Chapter 5Chapter 5

Electricity and Electricity and MagnetismMagnetism

OutlineOutline• ChargeCharge

­ Defining­chargeDefining­charge­ Forces­on­charges:­Coulomb’s­lawForces­on­charges:­Coulomb’s­law

• Electricity­and­matterElectricity­and­matter• CurrentCurrent

­ Potential­differencePotential­difference­ Ohm’s­LawOhm’s­Law­ Electric­powerElectric­power

• MagnetismMagnetism­ Magnetic­forcesMagnetic­forces­ Magnetic­fieldMagnetic­field­ The­connection­between­electricity­and­The­connection­between­electricity­and­magnetic­fieldsmagnetic­fields

• ApplicationsApplications

ChargeCharge

•We­have­been­discussing­a­basic­We­have­been­discussing­a­basic­property­of­matter­called­massproperty­of­matter­called­mass

• In­electricity,­the­basic­concept­is­called­In­electricity,­the­basic­concept­is­called­chargecharge

• Classical­mechanics­doesn’t­explain­what­Classical­mechanics­doesn’t­explain­what­mass­is,­just­how­it­behaves:­how­it­mass­is,­just­how­it­behaves:­how­it­moves,­etc.moves,­etc.

• Similarly,­classical­E­&­M­(electricity­Similarly,­classical­E­&­M­(electricity­and­magnetism)­does­not­tell­us­what­and­magnetism)­does­not­tell­us­what­charge­is­exactly,­only­how­it­behavescharge­is­exactly,­only­how­it­behaves

Positive and Negative Positive and Negative ChargesCharges

•Ben­Franklin­(1706-1790)­named­Ben­Franklin­(1706-1790)­named­the­two­kinds­of­charges:­Positive­the­two­kinds­of­charges:­Positive­and­Negativeand­Negative

•All­charge­is­either­positive­or­All­charge­is­either­positive­or­negativenegative

•Like­charges­repel­each­otherLike­charges­repel­each­other

oror•Opposite­charges­attract­each­Opposite­charges­attract­each­otherother

-- --++ ++

--++

Net ChargeNet Charge

• Net­Charge:­the­total­amount­of­Net­Charge:­the­total­amount­of­charge­on­an­objectcharge­on­an­object­ For­most­objects­the­net­charge­=­zeroFor­most­objects­the­net­charge­=­zero

• ““Charged”­meansCharged”­means­ Having­more­of­one­kind­of­charge­(­+­Having­more­of­one­kind­of­charge­(­+­or­-­)­or­-­)­

­ Moving­around­electronsMoving­around­electrons• Charges­(i.e.,­electrons)­can­be­Charges­(i.e.,­electrons)­can­be­moved­around­moved­around­­ By­frictionBy­friction­ By­inductionBy­induction

Friction and InductionFriction and Induction

• FrictionFriction­ Charges­from­one­Charges­from­one­object­are­object­are­transferred­to­transferred­to­anotheranother

• InductionInduction­ A­charged­object­A­charged­object­induces­a­charge­induces­a­charge­separation­in­a­separation­in­a­second­objectsecond­object

Conservation of ChargeConservation of Charge

• The­net­charge­in­an­isolated­system­The­net­charge­in­an­isolated­system­can­not­changecan­not­change­ This­means­that­the­sum­of­the­negative­This­means­that­the­sum­of­the­negative­and­positive­charges­in­any­system­and­positive­charges­in­any­system­remains­constantremains­constant

• One­can­move­charges­around,­but­One­can­move­charges­around,­but­cannot­create­or­destroy­chargecannot­create­or­destroy­charge

Defining ChargeDefining Charge

• All­matter­is­composed­of­atomsAll­matter­is­composed­of­atoms

• All­atoms­are­composed­of­three­All­atoms­are­composed­of­three­elementary­particleselementary­particles­ Protons­(mass­=­1.673­x­10Protons­(mass­=­1.673­x­10-27­-27­kg,­kg,­charge­=­1.06­x­10charge­=­1.06­x­10-19­-19­Coulombs)Coulombs)

