7/29/2019 Magnetic Fields - P2, week 3

1/18

Magnetism

Rocksthatcoulda-ractorpusheachotherwerefoundinancient5mesneartheGreekregionofMagnesia

Proper5esofmagne5teorealsoknowntoancientChinese PeterPeregrinusOnthemagnet(1269) WilliamGilbertDeMagnete(1600) HansOersteddemonstratesmagne5smisrelatedtoelectricity(1820)

JamesMaxwelldevelopselectromagne5ctheory

PublishesATrea'seonElectricityandMagne'smin1873

1

7/29/2019 Magnetic Fields - P2, week 3

2/18

Magnetic,ieldpatterns

Magne5cfieldsareproducedby Electriccurrentsflowingthroughconductors(electromagnets) Permanentmagnets

Thedirec5onofamagne5cfieldisthedirec5onoftheforceonanorthpoleifplacedinthefield

2

7/29/2019 Magnetic Fields - P2, week 3

3/18

Magnetic,ieldoftheEarth

Tiltedby11.degreesfromdirec5onofspinaxis

Magne5cfieldoftheEarthvariesoverlong5mescales3

Notethatthemagne5cpole

oftheearthclosetothe

geographicNorthPoleis

actuallyasouthmagne5c

pole!

7/29/2019 Magnetic Fields - P2, week 3

4/18

Magnetic,ieldoftheEarth

CausedbyelectriccurrentsinsidethemoltenironcoreoftheEarth

4

7/29/2019 Magnetic Fields - P2, week 3

5/18

Magnetic,ieldsfromcurrent-

carryingwires Currentflowingthroughawireproducesamagne5cfield Strongercurrentsproducestrongermagne5cfields

Direc5onofthefieldlinesdependsondirec5onofcurrentflow(userighthandcorkscrewrule)

7/29/2019 Magnetic Fields - P2, week 3

6/18

Solenoid

Asolenoidisalongcylindricalcoil Whencurrentflowsthroughwire,themagne5cfieldinsidethesolenoidisuniform

Solenoidbehaveslikeabarmagnet Thestrengthofthemagne5cfieldcanbeincreasedbyincreasingthecurrent

Direc5onofthefieldgivenbyrighthandrule

6

7/29/2019 Magnetic Fields - P2, week 3

7/18

Magnetic,ieldstrength

Magne5cfieldstrength(=fluxdensity)B Theforceonacurrentcarryingwireperpendiculartothefieldlinesofauniformmagne5cfieldis

Propor5onaltothecurrentI Propor5onaltothelengthofthewireL

Defini&on:Themagne5cfieldstrengthBistheforceac5ngperunitcurrentinawireofunitlengthwhenthewireis

perpendiculartothemagne5cfieldlines

Unit:Tesla(T)=Nm1A17

7/29/2019 Magnetic Fields - P2, week 3

8/18

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

8

7/29/2019 Magnetic Fields - P2, week 3

9/18

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

9

7/29/2019 Magnetic Fields - P2, week 3

10/18

Rememberthesepoints

1. Whenacurrentcarryingconduc5ngwireisturnedinauniformmagne5cfieldoffluxdensityBun5lnoforceis

ac5ngonit,thenthewirepointsinthedirec5onofB

2. Whenastraightconduc5ngwireoflengthLcarryingacurrentIisplacedperpendiculartoauniformfieldandaforceFactsonthewire,thenthemagnitudeBofthe

magne5cfieldstrength(fluxdensity)isdefinedas

3.Bisavector.Soitscomponentinadirec5onatanangletoBisBcos()

4. Thedirec5onoftheforceac5ngonthewirecanbefoundbyusingFlemingsLehandrule

B =F

IL

10

7/29/2019 Magnetic Fields - P2, week 3

11/18

S

Motors

N S N

I

F

F

X

W Z

Y

d

Torqueoncoil(90):C= BANI11

7/29/2019 Magnetic Fields - P2, week 3

12/18

S

Motors

N S N

I

Torqueoncoil(90): Torqueoncoil():

C= BANI

C= BANIcos()

12

7/29/2019 Magnetic Fields - P2, week 3

13/18

Chargedparticlesinmagnetic

,ields Chargesmovingthroughamagne5cfieldexperienceaforceduetothepresenceofthefield

F= BvQ

+Q

B

v

F

F= BvQsin

+Q v

FB

13

7/29/2019 Magnetic Fields - P2, week 3

14/18

Fieldinasolenoid

Themagne5cfieldofasolenoidcarryingacurrentis Propor5onaltothecurrent Propor5onaltothenumberofturnspermeter

Magne5cfieldstrengthgivenby 0=4x107Hm1(magne5cfieldconstant) Unitof0:Henrypermeter

B =0

IN

l

14

7/29/2019 Magnetic Fields - P2, week 3

15/18

Fieldnearalongstraightwire

Howdoesthefieldstrengthvarywithdistancefromthe

wire?

Magne5cfieldstrength Propor5onaltoI Propor5onalto1/r

Amperederived

B =

B = 0I2r

1

7/29/2019 Magnetic Fields - P2, week 3

16/18

De,initionoftheAmpere

WecannowdefinetheAmpere Twocurrentcarryingwiresplacedinparallelwill:

A-ractifthecurrentsflowinthesamedirec5on Repelisthecurrentsflowinoppositedirec5ons

Magne5cfieldstrengthproducedbywireYatX:

ForceonlengthLofwireX:

B =

0Iy

2d

F= BIxL =

0IyIxL

2d

16

7/29/2019 Magnetic Fields - P2, week 3

17/18

De,initionoftheAmpere

WecannowdefinetheAmpere Twocurrentcarryingwiresplacedinparallelwill:

A-ractifthecurrentsflowinthesamedirec5on Repelifthecurrentsflowinoppositedirec5ons

OneAmpereisdefinedasthecurrentintwoinfinitelylongstraightwires1mapartinavacuumwhichproducesaforceof

2x107N/moneachwire.

17

7/29/2019 Magnetic Fields - P2, week 3

18/18

ToDo

Readchapter14fromthebook[p.27p.272Magne5cfields] Donotreadsec5on14.6

Assignmentwk3:Prac&calthisWednesday! Where?Physicslabs(G.O.Jones2ndfloor) When?Wednesday23rdbetween1pm(makesureyougoattheright5me)

Handitinnolaterthan4:00pmnextWednesday(30thofJan)-LATEWORKWILLNOTBEACCEPTED

18