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Unit Ten:Magnetism And

Electromagnetism

John Elberfeld

JElberfeld@itt-tech.edu

WWW.J-Elberfeld.com

ET115 DC Electronics

ScheduleSchedule

Unit Unit Topic Topic Chpt LabsChpt Labs1.1. Quantities, Units, SafetyQuantities, Units, Safety 11 2 (13)2 (13)2.2. Voltage, Current, ResistanceVoltage, Current, Resistance 22 3 + 163 + 163.3. Ohm’s LawOhm’s Law 33 5 (35)5 (35)4.4. Energy and PowerEnergy and Power 33 6 (41)6 (41)

5.5. Series CircuitsSeries Circuits Exam IExam I 44 7 (49)7 (49)

6.6. Parallel CircuitsParallel Circuits 55 9 (65)9 (65)

7.7. Series-Parallel CircuitsSeries-Parallel Circuits 66 10 (75)10 (75)

8.8. Thevenin’s, Power Thevenin’s, Power Exam 2Exam 2 66 19 (133)19 (133)

9.9. Superposition Theorem Superposition Theorem 66 11 (81)11 (81)

10.10. Magnetism & Magnetic DevicesMagnetism & Magnetic Devices 77 Lab Final Lab Final 11.11. Course Review and Course Review and Final ExamFinal Exam

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Unit 10 Objectives - IUnit 10 Objectives - I

• Explain magnetic flux and magnetic flux Explain magnetic flux and magnetic flux density and cite the measurement units in density and cite the measurement units in the MKS system.the MKS system.

• Define permeability, reluctance and Define permeability, reluctance and magnetomotive force.magnetomotive force.

• Describe how a solenoid, speaker, relay, Describe how a solenoid, speaker, relay, and analog meter work.and analog meter work.

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Unit 10 Objectives – IIUnit 10 Objectives – II

• Construct basic DC circuits on a Construct basic DC circuits on a protoboard.protoboard.

• Use a digital multimeter (DMM) to measure Use a digital multimeter (DMM) to measure a predetermined low voltage on a power a predetermined low voltage on a power supply.supply.

• Measure resistances and voltages in a DC Measure resistances and voltages in a DC circuit using a DMM.circuit using a DMM.

Reading AssignmentReading Assignment

• Read and study Read and study

• Chapter 7: Magnetism and Chapter 7: Magnetism and Electromagnetism: Pages 277-292Electromagnetism: Pages 277-292

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Lab AssignmentLab Assignment

• ReviewReview

• Lab FinalLab Final

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Written AssignmentsWritten Assignments

• Complete the Unit 10 Homework Complete the Unit 10 Homework sheetsheet

• Show all your work!Show all your work!

• Be prepared for a Theory Be prepared for a Theory FINAL FINAL EXAMEXAM on questions similar to those on questions similar to those on the homework.on the homework.

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DefinitionsDefinitions

• Magnetism: Ability to attract Magnetism: Ability to attract specific material types using specific material types using magnetic forces.magnetic forces.

• Magnets: Materials that display Magnets: Materials that display phenomenon called magnetism.phenomenon called magnetism.

• Magnetic field: The invisible lines of Magnetic field: The invisible lines of influence that surround a magnet or influence that surround a magnet or a wire carrying current.a wire carrying current.

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Magnetic and Electromagnetic FieldsMagnetic and Electromagnetic Fields

• A body that attracts a piece of iron is A body that attracts a piece of iron is known as a magnet, and this property known as a magnet, and this property called magnetism.called magnetism.

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Natural MagnetsNatural Magnets

• Lodestone is a special iron ore that Lodestone is a special iron ore that is magnetized when lightening is magnetized when lightening strikes near itstrikes near it

• Lodestone has been studied for Lodestone has been studied for many hundreds of years and used many hundreds of years and used for navigationfor navigation

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Magnetic and Electromagnetic FieldsMagnetic and Electromagnetic Fields

• Artificial magnets are used in electric Artificial magnets are used in electric machines and equipments.machines and equipments.

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Properties of MagnetsProperties of Magnets

• The following figure shows the The following figure shows the attractive property of magnets.attractive property of magnets.– Iron filings are attracted in consistent Iron filings are attracted in consistent

patterns to a magnetpatterns to a magnet

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AttractionAttraction

• Unlike poles attract (N-S, S-N)Unlike poles attract (N-S, S-N)

• Like poles repel (N-N, S-S)Like poles repel (N-N, S-S)

• Forces can be VERY strongForces can be VERY strong

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Properties of MagnetsProperties of Magnets

• Iron filings show the lines of flux Iron filings show the lines of flux around a magnetaround a magnet

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DefinitionsDefinitions

• Lines of Force: That which makes up Lines of Force: That which makes up the magnetic field. Also called Flux the magnetic field. Also called Flux lines.lines.