­ Electrons­(mass­=­9.11­x­10Electrons­(mass­=­9.11­x­10-31­-31­kg,­kg,­charge­=­-1.06­x­10charge­=­-1.06­x­10-19­-19­Coulombs)Coulombs)

­ NeutronNeutrons s (mass­=­1.675­x­10(mass­=­1.675­x­10-27­-27­kg,­kg,­charge­=­0charge­=­0­­Coulombs)Coulombs)

• 1­Coulomb­=­6.25­×­101­Coulomb­=­6.25­×­1018­18­electrons­electrons­

Coulomb’s LawCoulomb’s Law

• Force­between­two­point­Force­between­two­point­chargescharges­ The­force­exerted­by­one­charged­The­force­exerted­by­one­charged­object­on­another­charged­object­is­object­on­another­charged­object­is­proportional­to­the­charge­of­each­proportional­to­the­charge­of­each­object­and­to­the­inverse­of­the­object­and­to­the­inverse­of­the­(distance)(distance)22­between­the­objects­between­the­objects

­ the­force­can­be­the­force­can­be­ Attractive­-­oppositely­charged­objectsAttractive­-­oppositely­charged­objects Repulsive­-­similarly­charges­objectsRepulsive­-­similarly­charges­objects

­ ““k”­is­called­“coulomb’s­constant”,­k”­is­called­“coulomb’s­constant”,­(k­=­9x10(k­=­9x1099­Nm­Nm22/C/C22))

€

F=kq1 × q2

r2

Another Inverse Square Another Inverse Square LawLaw

• Remember­we­have­already­seen­the­Remember­we­have­already­seen­the­inverse­square­distance­dependence­inverse­square­distance­dependence­in­the­universal­law­of­gravityin­the­universal­law­of­gravity

• In­many­ways,­the­force­of­In­many­ways,­the­force­of­electrostatic­interaction­is­very­electrostatic­interaction­is­very­similar­to­the­force­of­gravitational­similar­to­the­force­of­gravitational­interaction,­but­there­are­some­interaction,­but­there­are­some­major­differencesmajor­differences

€

Fe =kq1 × q2

r2

Electrostatic Electrostatic InteractionsInteractions

• The­force­due­to­electrostatic­interaction­is­The­force­due­to­electrostatic­interaction­is­much­larger­than­the­force­due­to­gravitymuch­larger­than­the­force­due­to­gravity­ The­ratio­of­the­electric­and­gravitational­The­ratio­of­the­electric­and­gravitational­forces­between­the­electron­and­proton­in­a­forces­between­the­electron­and­proton­in­a­hydrogen­atom­is­Fhydrogen­atom­is­Felecelec/F/Fgravgrav­=­2.2­×10­=­2.2­×103939

• So­why­don’t­we­notice­electrical­forces­So­why­don’t­we­notice­electrical­forces­more?more?­ Masses­can­not­cancel­each­other­while­Masses­can­not­cancel­each­other­while­charges­can­(1­g­+­1­g­=­2­g,­1­C­+­-1­C­=­0­C)charges­can­(1­g­+­1­g­=­2­g,­1­C­+­-1­C­=­0­C)

• The­force­due­to­gravity­can­only­be­The­force­due­to­gravity­can­only­be­attractiveattractive

• Electrostatic­forces­can­be­repulsive­or­Electrostatic­forces­can­be­repulsive­or­attractiveattractive

Material MattersMaterial Matters

• For­some­materials,­once­an­imbalance­For­some­materials,­once­an­imbalance­of­charge­is­produced,­the­charge­“sticks­of­charge­is­produced,­the­charge­“sticks­around”around”­ These­materials­are­called­electrical­These­materials­are­called­electrical­insulatorsinsulators

