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Electriccharge

Electricchargeisapropertyofma.erthatcausesthepar2clesorobjectsthatpossessittoexperienceaforcewhen

nearotherobjectsorpar2clesthatalsopossessit

Theelectricchargeisthesourceofelectricforce(canbea.rac2veorrepulsive)

Conductors:materialsthroughwhichelectronscanfloweasily Insulators:materialsthatopposethefreeflowofelectrons Electronaffinity:theamountofenergyreleasedwhenanelectronisaddedtoaneutralatomtoformanega2veion

ConductorsandInsulators

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Electriccurrent

Theflowofelectricchargethroughaconduc2ngmaterialiscalledtheelectriccurrent

Forcurrenttoflow,weneedapoten,aldifference Electronsflowfromthenega2veterminaloftheba.erytotheposi2veterminal(-to+)

Conven2onalcurrentflowstheotherwayround(+to-)

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DriJvelocity

ConductorlengthL

Cross-sec2onal

areaAe-

e-e-

e-

e-e-

Direc2onofflowofcurrent

L = vt

V= LA = vtA

Ne = nV= nvtA

Q = qNe = qnvtA

I= Qt= qnvtA

t= qnvA

Lengthoftheconductor

Volumeoftheconductor

Numberofelectronscrossing

Totalcharge

Current

TheCoulombistheunitofelectriccharge

TheAmpereistheunitofelectriccurrent I=Q

t

3

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Potentialdifference(Voltage)

Defni2on:Thepoten2aldifferenceVbetweentwopointsinacircuitistheelectricalenergyconvertedintootherformsof

energywhen1Coulombofchargepassesfromonepointto

theother

W=electricalenergydelivered=workdone Q=Chargepassed V=poten2aldifference(alsocalledvoltage) Measuredinoules/Coulomb=Volt

V=W

Q

4

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Electricalpowerande.m.f.

V=poten2aldifference I=current Q=It TotalenergyE=ItV ElectricalpowerP=energydeliveredpersecond=IV(Wa.s)

Theelectromo2veforce(emf)E=W/Qofasourceisdefinedasthe

electricalenergyproducedperunitchargeinsidethesource

Batteriesconnectedinseriesandparallel

1V1V

4V

1V

1V

1V

Inseries:

sumupemfs

Inparallel:

emfstaysthesame5

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OhmsLaw

OhmsLaw:Underconstantphysicalcondi2ons,theresistanceV/IisaconstantindependentofVorIandtheirdirec2ons

I=

V

R

V = IR R =V

I

or or

P = IV P = I2R P =

V2

Roror

R =L

A

R=resistance

L=lengthofwire

A=cross-sec2onalareaofwire

=resis2vityofthematerial Unitofresis2vity:Ohmmeter(m)

6

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ResistorsinSeries

Resistorsinanelectricalcircuitmaybearrangedisseries

Samecurrentthroughallresistors,differentpd(voltage)acrosseachresistor

Totalpoten2aldifferenceV=V1+V2+V3 Individualpoten2aldifferences,directlypropor2onaltoresistancesV1=IR1V2=IR2V3=IR3

TotalresistanceR=R1+R2+R3

R1 R R3

I

V1 V V3

V

7

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ResistorsinParallel

Resistorsinanelectricalcircuitmaybearrangedisseries

Samepdacrossallresistors,differentcurrentthrougheachresistor

TotalcurrentI=I1+I2+I3 Individualcurrent,inverselypropor2onaltoresistancesI1=V/R1I2=V/R2I3=V/R3

Totalresistance1/R=1/R1+1/R2+1/R3

R1

R

R3

V

I I

I1

I

I3

8

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InternalResistance

Powersourceshaveinternalresistances Someoftheenergyprovidedisconvertedtoheatinsidethesource

+ -

E

R

I

r

I=E

(R+ r)

E= IR+Ir

V = E Ir9

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ToDo

Fromchapterreadsec2ons1,,3andfirsthalfof4(uptoKirchhoffslaws)[p444-p457electricity]

HomeworkAssignmentwk1:ques2ons22.1,22.2,22.4,22.9,22.10

Handitinnolaterthan4:00pmnextWednesday(16Jan)-LATEWORKWILLNOTBEACCEPTED

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