• Magnet Flux Density: Quantity of lines Magnet Flux Density: Quantity of lines of force per given unit area.of force per given unit area.

• Flux = Flux = ФФ in webersin webers• AreaArea = A = A in min m22

• Magnetic Flux Density: B = Magnetic Flux Density: B = ФФ/A/A– Unit is Unit is teslatesla (T) (T)

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Force In MagnetismForce In Magnetism

Like poles repel Unlike poles attractLike poles repel Unlike poles attract

• Force is dependent on field strength.Force is dependent on field strength.

• Force is inversely dependent on Force is inversely dependent on proximity of poles.proximity of poles.

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strength2Polestrength1PoleForce

d

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Magnetized Versus NonmagnetizedMagnetized Versus Nonmagnetized

• Magnetic materials are made up of Magnetic materials are made up of billions of magnetic regions or billions of magnetic regions or domains all aligned in the same domains all aligned in the same directiondirection

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N and S PolesN and S Poles

• Every magnet has both an N and an S poleEvery magnet has both an N and an S pole• The Earth is a giant magnetThe Earth is a giant magnet• If allowed to spin freely, the N pole of a If allowed to spin freely, the N pole of a

small magnet always seeks or points small magnet always seeks or points toward the NORTH magnetic pole up in toward the NORTH magnetic pole up in CanadaCanada– Technically speaking, the north magnetic pole Technically speaking, the north magnetic pole

in Canada is really an S pole. in Canada is really an S pole. Why?Why?

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Properties of MagnetsProperties of Magnets

• If a magnet is cut or sliced into If a magnet is cut or sliced into pieces, each piece of the magnet pieces, each piece of the magnet will have a N and an S pole.will have a N and an S pole.

• You can’t isolate a single pole - You can’t isolate a single pole - ever.ever.

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Lines Of FluxLines Of Flux

• Flux lines go from N to S polesFlux lines go from N to S poles

• If you could get a single N pole, it If you could get a single N pole, it would move along a line of fluxwould move along a line of flux

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Characteristics of Magnetic Lines of ForceCharacteristics of Magnetic Lines of Force

• Outside a magnet, the magnetic Outside a magnet, the magnetic lines of force are from North to lines of force are from North to South while inside, they are from South while inside, they are from South to North.South to North.

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DefinitionsDefinitions

• Lines of Force: That which makes up Lines of Force: That which makes up the magnetic field. Also called Flux the magnetic field. Also called Flux lines.lines.

• Magnet Flux Density: Quantity of lines Magnet Flux Density: Quantity of lines of force per given unit area.of force per given unit area.

• Flux = Flux = ФФ in webersin webers• AreaArea = A = A in min m22

• Magnetic Flux Density: B = Magnetic Flux Density: B = ФФ/A/A– Unit is Unit is tesla tesla (T)(T)

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Permanent MagnetsPermanent Magnets

• Not as permanent as name implies.Not as permanent as name implies.

• Configured in different shapes.Configured in different shapes.

• Use of ‘Keeper Bar’ extends life.Use of ‘Keeper Bar’ extends life.

Damage to a MagnetDamage to a Magnet

• Heat and physical abuse can make Heat and physical abuse can make the magnetic domains become the magnetic domains become randomizedrandomized

• When the domains are NOT lined up, When the domains are NOT lined up, the material stops being a magnetthe material stops being a magnet

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• These are strongly magnetized in the direction These are strongly magnetized in the direction of the field when placed in a magnetic field, such of the field when placed in a magnetic field, such as, iron, steel, nickel, cobalt, and some of their as, iron, steel, nickel, cobalt, and some of their alloys.alloys.

• Each atom in these materials make a Each atom in these materials make a considerable contribution to establish an considerable contribution to establish an internal magnetic field.internal magnetic field.

• Depending upon there relative permeability, they Depending upon there relative permeability, they are further classified into two types:are further classified into two types:– Soft magnetic materialsSoft magnetic materials– Hard magnetic materialsHard magnetic materials

Ferromagnetic MaterialsFerromagnetic Materials

MagnetismMagnetism

• Materials with a high Materials with a high permeabilitypermeability can easily have a magnetic field can easily have a magnetic field established with themestablished with them– Similar to conductance in DC currentSimilar to conductance in DC current

• Materials with a high Materials with a high reluctance reluctance oppose or resist the establishment of oppose or resist the establishment of a magnetic fielda magnetic field– Similar to resistance in DC currentSimilar to resistance in DC current

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ElectromagnetsElectromagnets

• Moving electric current in a wire Moving electric current in a wire makes a magnetic fieldmakes a magnetic field

• Moving magnets near a wire makes Moving magnets near a wire makes an electric current flow in the wirean electric current flow in the wire

• Notice there must be motion in both Notice there must be motion in both situations.situations.