• For­other­materials,­the­charge­For­other­materials,­the­charge­imbalance­is­very­difficult­to­achieve---it­imbalance­is­very­difficult­to­achieve---it­seems­as­thought­the­charge­just­passes­seems­as­thought­the­charge­just­passes­through­the­material­(e.g.­metals)through­the­material­(e.g.­metals)­ These­materials­are­called­electrical­These­materials­are­called­electrical­conductorsconductors

Levels of ConductivityLevels of Conductivity

• ConductorConductor­ Material­through­which­electrons­can­easily­Material­through­which­electrons­can­easily­movemove

• InsulatorInsulator­ Material­in­which­electrons­remain­fixed­in­Material­in­which­electrons­remain­fixed­in­one­location­under­normal­circumstancesone­location­under­normal­circumstances

• But­not­all­materials­can­be­distinctly­But­not­all­materials­can­be­distinctly­characterized­by­these­simple­definitionscharacterized­by­these­simple­definitions­ Semiconductors­–­Have­conductivity­between­Semiconductors­–­Have­conductivity­between­conductor­and­insulator­-­used­in­transistorsconductor­and­insulator­-­used­in­transistors

­ Superconductors­–­Electrons­flow­through­Superconductors­–­Electrons­flow­through­these­materials­with­no­resistancethese­materials­with­no­resistance

IonsIons• The­conduction­of­electricity­through­liquids­The­conduction­of­electricity­through­liquids­and­gases­involves­the­movement­of­ionsand­gases­involves­the­movement­of­ions

• Whenever­an­atom­gains­or­loses­an­electron,­Whenever­an­atom­gains­or­loses­an­electron,­we­say­that­the­atom­is­ionizedwe­say­that­the­atom­is­ionized

• An­ion­is­simply­an­atom­with­excess­chargeAn­ion­is­simply­an­atom­with­excess­charge­ The­ion­can­be­positive­or­negativeThe­ion­can­be­positive­or­negative

• Ionization­can­be­accomplished­in­several­Ionization­can­be­accomplished­in­several­waysways­ SparksSparks­ FlameFlame­ UV­and­X-raysUV­and­X-rays

Electric PotentialElectric Potential

• Whenever­a­potential­difference­Whenever­a­potential­difference­exists,­any­charge­will­move­to­the­exists,­any­charge­will­move­to­the­position­with­the­lowest­potentialposition­with­the­lowest­potential

• Differences­in­potential­are­caused­Differences­in­potential­are­caused­by­differences­in­the­net­value­of­by­differences­in­the­net­value­of­electric­field­that­varies­from­point­electric­field­that­varies­from­point­to­point­in­spaceto­point­in­space

Electric PotentialElectric Potential

• Unit:­Volt­­Unit:­Volt­­=­Joule­/­=­Joule­/­CoulombCoulomb

=­kg­m=­kg­m22/s/s22CC€

Electric Potential =

Potential Energy

Charge

Voltage of BatteriesVoltage of Batteries• ­­Just­as­every­liter­of­water­can­do­8­times­more­Just­as­every­liter­of­water­can­do­8­times­more­work­if­dropped­from­8­times­as­high,­every­work­if­dropped­from­8­times­as­high,­every­Coulomb­does­8­times­more­work­if­the­voltage­is­8­Coulomb­does­8­times­more­work­if­the­voltage­is­8­times­as­greattimes­as­great

• ­­The­12V­battery­will­push­8­times­more­electrons­The­12V­battery­will­push­8­times­more­electrons­through­the­same­circuit­in­a­given­time.­through­the­same­circuit­in­a­given­time.­

12m12m

1.5m1.5m

12V12V

1.5V1.5V

Electric CurrentElectric Current

• ­­When­charge­flows­from­one­place­to­When­charge­flows­from­one­place­to­another,­an­electric­current­is­formedanother,­an­electric­current­is­formed

• ­­Most­practical­applications­of­electrical­Most­practical­applications­of­electrical­phenomena­are­based­on­the­phenomena­are­based­on­the­understanding­of­electric­currentunderstanding­of­electric­current

• ­­Unit­of­current:­Ampere­=­Coulomb­per­Unit­of­current:­Ampere­=­Coulomb­per­secondsecond

