the conductor

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Faraday’s LawThe voltage induced across a coil of wire equals the number of turns in the coil times the rate of change of the magnetic flux

dtdN BΦ

−=ε

MOVIE ACTIVE

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Lenz’s LawLenz’ Law states the polarity of the induced voltage in a loop is such that it produces a current whose magnetic field opposes the change in magnetic flux through the loop

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DC GeneratorBasic operation of a dc generator

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Summary

Unlike magnetic poles attract each other, and like poles repel each otherMaterials that can be magnetized are called ferromagneticWhen there is current through a conductor, it produces an electromagnetic field around the conductorThe right-hand rule can be used to establish the direction of the electromagnetic lines of force around a conductor

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Summary

When a conductor moves within a magnetic field, or when a magnetic field moves relative to a conductor, a voltage is induced across the conductorThe faster the relative motion between a conductor and a magnetic field, the greater is the induced voltageThe polarity of the induced voltage in a loop is such that it produces a current whose magnetic field opposes the change in magnetic flux through the loop