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Electromagnetic FieldsElectromagnetic Fields

• Most electric machines and motors Most electric machines and motors depend on electromagnetic fields for depend on electromagnetic fields for forces that cause motionforces that cause motion

• Motors use current moving through a wire Motors use current moving through a wire to generate an electromagnetic fieldto generate an electromagnetic field

• The magnetic field exerts a force that The magnetic field exerts a force that make the motor rotate and do workmake the motor rotate and do work

• The direction of the current flow The direction of the current flow determines the orientation of the fielddetermines the orientation of the field

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Direction Of Field In ConductorDirection Of Field In Conductor

• Current-carrying wire has an Current-carrying wire has an associated magnetic field.associated magnetic field.

• Left-Hand rule determines direction Left-Hand rule determines direction of field.of field.

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Left hand ruleLeft hand rule

• If you grab a wire with your LEFT If you grab a wire with your LEFT hand, with you thumb pointing in the hand, with you thumb pointing in the direction of the electron flow, your direction of the electron flow, your fingers will point in the same fingers will point in the same direction that a compass (N) would direction that a compass (N) would pointpoint

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Try itTry it

• Dot – at youDot – at you Cross - awayCross - away

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Forces In Parallel ConductorsForces In Parallel Conductors

• Two wires placed side by side and Two wires placed side by side and carrying current in the same carrying current in the same direction will attract.direction will attract.

• Two wires placed side by side and Two wires placed side by side and carrying current in opposite carrying current in opposite directions will repel.directions will repel.

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Coiled WireCoiled Wire

• A coil of wire has a more intense A coil of wire has a more intense magnetic field than a single wire.magnetic field than a single wire.

• One end of the coil is N, the other end SOne end of the coil is N, the other end S

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Magnetic Polarity Of CoilMagnetic Polarity Of Coil

• Left-hand rule determines North pole.Left-hand rule determines North pole.• Coil your fingers in the direction of the Coil your fingers in the direction of the

ELECTRON flowELECTRON flow• Your thumb points toward the N poleYour thumb points toward the N pole

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Factors For Field Strength Of Coil

FACTOR

1. Number of turns

2. Amount of current

3. Cross-sectional area

4. Core material

RELATIONSHIP

Direct

Direct

Direct

Direct

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Magnetic FluxMagnetic Flux

• It is the total number of magnetic lines of It is the total number of magnetic lines of force in a magnetic field, represented by force in a magnetic field, represented by ΦΦ and measured in and measured in WeberWeber..

• The closer the lines are together, the The closer the lines are together, the higher the magnetic flux densityhigher the magnetic flux density

• Magnetic flux density is also known as Magnetic flux density is also known as magnetic induction represented magnetic induction represented by by B B and is measured in and is measured in TeslaTesla (weber/m(weber/m22) as:) as:

aB

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Basic Magnetic TermsBasic Magnetic Terms

TermTerm SymbolSymbol SI UnitSI Unit

FluxFlux Weber Weber

Flux DensityFlux Density ßß TeslaTesla

MagnetomotiveMagnetomotiveforceforce mmfmmf Ampere-Ampere-

turnsturns

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Compared to Ohm’s LawCompared to Ohm’s Law

• Magnetomotive (mmf) force in magnetism Magnetomotive (mmf) force in magnetism compares to electromotive force (emf or compares to electromotive force (emf or voltage) in electrical circuits.voltage) in electrical circuits.

• Reluctance (how much a material oppose Reluctance (how much a material oppose magnetic flux) compares to resistance in magnetic flux) compares to resistance in electrical circuitselectrical circuits

• Flux tells how strong the magnetic field is Flux tells how strong the magnetic field is and compares to current in electrical and compares to current in electrical circuitscircuits

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FormulaFormula

• FFmm = = ΦΦ R or R or ΦΦ = F = Fmm/ R / R or or

• FFmm = mmf = = mmf = ampere turns ampere turns = At (Voltage)= At (Voltage)

• ΦΦ = magnetic flux = weber (Current) = magnetic flux = weber (Current)

• R = Reluctance = At / weber (Ohms)R = Reluctance = At / weber (Ohms)

• Compare:Compare:

• V = I R V = I R I = V / RI = V / R

• FFmm = = ΦΦ R R Φ Φ = F = Fmm/ R / R

SolenoidSolenoid

• A solenoid is an electromagnetic device A solenoid is an electromagnetic device that converts an electrical input to that converts an electrical input to mechanical motionmechanical motion

• When current is applied, the magnet pulls When current is applied, the magnet pulls the plunger inthe plunger in

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SolenoidSolenoid

• When the electricity is turned off, a When the electricity is turned off, a spring or other force is needed to spring or other force is needed to make move back out to its original make move back out to its original positionposition