• ­­Some­rules­for­the­flow­of­charge:Some­rules­for­the­flow­of­charge:­ ­­Current­can­only­exist­between­two­points­of­Current­can­only­exist­between­two­points­of­different­potential­connected­by­a­conductordifferent­potential­connected­by­a­conductor

­ ­­Current­can­only­exist­whenever­a­conductive­Current­can­only­exist­whenever­a­conductive­path­with­no­breaks­is­providedpath­with­no­breaks­is­provided

AC/DCAC/DC

• AC:­Alternating­Current­(wall­AC:­Alternating­Current­(wall­socket)socket)

• DC:­Direct­Current­(battery)DC:­Direct­Current­(battery)

• What’s­the­difference?­What’s­the­difference?­­ AC:­AC:­time­dependent­voltagetime­dependent­voltage

­ DC:­DC:­constant­voltageconstant­voltage

Current: “Flowing” Current: “Flowing” ChargeCharge

• Flow­of­charge­is­similar­to­flowing­waterFlow­of­charge­is­similar­to­flowing­water

­ If­the­faucet­is­closed,­the­flow­stopsIf­the­faucet­is­closed,­the­flow­stops­ If­the­pump­is­turned­off,­the­flow­stopsIf­the­pump­is­turned­off,­the­flow­stops

PUMPPUMP

FAUCETFAUCETHOSE OR FILTERHOSE OR FILTER

Electric Circuit ElementsElectric Circuit Elements

• Battery:­source­of­potential­differenceBattery:­source­of­potential­difference

• Resistor:­the­resistance­in­an­electric­Resistor:­the­resistance­in­an­electric­circuit­is­a­measure­of­how­“difficult”­circuit­is­a­measure­of­how­“difficult”­is­is­to­move­charge­through­the­circuitis­is­to­move­charge­through­the­circuit

SWITCHSWITCH(like the faucet)(like the faucet)

RESISTORRESISTOR(like crimps in the hose)(like crimps in the hose)

BATTERYBATTERY(like the pump)(like the pump)

Ohm’s LawOhm’s Law

• There­is­a­relationship­between­the­There­is­a­relationship­between­the­current,­voltage,­and­resistance­in­current,­voltage,­and­resistance­in­an­electric­circuitan­electric­circuit

• These­parameters­are­usually­These­parameters­are­usually­represented­by­the­letters:represented­by­the­letters:­ I­­currentI­­current­ V­­voltageV­­voltage­ R­­resistance­(unit­=­ohm,­symbol­=­R­­resistance­(unit­=­ohm,­symbol­=­))

• They­are­related­by­Ohm’s­law:They­are­related­by­Ohm’s­law:­ V­=­I­×­RV­=­I­×­R

Electric ShockElectric Shock

• Electric­shock­is­governed­by­Ohm’s­law­Electric­shock­is­governed­by­Ohm’s­law­­ Lowered­resistance­means­higher­currentLowered­resistance­means­higher­current­ High­current­is­deadly!High­current­is­deadly!

• A­“high­voltage”­does­not­directly­harm­youA­“high­voltage”­does­not­directly­harm­you

• Current­is­what­passes­through­objects­Current­is­what­passes­through­objects­when­charges­flow­toward­a­lower­potentialwhen­charges­flow­toward­a­lower­potential

• According­to­Ohm’s­law:According­to­Ohm’s­law:­ Low­resistance­with­high­voltage­means­large­Low­resistance­with­high­voltage­means­large­currentcurrent

Electric ShockElectric Shock• Typical­Resistance­of­the­Human­SkinTypical­Resistance­of­the­Human­Skin

­ Dry:­­as­much­as­500,000­Ohms­(500k)Dry:­­as­much­as­500,000­Ohms­(500k)­ Wet:­­as­little­as­100­Ohms­(if­wet­with­salt­Wet:­­as­little­as­100­Ohms­(if­wet­with­salt­water)water)

