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General Physical General Physical

ScienceScience

Chapter 8Chapter 8

Electricity and MagnetismElectricity and Magnetism

Electrical ChargeElectrical Charge

Electrical charge is one of the 7 Electrical charge is one of the 7

fundamental quantities.fundamental quantities.

Two types of electrical chargeTwo types of electrical charge

–– Positive (+)Positive (+)

––Negative (Negative (--))

–– 3 types of sub3 types of sub--atomic particlesatomic particles

Protons (+)Protons (+)

Electrons (Electrons (--))

Neutrons (no charge)Neutrons (no charge)

Electrical ChargeElectrical Charge

The magnitude of the proton charge The magnitude of the proton charge

is equal to the magnitude of the is equal to the magnitude of the

electron charge.electron charge.

––Charges will exactly cancel.Charges will exactly cancel.

CoulombCoulomb

––Unit of electrical chargeUnit of electrical charge

––+q = greater + charge = electron +q = greater + charge = electron

deficientdeficient

–– --q = greater q = greater -- charge = electron richcharge = electron rich

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Electrical ForceElectrical Force

Exists between two charged particlesExists between two charged particles

Law of chargesLaw of charges–– Unlike charges attract and like charges Unlike charges attract and like charges

repel.repel.

Magnitude of the force = CoulombMagnitude of the force = Coulomb’’s s LawLaw–– The force of attraction or repulsion between The force of attraction or repulsion between

two charged bodies is directly proportional two charged bodies is directly proportional to the product of the two charges and to the product of the two charges and inversely proportional to the square of the inversely proportional to the square of the distance between them. distance between them.

CoulombCoulomb’’s Laws Law

Mathematically F = kqMathematically F = kq11qq22 / r/ r22

–– F = Force (N)F = Force (N)

–– k = proportionality constantk = proportionality constant9.0 x 109.0 x 1099 N mN m22 / C/ C22

Similar to gravitational law, Similar to gravitational law, howeverhowever……

––Much higher value for the constant.Much higher value for the constant.

–– Based on charges, not masses.Based on charges, not masses.

–– can be an attraction or repulsioncan be an attraction or repulsion

Static ChargeStatic Charge

Excess or deficiency of electronsExcess or deficiency of electrons–– Excess; negative chargeExcess; negative charge

–– DefiencyDefiency; positive charge; positive charge

Walk across carpet in winterWalk across carpet in winter

PolarizationPolarization–– separation of chargeseparation of charge

–– rubbing rubber balloon in hairrubbing rubber balloon in hair

Induced polarizationInduced polarization–– A charge causes charge separationA charge causes charge separation

–– reason balloon sticks to wallreason balloon sticks to wall

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PhotocopierPhotocopier

Se drum + chargeSe drum + charge

Light reflected off paper Light reflected off paper removes some of the removes some of the drum chargedrum charge

Toner has a Toner has a -- chargecharge–– sticks to + areas on sticks to + areas on

drumdrum

Paper + charge gets Paper + charge gets toner from drumtoner from drum

Toner Toner ‘‘fusedfused’’ by heatby heat

Electrical Charge and Electrical Electrical Charge and Electrical

ForceForce

Learning GoalsLearning Goals

––Describe electrical charge in terms of Describe electrical charge in terms of

protons and electronsprotons and electrons

–– State similarities and differences State similarities and differences

between Newtonbetween Newton’’s Law of Universal s Law of Universal

Gravitation and CoulombGravitation and Coulomb’’s Laws Law

Questions: 1Questions: 1--44

Problems: 1, 3Problems: 1, 3

Electrical CurrentElectrical Current

CurrentCurrent

––Rate of flow of electrical chargeRate of flow of electrical charge

––Measured in Amperes (A)Measured in Amperes (A)

