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Magnetism
Rocksthatcoulda-ractorpusheachotherwerefoundinancient5mesneartheGreekregionofMagnesia
Proper5esofmagne5teorealsoknowntoancientChinese PeterPeregrinusOnthemagnet(1269) WilliamGilbertDeMagnete(1600) HansOersteddemonstratesmagne5smisrelatedtoelectricity(1820)
JamesMaxwelldevelopselectromagne5ctheory
PublishesATrea'seonElectricityandMagne'smin1873
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Magnetic,ieldpatterns
Magne5cfieldsareproducedby Electriccurrentsflowingthroughconductors(electromagnets) Permanentmagnets
Thedirec5onofamagne5cfieldisthedirec5onoftheforceonanorthpoleifplacedinthefield
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Magnetic,ieldoftheEarth
Tiltedby11.degreesfromdirec5onofspinaxis
Magne5cfieldoftheEarthvariesoverlong5mescales3
Notethatthemagne5cpole
oftheearthclosetothe
geographicNorthPoleis
actuallyasouthmagne5c
pole!
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Magnetic,ieldoftheEarth
CausedbyelectriccurrentsinsidethemoltenironcoreoftheEarth
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Magnetic,ieldsfromcurrent-
carryingwires Currentflowingthroughawireproducesamagne5cfield Strongercurrentsproducestrongermagne5cfields
Direc5onofthefieldlinesdependsondirec5onofcurrentflow(userighthandcorkscrewrule)
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Solenoid
Asolenoidisalongcylindricalcoil Whencurrentflowsthroughwire,themagne5cfieldinsidethesolenoidisuniform
Solenoidbehaveslikeabarmagnet Thestrengthofthemagne5cfieldcanbeincreasedbyincreasingthecurrent
Direc5onofthefieldgivenbyrighthandrule
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Magnetic,ieldstrength
Magne5cfieldstrength(=fluxdensity)B Theforceonacurrentcarryingwireperpendiculartothefieldlinesofauniformmagne5cfieldis
Propor5onaltothecurrentI Propor5onaltothelengthofthewireL
Defini&on:Themagne5cfieldstrengthBistheforceac5ngperunitcurrentinawireofunitlengthwhenthewireis
perpendiculartothemagne5cfieldlines
Unit:Tesla(T)=Nm1A17
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Magnetic,ieldstrength
B=magne5cfieldstrength(T) F=forceonawireperpendiculartothefieldlines(N) I=current(A) L=lengthofwire(m)
B =F
IL
B
I
L
F= BIL
F
Flemings
Le.-hand
rule
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Magnetic,ieldstrength
B=magne5cfieldstrength(T) F=forceonawireperpendiculartothefieldlines(N) I=current(A) L=lengthofwire(m)
B =F
IL
I
F= BILsin
B
I
F= 0
B
I
L
F= BIL
B
l
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Rememberthesepoints
1. Whenacurrentcarryingconduc5ngwireisturnedinauniformmagne5cfieldoffluxdensityBun5lnoforceis
ac5ngonit,thenthewirepointsinthedirec5onofB
2. Whenastraightconduc5ngwireoflengthLcarryingacurrentIisplacedperpendiculartoauniformfieldandaforceFactsonthewire,thenthemagnitudeBofthe
magne5cfieldstrength(fluxdensity)isdefinedas
3.Bisavector.Soitscomponentinadirec5onatanangletoBisBcos()
4. Thedirec5onoftheforceac5ngonthewirecanbefoundbyusingFlemingsLehandrule
B =F
IL
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S
Motors
N S N
I
F
F
X
W Z
Y
d
Torqueoncoil(90):C= BANI11
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S
Motors
N S N
I
Torqueoncoil(90): Torqueoncoil():
C= BANI
C= BANIcos()
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Chargedparticlesinmagnetic
,ields Chargesmovingthroughamagne5cfieldexperienceaforceduetothepresenceofthefield
F= BvQ
+Q
B
v
F
F= BvQsin
+Q v
FB
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Fieldinasolenoid
Themagne5cfieldofasolenoidcarryingacurrentis Propor5onaltothecurrent Propor5onaltothenumberofturnspermeter
Magne5cfieldstrengthgivenby 0=4x107Hm1(magne5cfieldconstant) Unitof0:Henrypermeter
B =0
IN
l
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Fieldnearalongstraightwire
Howdoesthefieldstrengthvarywithdistancefromthe
wire?
Magne5cfieldstrength Propor5onaltoI Propor5onalto1/r
Amperederived
B =
B = 0I2r
1
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De,initionoftheAmpere
WecannowdefinetheAmpere Twocurrentcarryingwiresplacedinparallelwill:
A-ractifthecurrentsflowinthesamedirec5on Repelisthecurrentsflowinoppositedirec5ons
Magne5cfieldstrengthproducedbywireYatX:
ForceonlengthLofwireX:
B =
0Iy
2d
F= BIxL =
0IyIxL
2d
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De,initionoftheAmpere
WecannowdefinetheAmpere Twocurrentcarryingwiresplacedinparallelwill:
A-ractifthecurrentsflowinthesamedirec5on Repelifthecurrentsflowinoppositedirec5ons
OneAmpereisdefinedasthecurrentintwoinfinitelylongstraightwires1mapartinavacuumwhichproducesaforceof
2x107N/moneachwire.
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ToDo
Readchapter14fromthebook[p.27p.272Magne5cfields] Donotreadsec5on14.6
Assignmentwk3:Prac&calthisWednesday! Where?Physicslabs(G.O.Jones2ndfloor) When?Wednesday23rdbetween1pm(makesureyougoattheright5me)
Handitinnolaterthan4:00pmnextWednesday(30thofJan)-LATEWORKWILLNOTBEACCEPTED
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