• Currents­in­the­human­bodyCurrents­in­the­human­body

0.001­Amp­-­you­feel­it0.001­Amp­-­you­feel­it0.005­Amp­-­you­feel­pain0.005­Amp­-­you­feel­pain0.010­Amp­-­muscle­spasm­with­the­0.010­Amp­-­muscle­spasm­with­the­

painpain0.015­Amp­-­loss­of­muscle­control0.015­Amp­-­loss­of­muscle­control0.070­Amp­-­serious­injury0.070­Amp­-­serious­injury

• A­current­of­only­0.070­(or­70­milli-Amps)­A­current­of­only­0.070­(or­70­milli-Amps)­passing­through­the­heart­muscle­for­more­passing­through­the­heart­muscle­for­more­than­one­second­can­cause­the­heart­to­stop­than­one­second­can­cause­the­heart­to­stop­beatingbeating

Electric ShockElectric Shock

• If­your­skin­is­dry,­then­R­­500,000­If­your­skin­is­dry,­then­R­­500,000­

• Since­V­=­120­V­for­most­outlets­in­Since­V­=­120­V­for­most­outlets­in­the­US,­Ohm’s­Law­predicts:the­US,­Ohm’s­Law­predicts:

I­=­V/R­=­120­V­­500,000­I­=­V/R­=­120­V­­500,000­

=­0.0002­Amps=­0.0002­Amps

Painful,­but­not­deadlyPainful,­but­not­deadly

Electric ShockElectric Shock

• If­your­skin­is­wet,­then­R­can­be­as­low­If­your­skin­is­wet,­then­R­can­be­as­low­asas1,000­1,000­

• So­for­a­V­=­120­V­outlet,­we­have:So­for­a­V­=­120­V­outlet,­we­have:

I­=­V/R­=­120­V­­1,000­I­=­V/R­=­120­V­­1,000­

=­0.12­Amps=­0.12­Amps

Which­can­be­fatal­if­the­current­Which­can­be­fatal­if­the­current­passes­through­the­heartpasses­through­the­heart

Electric PowerElectric Power• The­flow­of­current­is­the­source­of­The­flow­of­current­is­the­source­of­electric­energyelectric­energy­ Whenever­a­current­flows,­some­energy­Whenever­a­current­flows,­some­energy­is­being­transformed­into­heatis­being­transformed­into­heat

­ The­remainder­of­the­energy­can­be­The­remainder­of­the­energy­can­be­used­to­do­workused­to­do­work

• The­rate­at­which­electric­current­is­The­rate­at­which­electric­current­is­doing­work­is­called­electric­powerdoing­work­is­called­electric­power

• Electric­power­is­proportional­to­Electric­power­is­proportional­to­both­the­current­and­the­potential­both­the­current­and­the­potential­difference:difference:

€

Power = Current × Voltage = IV

Electric CircuitsElectric Circuits

• ­­The­amount­of­current­flowing­through­The­amount­of­current­flowing­through­the­circuit­paths­and­the­effective­the­circuit­paths­and­the­effective­resistance­of­the­circuit­depends­upon­resistance­of­the­circuit­depends­upon­how­the­circuit­elements­are­arrangedhow­the­circuit­elements­are­arranged

• ­­Most­circuits­are­arranged­in­two­Most­circuits­are­arranged­in­two­basic­ways:basic­ways:­ Series­(voltage­split­among­elements)Series­(voltage­split­among­elements)­ Parallel­(current­split­among­elements)Parallel­(current­split­among­elements)

• ­­The­amount­of­current­that­can­flow­in­The­amount­of­current­that­can­flow­in­a­circuit­depends­on­the­arrangement­a­circuit­depends­on­the­arrangement­of­the­resistances­in­the­circuitof­the­resistances­in­the­circuit

Series CircuitsSeries Circuits

• ­­Series­(voltage­split­Series­(voltage­split­among­elements)among­elements)

• ­­Resistance­is­Resistance­is­higher­than­for­a­higher­than­for­a­single­bulb,­so­single­bulb,­so­current­through­the­current­through­the­circuit­is­reducedcircuit­is­reduced