1 A = 1C / 1s1 A = 1C / 1s

General: I = q / tGeneral: I = q / t

–– I=CurrentI=Current

–– q=chargeq=charge

–– t=timet=time

q = I x tq = I x t

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CurrentCurrent

ConductorsConductors

–– current flows readilycurrent flows readily

Does not Does not ‘‘flowflow’’ in the traditional sensein the traditional sense

––metals are good electrical conductorsmetals are good electrical conductors

–– ‘‘looseloose’’ electronselectrons

Also reason for good thermal conductivityAlso reason for good thermal conductivity

InsulatorsInsulators

–– current d/n flowcurrent d/n flow

–– electrical wire coatingelectrical wire coating

CurrentCurrent

SemiconductorsSemiconductors

–– intermediate between conductor and intermediate between conductor and

insulatorinsulator

–– allows some (but not free) current flowallows some (but not free) current flow

–– graphite (resistors)graphite (resistors)

–– Silicon/Germanium mixturesSilicon/Germanium mixtures

computer chipscomputer chips

the the ‘‘semiconductorsemiconductor’’ industryindustry

ExampleExample

A current of 0.50 A flows in a wire A current of 0.50 A flows in a wire

for 2.0 minutes. (a) How much (net) for 2.0 minutes. (a) How much (net)

charge goes past a point in the wire charge goes past a point in the wire

in this time? (b) How many in this time? (b) How many

electrons make up this charge?electrons make up this charge?

What do we know?What do we know?

–– a) 0.50 A (C/s), 2.0 min (120 s); q = I a) 0.50 A (C/s), 2.0 min (120 s); q = I

x tx t

–– q = 0.50 C/s x 120 s = 60Cq = 0.50 C/s x 120 s = 60C

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ExampleExample

60 C60 C

Charge on a single electron is 1.6 x Charge on a single electron is 1.6 x

1010--1919CC

How many electrons = 60 CHow many electrons = 60 C

–– 60 C = x(1.6 x 1060 C = x(1.6 x 10--1919C)C)

–– x = 60 C / 1.6 x 10x = 60 C / 1.6 x 10--1919CC

–– x = 3.8 x 10x = 3.8 x 102020 electronselectrons

–– 380,000,000,000,000,000,000 380,000,000,000,000,000,000

electrons!electrons!

VoltageVoltage

Greater separation Greater separation of charges means of charges means more work more work required to required to separate more separate more charges.charges.

Electrical potential Electrical potential energyenergy–– Voltage (potential)Voltage (potential)

–– Work per unit Work per unit chargecharge

–– PE per unit chargePE per unit charge

VoltageVoltage

Measure of electrical potential is the Measure of electrical potential is the volt. (Work per unit charge)volt. (Work per unit charge)

–– 1 V = 1 J / 1 C1 V = 1 J / 1 C

Measurement of the Measurement of the ‘‘frictionfriction’’ for for electrical current flow is called electrical current flow is called resistance.resistance.–– Ohm Ohm -- ΩΩ

Ohms Law Ohms Law -- relationship between relationship between voltage, current, and resistancevoltage, current, and resistance

–– V = IRV = IR

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Current FlowCurrent Flow

Voltage Voltage -- water pressure; Current = water pressure; Current =

water flow; Resistance = water water flow; Resistance = water

wheelwheel

Current FlowCurrent Flow

You must have a complete circuit for You must have a complete circuit for

electricity to flowelectricity to flow

By convention, current flow is the By convention, current flow is the

way that positive charges would way that positive charges would

flow.flow.

–– convention was developed before our convention was developed before our

understanding of protons and neutrons.understanding of protons and neutrons.

––No practical difference because of this No practical difference because of this

convention.convention.

Electrical PowerElectrical Power

Power = current x voltagePower = current x voltage

–– P = IVP = IVBut V = IRBut V = IR

–– P = IP = I22RR

Heat lossHeat loss

–– All metals have some resistanceAll metals have some resistanceLight bulbs (watts)Light bulbs (watts)

–– Lower T, lower resistanceLower T, lower resistance

–– Superconductivity at very low Superconductivity at very low temperatures.temperatures.

