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Physics (9PH01) LECTURE NOTES
PHYSICS 9PH01)
LECTURE NOTES
Unit 4: Physics on the Move 6PH04/01)
TOPIC 2 Electric and Magnetic field
2.1 Electric Fields
2.2 Capacitors
2.3 Electromagnetic Effects
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Physics (9PH01) Unit 4: Physics on the Move
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Unit 4: TOPIC 2 Electric and Magnetic Field
2.1 Electric Fields2.1.1 Pushing Charges
1. Anelectricfieldcanbeusedtoaccelerateachargeparticle.
2. InX-raymachine,X-rayisproducedwhenthetungstentarget(anode)isbombardedbyelectronsathighspeed.
3. Averyhighelectricfieldisappliedonelectronsinordertoacceleratetheelectronstomoveathighspeed.
Figure 2.1 X-ray [source a)]
Electrical FieldDefinition
4. Regionwherethereareelectricalforcesactingoncharges
Electrical Field StrengthDefinition
5. Electrical field strength, E atapointinanelectricfieldisdefinedaselectricalforceofattractionexertedbytheelectricfieldperunitcharge.
= E - 2.1)
where − F Electrostaticforce
−q Charge
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Physics (9PH01) Unit 4: Physics on the Move
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AccordingtoNewton’sSecondLawofmotion,
ma F =
Electrostaticforce,
qE F =
Hence,theaccelerationoftheelectron,
=a - 2.2)
where − E Electricalfieldstrength−q Chargeofelectron
2.1.2 Uniform and radial fieldsUniform Electric Field
6. Theelectricalfieldstrengthinequalatallthepointthelinesbetweentwoplatesareparallelandequallyspaced.
Figure 2.2 Uniform electric field [Source: e)]
Electric potential difference.
Electrical Potential
7. Electrical potential,V atapointinanelectricfieldisdefinedaselectricalpotentialenergyperunitcharge.
=V - 2.3)where −W Electrostaticenergy
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Physics (9PH01) Unit 4: Physics on the Move
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8. Aswehavelearninunit2,workdonetobringcharge,Q fromonepointtoanother
pointwherethepotentialbetweentwopoint,V :
QV W =
9. Electrical potential for radial electric field,
=V - 2.4)
where −r distance
Page 41, question
Answer:
Page 41, question
Answer
:
Page 41, question
Answer:
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Physics (9PH01) Unit 4: Physics on the Move
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Equipotential
10. Linejoiningpointsofequalpotentialenergyperunitcharge.
Figure 2.3 Equipotential curves and electric field lines for a) a constant E field, b) a
point charge, and c) an electric dipole. [source e)]
Potential Gradient
11. Rateatwhichpotentialdifferencechangeswithdistance.12. Asthedistance(frompositiveplate)increases,thepotentialdecreasesuniformly.
Figure 2.4 Potential-distance graph
13. Workdonetomoveacharge,Q underpotentialdifferenceofV isequaltothe
electricalforce, F appliedonthechargetomovetoadistanceof x .
gradient xV
Q F
QV Fx
==⇒
=
14. Thepotentialgradientisequaltotheelectricalfieldstrength.
UnitofQ
F E = is 1−NC andunitsof
x
V is 1−Vm .Theyarethesame.
0
Potential,V
Distance, x
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Physics (9PH01) Unit 4: Physics on the Move
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Page 41, question
Answer:
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Physics (9PH01) Unit 4: Physics on the Move
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Practice Questions
Solutions
Page 43, question
Answer:
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Physics (9PH01) Unit 4: Physics on the Move
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Page 43, question
Answer:
2.1.3 Coulomb’s Law15. Considerasystemoftwopointcharges, 1Q and 2Q ,separatedbyadistance r in
vacuum.Theforceexertedby 2Q on 1Q isgivenbyCoulomb'slaw:
Theforcebetweentwochargesisdirectlyproportionaltotheproductoftheirchargeandinverselyproportionaltothesquareofthedistancebetweenthem.
2
21
r
QQ F α
- 2.6)
where − F Electrostaticforce
−21,QQ Charges
−r distancebetween_____________−k constant
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Physics (9PH01) Unit 4: Physics on the Move
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22
4
1 −== CmN
o
k
πε
where −oε ________________________, 1121085.8 −−
× mF .
= F - 2.7)
Page 46, question
Answer:
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Physics (9PH01) Unit 4: Physics on the Move
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2.2 Capacitors
2.2.1 Storing Charge
16. Acapacitorcanbechargedbyconnectingabatteryacrossit.
Figure 2.5 Charging a Capacitor [source b)]
17. ElectronsarerepelledbythenegativeterminalofthebatterytoplateYandelectrons
onplateXareattractedtothepositiveterminalofthebattery.