Parallel CircuitsParallel Circuits

• ­­Current­split­Current­split­among­elementsamong­elements

• ­­Because­the­Because­the­elements­in­a­elements­in­a­parallel­circuit­have­parallel­circuit­have­the­same­voltage­the­same­voltage­across­them,­the­across­them,­the­current­through­current­through­each­bulb­is­greater­each­bulb­is­greater­than­in­the­series­than­in­the­series­casecase

MagnetismMagnetism

• ­­At­one­point,­electricity­and­At­one­point,­electricity­and­magnetism­were­understood­to­be­magnetism­were­understood­to­be­completely­separate­in­naturecompletely­separate­in­nature

• ­­Scientists­discovered­in­the­nineteenth­Scientists­discovered­in­the­nineteenth­century­that­electrical­and­magnetic­century­that­electrical­and­magnetic­phenomena­are­actually­closely­relatedphenomena­are­actually­closely­related

• All­magnets­have­two­poles,­All­magnets­have­two­poles,­north­and­southnorth­and­south­ Like­poles­repelLike­poles­repel­ Opposite­poles­attractOpposite­poles­attract

Electric and Magnetic Electric and Magnetic ForcesForces

• ­­Both­electric­and­magnetic­forces­can­Both­electric­and­magnetic­forces­can­have­an­effect­on­objects­without­being­in­have­an­effect­on­objects­without­being­in­direct­contact­with­the­objects­(Similar­to­direct­contact­with­the­objects­(Similar­to­the­effect­of­gravity)the­effect­of­gravity)

• ­­We­explain­this­ability­to­do­work­‘at­a­We­explain­this­ability­to­do­work­‘at­a­distance’­with­the­concept­of­“force­fields”distance’­with­the­concept­of­“force­fields”

• ­­Anything­with­mass­fills­all­space­with­a­Anything­with­mass­fills­all­space­with­a­gravitational­fieldgravitational­field­ ­­It­is­the­field­that­causes­the­forces­on­any­It­is­the­field­that­causes­the­forces­on­any­mass­that­lies­in­the­fieldmass­that­lies­in­the­field

• ­­Similarly,­every­charged­particle­fills­all­Similarly,­every­charged­particle­fills­all­space­with­an­electric­fieldspace­with­an­electric­field­ ­­Whenever­another­charge­lies­in­the­electric­Whenever­another­charge­lies­in­the­electric­field,­electric­forces­act­on­itfield,­electric­forces­act­on­it

The Connection between The Connection between Electricity and Electricity and

MagnetismMagnetism• ­­Oersted­showed­that­Oersted­showed­that­electricity­and­magnetism­electricity­and­magnetism­were­inseparable­phenomenawere­inseparable­phenomena

• ­­He­showed­that­a­compass­He­showed­that­a­compass­could­be­used­to­detect­the­could­be­used­to­detect­the­presence­of­a­magnetic­fieldpresence­of­a­magnetic­field

• ­­Oersted­held­a­compass­near­Oersted­held­a­compass­near­a­wire­through­which­he­a­wire­through­which­he­passed­a­current,­and­found­passed­a­current,­and­found­that­the­compass­needle­was­that­the­compass­needle­was­deflected­by­the­current­in­the­deflected­by­the­current­in­the­wirewire

Direction of the Direction of the Magnetic FieldMagnetic Field

• A­simple­rule,­called­the­right­hand­A­simple­rule,­called­the­right­hand­rule­determines­the­direction­of­the­rule­determines­the­direction­of­the­magnetic­field­produced­by­a­wiremagnetic­field­produced­by­a­wire

The Current Loop The Current Loop Magnet Magnet

• Soon­scientists­understood­that­Soon­scientists­understood­that­current­loop­could­be­considered­as­current­loop­could­be­considered­as­a­simple­magneta­simple­magnet

Major Discovery - Major Discovery - InductionInduction

• All­moving­electric­All­moving­electric­charges­produce­charges­produce­magnetic­fieldsmagnetic­fields