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ExampleExample

Find the current and resistance of a Find the current and resistance of a 60W, 120V light bulb in operation.60W, 120V light bulb in operation.

What do we know?What do we know?

–– Power = 60W, Voltage = 120 VPower = 60W, Voltage = 120 V

–– P = IV means 60 = I x 120P = IV means 60 = I x 120

–– I = 60/120 = 0.50 AI = 60/120 = 0.50 A

–– P = IP = I22R means 60 = 0.50R means 60 = 0.5022 x x ΩΩ

–– ΩΩ = 60 / 0.50= 60 / 0.5022 = 240= 240ΩΩ

Current, Voltage and Electrical Current, Voltage and Electrical

PowerPower

Learning GoalsLearning Goals

––Define Define currentcurrent..

––Define Define voltagevoltage, and state how Ohm, and state how Ohm’’s s

law relates it to current and resistance.law relates it to current and resistance.

–– Explain electrical power and identify the Explain electrical power and identify the

parameters used to describe it.parameters used to describe it.

Questions: 5Questions: 5--88

Exercises: 5Exercises: 5--23 odd 23 odd

Simple Electrical CircuitsSimple Electrical Circuits

Two types of currentTwo types of current

––Direct (battery)Direct (battery)

–– Alternating (wall outlet)Alternating (wall outlet)

US ac typically 60 HzUS ac typically 60 Hz

Two basic types of electrical circuitsTwo basic types of electrical circuits

–– SeriesSeries

–– ParallelParallel

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Series CircuitSeries Circuit

Hooked up as a Hooked up as a

chainchain

–– Same current at all Same current at all

points.points.

–– Total resistance Total resistance

sum of individual sum of individual

resistances.resistances.

–– Total voltage drop Total voltage drop sum of individual sum of individual

drops.drops.

–– RRtt = = ΣΣRRii

Parallel CircuitParallel Circuit

Hooked up like a Hooked up like a

rope.rope.

–– Voltage across each Voltage across each

resistance the resistance the

same.same.

–– Current will vary in Current will vary in

each branch.each branch.

–– 1/R1/Rtt = = ΣΣ(1/R(1/Rii))

–– For two resistors:For two resistors:

RRtt= (R= (R11RR22) / (R) / (R11+R+R22))

ExampleExample

Three resistors have values of 6.0Three resistors have values of 6.0Ω, Ω,

6.0Ω, 6.0Ω, and 3.0and 3.0Ω. Ω. What is their total What is their total resistance when connected in parallel, resistance when connected in parallel, and how much current will be drawn and how much current will be drawn from a 12V battery if it is connected to from a 12V battery if it is connected to the circuit?the circuit?

What do we know?What do we know?–– 6.06.0Ω, 6.0Ω, Ω, 6.0Ω, and 3.0and 3.0ΩΩ

–– 1/R1/Rtt=1/R=1/R11 + 1/R+ 1/R22 + 1/R+ 1/R33 = 1/6 + 1/6 + 1/3 = 1/6 + 1/6 + 1/3 = 2/3= 2/3

–– RRtt = 3/2= 3/2ΩΩ

–– V = IR means I=V/R = 12/1.5 = 8.0AV = IR means I=V/R = 12/1.5 = 8.0A

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Parallel CircuitsParallel Circuits

Total resistance Total resistance

always less than always less than

the lowest the lowest

resistanceresistance

Household wiringHousehold wiring

–– all in parallelall in parallel

–– rest will work if one rest will work if one

failsfails

–– Christmas lightsChristmas lights

‘‘Shunt resistorShunt resistor’’