18. Duringthechargingprocess,therewillbeamomentarycurrentastheelectronsflow.
Thereadingwillreturntozerowhenvoltagebetweenthecapacitorthebatteryisthesame.
Capacitance Definition
19. Capacitor
20. Acapacitorisconsistedoftwoconductiveplatesseparatedbyaninsulatorcalledas
dielectric.
21. Symbol of a capacitor
Figure 2.6 Symbol of a) unpolarized capacitor b) polarized capacitor
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Physics (9PH01) Unit 4: Physics on the Move
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22. Capacitance
=C - 2.8)
where −Q Charge
−V potentialdifference
Page 53, question
Answer:
Energy stored on a charged capacitor
23. Duringchargingprocess,thepotentialdifferencebetweentwoplatesincreasesaccordingtotheamountofchargestored.
C QV =
24. Hence,thepotentialdifferenceagainstchargestoredgraphasbelow:
Figure 2.9 Energy stored in a charged capacitor
25. Energystoredincapacitor, E =Areaundergraph
2
1= E - 2.11)
0
Potential
difference,
Charge, Q
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Physics (9PH01) Unit 4: Physics on the Move
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OR
2
1= E
- 2.12)
Page 53, question
Answer:
2.2.2 Charging and discharging capacitorsGrowth and decay curves
Discharging capacitor maths
26. Whenaconductorisconnectedacrossachargedcapacitor,therewillbeflowofelectronsfromplateYtoplateX.Thecurrentwillstopwhenthevoltageacrossthetwoplatesiszero.
Figure 2.10 Discharging a Capacitor [source e)]
Exponential decay27. Charginganddischargingofcapacitorfollowtheexponentialdecay.
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Physics (9PH01) Unit 4: Physics on the Move
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Charging of Capacitor28. Forcharging,thecurrentaftertime t ,
RC t
oe I I −
=
where −o I initialcurrent R
E I o =
− R resistance−C capacitance
29. Voltageaftertime t ,
)1( RC
t
e E V
−
−=
where − E electromotiveforce
30. Electric chargestoredincapacitoraftertime t
)1( RC
t
o eQQ−
−=
where CE Qo =
Graphs
Figure 2.11 Graph of charging a Capacitor
Discharging the Capacitor31. Fordischarging,thecurrentaftertime t ,
RC
t
oe I I −
= - 2.13)
where −o I initialcurrent
− R resistance−C capacitance
I
t0
V
t0
Q
t0
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Physics (9PH01) Unit 4: Physics on the Move
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32. Voltageaftertime t ,
RC
t
EeV
−
= - 2.14)
where − E electromotiveforce
33. Electric chargestoredincapacitoraftertime t
RC
t
oeQQ−
= - 2.15)
where CE Qo =
Graphs
Figure 2.12 Graph of discharging a Capacitor
Time Constant,
34. Definition
Timetakenfor
=τ - 2.16)
where − R resistance
−C capacitance
Practice Questions
I
t0
V
t0
Q
t0
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Physics (9PH01) Unit 4: Physics on the Move
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Solutions,
Page 53, question
Answer:
Page 53, question
Answer:
Page 59, question
Answer:
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Page 59, question
Answer:
Page 59, question
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Physics (9PH01) Unit 4: Physics on the Move
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Answer:
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Physics (9PH01) Unit 4: Physics on the Move
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2.3 Electromagnetic Effects
2.3.1 Magnetic Fields
Pushing Poles
Permanent magnets.Magnetic field
35. Regionwheremagneticforcesareexperienced.
36. Likepolesrepelbutunlikepolesattract.
37. Themagneticfieldcanberepresentedbymagneticfieldlines.
38. Figure 2.13 showsthedirectionsmagneticfieldlinesareclearlyshownusingcompass.
Figure 2.13 Magnetic Field [source e)]
Fleming’s Left Hand Rules
Figure 2.14 Fleming’s Left-hand Rule [source f)]
39. Fleming’s Left-hand Rule helpsinidentifythedirectionofthemagneticforce(output)wherethedirectionofthemagneticfieldofpermanentmagnetandthedirectionof
3) THUMB – THRUST (Output), F
1) First Finger – Field, B (Point to South)
2) SeCond Finger – Electric Current, I
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electriccurrentareidentifiedbeforehand.Figure 5.29 showsthemethodstoapplyFleming’sLeft-handRule.