• Important­Important­Application:Application:­ By­using­a­coil­of­By­using­a­coil­of­wire­and­high­wire­and­high­current,­strong­current,­strong­magnetic­field­can­magnetic­field­can­be­produced.­This­be­produced.­This­is­called­an­is­called­an­electromagnetelectromagnet

MagnetismMagnetism• ­­All­magnetic­phenomena­is­All­magnetic­phenomena­is­produced­by­moving­chargesproduced­by­moving­charges

• Currents­are­moving­chargesCurrents­are­moving­charges­ ­­So­a­magnet­exerts­force­on­a­So­a­magnet­exerts­force­on­a­current-carrying­wirecurrent-carrying­wire

­ ­­And­the­current­carrying­wire­And­the­current­carrying­wire­produces­a­magnetic­fieldproduces­a­magnetic­field

• Inside­materials­(e.g.­–­Inside­materials­(e.g.­–­permanent­magnets)permanent­magnets)­ ­­The­moving­charge­is­in­the­form­of­The­moving­charge­is­in­the­form­of­electrons­spinning­around­nucleuselectrons­spinning­around­nucleus

The Electric MotorThe Electric Motor

• Makes­use­of­the­force­on­a­current­Makes­use­of­the­force­on­a­current­carrying­wire­by­a­magnetic­fieldcarrying­wire­by­a­magnetic­field

• Important­component:­CommutatorImportant­component:­Commutator

The­commutator­The­commutator­changes­the­direction­changes­the­direction­of­the­current­in­the­of­the­current­in­the­loop­so­that­the­force­loop­so­that­the­force­continues­to­push­in­continues­to­push­in­the­same­directionthe­same­direction

GeneratorsGenerators

• The­basic­idea:­whenever­there­is­relative­The­basic­idea:­whenever­there­is­relative­motion­between­a­wire­loop­and­a­magnetic­motion­between­a­wire­loop­and­a­magnetic­field,­a­current­is­producedfield,­a­current­is­produced

• This­is­called­induced­currentThis­is­called­induced­current

• When­the­direction­of­the­current­is­changed,­When­the­direction­of­the­current­is­changed,­the­current­is­called­an­alternating­currentthe­current­is­called­an­alternating­current

TransformersTransformers

• The­idea­of­induced­current­led­to­the­The­idea­of­induced­current­led­to­the­development­of­transformersdevelopment­of­transformers­ When­current­is­alternating­in­a­loop­of­wire­When­current­is­alternating­in­a­loop­of­wire­(called­the­primary­loop),­a­secondary­current­(called­the­primary­loop),­a­secondary­current­can­be­induced­in­a­nearby­loop­of­wire­(the­can­be­induced­in­a­nearby­loop­of­wire­(the­secondary­loop)secondary­loop)

• Transformers­are­very­useful,­since­the­Transformers­are­very­useful,­since­the­voltage­of­the­secondary­induced­current­voltage­of­the­secondary­induced­current­can­be­controlled­by­the­number­of­turnscan­be­controlled­by­the­number­of­turns­ This­is­the­way­electric­This­is­the­way­electric­energy­(at­about­120­Volts)energy­(at­about­120­Volts)is­delivered­to­your­home­is­delivered­to­your­home­from­high­voltage­power­linesfrom­high­voltage­power­lines

Transformer MathTransformer Math

• ­­The­relationship­between­the­voltages­in­The­relationship­between­the­voltages­in­the­loops­of­a­transformer­is­given­bythe­loops­of­a­transformer­is­given­by

• ­­Large­transformers­can­decrease­the­Large­transformers­can­decrease­the­voltage­by­several­thousand­timesvoltage­by­several­thousand­times

1 1 2

2 2 1

primary turns primary voltage secondary current

secondary turns secondary voltage primary current

N V I

N V I

or for any two nearby loops

Applications of Applications of MagnetismMagnetism

•Magnetic­RecordingMagnetic­Recording­ Video­tapesVideo­tapes­ Audio­tapesAudio­tapes­ Computer­hard­drivesComputer­hard­drives

• CRT­Television­screensCRT­Television­screens­ Use­magnets­to­direct­the­electrons­that­Use­magnets­to­direct­the­electrons­that­provide­the­source­of­energy­that­makes­the­provide­the­source­of­energy­that­makes­the­light­you­viewlight­you­view

• NavigationNavigation­ Compasses­–­man­made­and­biological!Compasses­–­man­made­and­biological!