Electrical SafetyElectrical Safety

FusesFuses

–– Edison baseEdison base

end similar to end similar to lightbulblightbulb

can switch fuse ratingscan switch fuse ratings

–– TypeType--SS

bases differentbases different

cannot switch fuse ratingscannot switch fuse ratings

––Circuit breakersCircuit breakers

–– Fuses/CB always on Fuses/CB always on ‘‘hothot’’ sideside

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ElectricalElectrical

Safety Safety Case groundingCase grounding

–– ‘‘33rdrd’’ wirewire

Polarized PlugPolarized Plug

Simple Electrical Circuits and Simple Electrical Circuits and

Electrical SafetyElectrical Safety Learning GoalsLearning Goals

––Define Define voltagevoltage and state how Ohmand state how Ohm’’s s

Law relates it to current and resistanceLaw relates it to current and resistance

–– Explain electrical power and identify the Explain electrical power and identify the

parameters used to describe it.parameters used to describe it.

Questions: 9Questions: 9--1313

MagnetismMagnetism

LodestonesLodestones

–– 66thth Century BCCentury BC

––MagicMagic

Artificial MagnetsArtificial Magnets

––ChineseChinese

––Made from natural magnetsMade from natural magnets

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MagnetismMagnetism

Two regions of magnetic strengthTwo regions of magnetic strength

––North pole (N) points northNorth pole (N) points north

–– South pole (S) points southSouth pole (S) points south

Law of PolesLaw of Poles

–– Like poles repel and unlike poles attract.Like poles repel and unlike poles attract.

–– Attraction/repulsion inversely related to Attraction/repulsion inversely related to

the square of the distance between the the square of the distance between the

poles.poles.

MagnetismMagnetism

All magnets contain both N and S All magnets contain both N and S

polepole

––DipolesDipoles

Magnetic fieldMagnetic field

–– Imaginary lines indicating the direction Imaginary lines indicating the direction

a compass would point if it were placed a compass would point if it were placed

near a magnet.near a magnet.

–– Lines of forceLines of force

MagnetismMagnetism

Use metal filings to Use metal filings to

‘‘seesee’’ magnetic magnetic

field.field.

–– iron filings become iron filings become

magnetizedmagnetized

induced magnetisminduced magnetism

ScrewdriversScrewdrivers

–– serve to show lines serve to show lines

of force.of force.

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MagnetismMagnetism

Source of magnetism is Source of magnetism is

moving/spinning electrons!moving/spinning electrons!

––Can generate magnetic field with Can generate magnetic field with

moving electric fieldmoving electric field

Strength directly proportional to currentStrength directly proportional to current

–– ElectromagnetsElectromagnets

Electromagnetic FieldsElectromagnetic Fields

MagnetismMagnetism

Ferromagnetic Ferromagnetic materialsmaterials–– internal magnetic internal magnetic

domainsdomains

–– Random Random arrangement means arrangement means nonnon--magneticmagnetic

–– Can induce Can induce alignment with alignment with external magnetexternal magnet Tends to randomize Tends to randomize

with removal of with removal of external magnet.external magnet.

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MagnetsMagnets

–– ‘‘SoftSoft’’ iron iron --temporary temporary

magnetismmagnetism

–– ‘‘HardHard’’ magnetic magnetic

material = material = permanent magnetpermanent magnet

‘‘HardHard’’ ironiron

Ni, Co, some other Ni, Co, some other

elementselements

CeramicsCeramics

–– ElectromagnetsElectromagnets

MagnetsMagnets

Curie TemperatureCurie Temperature–– will convert a permanent will convert a permanent

magnet to a nonmagnet to a non--magnetic material.magnetic material.

EarthEarth’’s magnetic fields magnetic field–– Proposed by William Proposed by William

GilbertGilbert

–– Aurora Borealis and Aurora Borealis and Aurora Aurora AustralisAustralis

–– Origin unknownOrigin unknown Theorize due to EarthTheorize due to Earth’’s s

rotationrotation

EarthEarth’’s Magnetic Fields Magnetic Field

Approximates bar Approximates bar

magnetmagnet

Known to Known to ‘‘shiftshift’’

with timewith time

Magnetic and true Magnetic and true

poles d/n coincidepoles d/n coincide

–– must apply must apply

magnetic correction magnetic correction

for for ‘‘truetrue’’ directiondirection

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IsogonicIsogonic Lines in AviationLines in Aviation

MagnetismMagnetism

Learning GoalsLearning Goals

–– State the law of poles and describe State the law of poles and describe

the magnetic field.the magnetic field.