40. Usingyourlefthand–firstfingershowsdirectionofmagneticfield,secondfinger
currentandthumbthethrust(offorce)ontheconductor.Page 63, question
Page 63, question
Answer
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Physics (9PH01) Unit 4: Physics on the Move
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Practice Questions
Solutions ,
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Physics (9PH01) Unit 4: Physics on the Move
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The Strength of Electromagnetic Force
Magnetic flux density (B-field).
Definition41. Forceperunitlengthperunitcurrentonacurrent-carryingconductoratrightanglestothefieldlines. - 2.17)
where − B magneticfieldstrength − F forceonthewireatrightangletofieldlines − I current −l lengthofwire
42. S.IunitofB istesla,T.
Notes:(a) BIl F =
(b) I B F ⊥⊥
43. Inmoregeneral,theforceproducedbycurrentandmagneticfieldonawire,
- 2.18)where −θ anglebetweenthecurrentandmagneticfield.
Figure 2.15 Force on a charged particle [source a)]
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Physics (9PH01) Unit 4: Physics on the Move
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44. Theforce, F actingonachargedparticle, q inamagneticfieldwithstrength, B and
movingwithvelocity, v
- 2.19)
where − F forceonthechargedparticle− B magneticfieldstrength
−q chargeoftheparticle
−v velocityoftheparticle
Practice Questions
Solutions ,
Page 67, question
Answer
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Physics (9PH01) Unit 4: Physics on the Move
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Page 67, question
Answer
Mass Spectrometer
45. Massspectrometeristheequipmentusedtoidentifyunknownchemicals.
Figure 2.16 Spectrometer [source a)]
46. Thechemicalischargedwhenenterthemachineandforceactsonthecharged
particleisamagneticfieldwhichisperpendiculartothedirectionmotionoftheparticle.
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Physics (9PH01) Unit 4: Physics on the Move
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47. Thisforcealsomakestheparticlemovesincircularmotion.Therefore,
Bqv F = , ⇒= r
mv F
2
r
mv Bqv
2
=
Thecharge/massratio =m
q - 2.20)
48. Thekineticenergyoftheparticleisgainedbyapplyingpotentialdifference, V across
theparticle.
qV mv =2
2
1,
m
qV v
2=
=r - 2.21)
where −r radius−V potentialdifferenceapplied
−q chargeoftheparticle
−m massoftheparticle − B magneticfieldstrength
Ideas:
Page 67, question
[Given V=3000V,B=3T,e=1.6x10-19C,mp=mn=1.67x10-27kg]
Answer
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Physics (9PH01) Unit 4: Physics on the Move
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2.3.2 Generating ElectricityElectromagnetic Induction
Phenomenon of Electromagnetic Induction
49. Wheneverthereisarelativemotionbetweenawireandamagnet,ane.m.f.willbeinducedinthewire. Flux linkage
Magnetic flux, flux linkage.Magnetic flux density
50. Ameasureofthestrengthofamagneticfield.
51. Tesla,T:unitofmagneticfluxdensity,where1Tproducesaforceof1N
52. Magneticfluxφ througharegionisameasureofthenumberofmagneticfieldlinethrougharegion.
- 2.22)
where − B magneticfluxdensity
− area
53. S.I.UnitofmagneticfluxisWeber(Wb).
Figure 2.17 Magnetic flux of single loop [source e)]
Page 71, question
Answer
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Physics (9PH01) Unit 4: Physics on the Move
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Magnetic flux linkage54. Aproductofmagneticfieldandthenumberofturns.
55. Fluxlinkageisdefinedastheproductofthenumberofturns, N andthefluxthroughthecoil,
- 2.23)
Lenz’s Law
Definition
56. Thedirectionofaninducedemfissuchas
Figure 2.17 Lenz Law [source e)]
Calculating induced emfs
Faraday’s and Lenz’s laws of electromagnetic induction.Faraday’s Law
57. Magnitudeofinducede.m.finacircuitisdirectlyproportionaltotherateofchangeofmagneticfluxlinkagethroughthatcircuit.
- 2.24)
where −ε Inducedemf
−Φ∆ )( N Changeinfluxlinkage
−∆t Timetaken
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Page 71, question
Answer
Page 71, question
Answer
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Reference:
(a) EdexcelA2Physics,MilesHudson,Pearson(b) Fields,ForcesandSynthesis(Revisededition),MarkEllse&ChrisHoneywill.
Publisher:nelsonthornes(c) AdvancedPhysicsforYou,KeithJohnson,SimmoneHewett,SueHort,John
Miller.Publisher:nelsonthornes(d) IGCSEPhysics,TomDuncan,HeatherKennett,Publisher:HodderMurray(e) NotesfromMITOpenCourseware(http://ocw.mit.edu/)(f) Internet