The CompassThe Compass

• The­earth­is­a­giant­magnetThe­earth­is­a­giant­magnet

• Useful­for­navigationUseful­for­navigation­ CompassesCompasses­ BirdsBirds­ BacteriaBacteria­ Cows!Cows!­ Humans?Humans?

Application of Application of ElectrostaticsElectrostatics

1.1. ChargingCharging­ The­surface­of­a­cylindrical­drum­The­surface­of­a­cylindrical­drum­

is­electrostatically­chargedis­electrostatically­charged2.2. ExposureExposure

­ Document­is­illuminated,­bright­Document­is­illuminated,­bright­areas­reflect­light­which­create­areas­reflect­light­which­create­charge­conduits­in­the­charge­conduits­in­the­photoconductor­photoconductor­

3.3. DevelopingDeveloping­ ­­Positively­charged­toner­sticks­to­Positively­charged­toner­sticks­to­

negatively­charged­areasnegatively­charged­areas4.4. TransferTransfer

­ ­­The­toner­on­the­drum­is­The­toner­on­the­drum­is­transferred­to­a­piece­of­paper­transferred­to­a­piece­of­paper­with­a­higher­negative­chargewith­a­higher­negative­charge

5.5. FusingFusing­ Toner­is­melted­and­bonded­to­the­Toner­is­melted­and­bonded­to­the­

paperpaper6.6. CleaningCleaning

E&M SummaryE&M Summary

• Charge­is­a­fundamental­property­of­Charge­is­a­fundamental­property­of­elementary­particleselementary­particles­ Two­types:­negative­and­positiveTwo­types:­negative­and­positive­ Unit:­Coulomb­(C)Unit:­Coulomb­(C)­ Fundamental­charge:­1.06Fundamental­charge:­1.06 1010-19­-19­C,­called­“C,­called­“ee””

• Atoms­are­composed­of­electrons,­protons­Atoms­are­composed­of­electrons,­protons­and­neutronsand­neutrons­ Electrons­have­Electrons­have­–e–e,­protons­have­,­protons­have­+e, neutrons +e, neutrons

are neutralare neutral­ An­atom­with­a­deficit­of­electrons­is­a­positive­An­atom­with­a­deficit­of­electrons­is­a­positive­ionion

• Current­is­the­flow­of­chargeCurrent­is­the­flow­of­charge­ Charge­flows­through­a­conductor,­not­Charge­flows­through­a­conductor,­not­through­an­insulatorthrough­an­insulator

E&M SummaryE&M Summary• Voltage­is­the­term­given­to­the­potential­Voltage­is­the­term­given­to­the­potential­difference­between­two­pointsdifference­between­two­points­ Voltage­Unit:­Volt­(V)Voltage­Unit:­Volt­(V)

• Relationship­between­current­and­voltage:­Relationship­between­current­and­voltage:­Ohm’s­Law:­Ohm’s­Law:­

V­=­IRV­=­IR (R­is­resistance)(R­is­resistance)• Power:­rate­of­doing­work;­P=IVPower:­rate­of­doing­work;­P=IV• Permanent­magnets­when­the­magnetic­Permanent­magnets­when­the­magnetic­field­can­be­lined­upfield­can­be­lined­up

• Electromagnetic­inductionElectromagnetic­induction­ Current­produced­when­there­is­relative­motion­Current­produced­when­there­is­relative­motion­between­a­current­loop­and­a­magnetic­fieldbetween­a­current­loop­and­a­magnetic­field

­ Basis­for­the­transformerBasis­for­the­transformer