–– Identify the cause of magnetism, and Identify the cause of magnetism, and

tell why some materials can be tell why some materials can be

magnetized and others cannot.magnetized and others cannot.

–– Analyze some aspects of EarthAnalyze some aspects of Earth’’s s

magnetic field.magnetic field.

Questions: 14Questions: 14--1818

ElectromagnetismElectromagnetism

Interaction of electrical and magnetic Interaction of electrical and magnetic

effects.effects.

––Moving electric fields generate magnetic Moving electric fields generate magnetic

fieldsfields

––Magnetic fields deflect a moving electric Magnetic fields deflect a moving electric

charge.charge.

Telephone receiver.Telephone receiver.

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Telephone ReceiverTelephone Receiver

MicrophoneMicrophone–– Diaphragm that can Diaphragm that can

vibrate and change vibrate and change resistance (carbon resistance (carbon micmic))

Varies electric Varies electric currentcurrent

SpeakerSpeaker–– electromagnetic coilelectromagnetic coil

–– current varies current varies strengthstrength

–– diaphragm vibratesdiaphragm vibrates

Magnetic Force on a Moving Magnetic Force on a Moving

Electric Charge.Electric Charge.

Motors and GeneratorsMotors and Generators

No moving current No moving current -- no forceno force

Moving current Moving current -- force on wireforce on wire

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Motors and GeneratorsMotors and Generators

BrushesBrushes

–– reverse currentreverse current

–– cause attraction to cause attraction to

become repulsion become repulsion

and repulsion to and repulsion to

become and become and

attraction.attraction.

GeneratorsGenerators

Same basic Same basic

principle as motorsprinciple as motors

AC AC -- rotate a wire rotate a wire

loop between loop between

magnetsmagnets

–– FaradayFaraday

How do we How do we

transmit? transmit?

Electrical TransmissionElectrical Transmission

TransformersTransformers

–– Cause change in Cause change in

voltagevoltage

–– windingswindings

–– iron coreiron core

Use to decrease Use to decrease

currentcurrent

–– Decreases IDecreases I22R lossR loss

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TransformersTransformers

For a transformer:For a transformer:

–– VV22 = (N= (N22 / N/ N11) x V) x V11

NN11 = number of windings in primary coil= number of windings in primary coil

NN22 = number of windings in secondary coil= number of windings in secondary coil

VV11 = voltage at primary coil= voltage at primary coil

VV22 = voltage at secondary coil= voltage at secondary coil

Will not work with dcWill not work with dc

ElectronicsElectronics

Emission and control of electronsEmission and control of electrons

TelevisionTelevision

Diodes and TransistorsDiodes and Transistors

Integrated circuitsIntegrated circuits

ElectromagnetismElectromagnetism

Learning GoalsLearning Goals

–– Identify some electromagnetic Identify some electromagnetic

interactions and applications.interactions and applications.

––Distinguish between Distinguish between motorsmotors and and

generatorsgenerators..

–– Explain the principle and use of Explain the principle and use of

transformers.transformers.

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ElectromagnetismElectromagnetism

Questions: 19Questions: 19--2424

Exercises: 25, 27Exercises: 25, 27

Key Terms; Matching, Multiple Key Terms; Matching, Multiple

Choice, and FillChoice, and Fill--inin--thethe--Blank Blank

Questions; Visual Connection and Questions; Visual Connection and

Applying your KnowledgeApplying your Knowledge

General Physical General Physical

ScienceScience

Chapter 8Chapter 8

Electricity and MagnetismElectricity and Magnetism