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RelaysRelays

• Relays use magnetic action Relays use magnetic action specifically to open or close electric specifically to open or close electric circuitscircuits

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SpeakersSpeakers• Speakers use electro-magnets to move a Speakers use electro-magnets to move a

paper cone and generate soundpaper cone and generate sound

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Motor ActionMotor Action

• The magnetic forces on coils of wire The magnetic forces on coils of wire can make coils move rapidly like in a can make coils move rapidly like in a motormotor

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Generator ActionGenerator Action

• Similarly, forcing a coil of wire to Similarly, forcing a coil of wire to move through a magnetic field move through a magnetic field generates electricitygenerates electricity

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Faraday’s LawFaraday’s Law

Factors influencing induced voltage:Factors influencing induced voltage:1. Amount of flux.1. Amount of flux.2. Number of turns linked by flux.2. Number of turns linked by flux.3. Angle of cutting.3. Angle of cutting.4. Rate of relative motion.4. Rate of relative motion.• To generate more electricity, To generate more electricity,

increase the magnetic field, the increase the magnetic field, the number of coils, and the rate the number of coils, and the rate the coils movecoils move

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Faraday’s Laws of Electromagnetic InductionFaraday’s Laws of Electromagnetic Induction

• Faraday gave two laws of Faraday gave two laws of electromagnetic induction:electromagnetic induction:– First Law: First Law: An induced emf is established in An induced emf is established in

a circuit whenever a magnetic field linking a circuit whenever a magnetic field linking a circuit is a circuit is changedchanged..

– Second law: The magnitude of emf induced Second law: The magnitude of emf induced in a circuit is directly proportional to the in a circuit is directly proportional to the rate of change of magnetic flux. For a coil rate of change of magnetic flux. For a coil with N turns, the flux changes from with N turns, the flux changes from ΦΦ11 to to ΦΦ22 in in tt time, then emf time, then emf ee produced is given by: produced is given by:

t

Ne

)( 12

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Faraday’s LawFaraday’s Law

TVinduced

• Another way of saying the same Another way of saying the same thingthing

• Voltage is proportional to the rate of Voltage is proportional to the rate of change of the magnetic fluxchange of the magnetic flux

• You need CHANGES in the magnetic You need CHANGES in the magnetic field to generate voltagefield to generate voltage

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DC GeneratorDC Generator

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Lenz’s LawLenz’s Law

• Direction of an induced voltage is Direction of an induced voltage is such that it tends to oppose the such that it tends to oppose the change that caused itchange that caused it

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Lenz LawLenz Law

• This law states that the direction of the This law states that the direction of the induced current is such that it opposes induced current is such that it opposes the very cause which produced the the very cause which produced the induced current.induced current.

• This law is applicable This law is applicable only to closed circuits.only to closed circuits.

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Practical ApplicationsPractical Applications

• Analog meters depend on the Analog meters depend on the magnetic fields generated by electric magnetic fields generated by electric current moving through the meter to current moving through the meter to exert a force and rotate a needle a exert a force and rotate a needle a distance that is proportional to the distance that is proportional to the currentcurrent

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D’Arsonval MeterD’Arsonval Meter

• It works when a It works when a current carrying coilcurrent carrying coil is placed in a is placed in a magnetic field and magnetic field and it experiences a it experiences a torque.torque.

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AmmeterAmmeter

• It is a low-resistance galvanometer used It is a low-resistance galvanometer used to measure current in a circuit in amperes.to measure current in a circuit in amperes.

• A galvanometer can be converted into an A galvanometer can be converted into an ammeter by using a low-resistance wire in ammeter by using a low-resistance wire in parallel with the galvanometer.parallel with the galvanometer.

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VoltmeterVoltmeter

• An analog voltmeter is a high-resistance An analog voltmeter is a high-resistance galvanometer. It is used to measure the galvanometer. It is used to measure the potential difference between two points of potential difference between two points of a circuit in volts.a circuit in volts.

• A galvanometer can be converted into a A galvanometer can be converted into a voltmeter by connecting a high resistance voltmeter by connecting a high resistance in series with the galvanometer.in series with the galvanometer.

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FINAL AdviceFINAL Advice

• Study all your homework papers and Study all your homework papers and quizzesquizzes

• Go over the practice test in detailGo over the practice test in detail

• Give yourself plenty of time to Give yourself plenty of time to practice and review all the material practice and review all the material we have coveredwe have covered

Unit 10 SummaryUnit 10 Summary

• Magnetic flux and flux densityMagnetic flux and flux density

• Permeability, reluctance, and Permeability, reluctance, and magnetomotive forcemagnetomotive force

• Solenoids, speakers, relays, meter Solenoids, speakers, relays, meter movementsmovements

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