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ResistorColourCodes
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Meters
Movingcoilmetersmusttakeacurrenttogiveanon-zeroreading
(a)Ammetermeasurescurrent (b)Voltmetermeasuresvoltage
A V
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Ammetershuntresistance
FortheAmmeter,theshuntresistanceisgivenby
where
iisthefullscaledeflec4oncurrent Iisthecurrentthroughthecircuit
R
cistheresistanceofthemetercoil
Rsh=
iRC
(I i)
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Voltmetermultiplierresistance
FortheVoltmeter,themulplierresistanceisgivenby
where
iisthefullscaledeflec4oncurrent Visthevoltage R
Cistheresistanceofthemetercoil
Rs=
V
i R
c
4
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Potentiometer
Apotenometerisusedtomeasuree.m.fsandp.d.s
Consistsofresistor(s)andavoltmeter V
BC=kLwherek=constant
TodetermineEtestmeasurelengthofwirewhenvoltmetershowsnodeflecon(0reading)
+-
A
B
CE
L V
+-
A
B
CE
V+- Etest
+-
A
B
C
E
V+- Etest
II
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Measuringthee.m.fofa
batterywithavoltmeter
VoltmeterhasresistanceR Powercellhasinternalresistancer
Currentthroughcircuit:I=(e.m.f.)/(R+r)=E/(R+r)
P.dacrossvoltmeter:V=IR=ER/(R+r)
Measuredvalueforthevoltagealwayslessthanthee.m.fofthecell
+ -BaWery
r
V
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ConvertingAmmeters
AGalvanometerisatypeofsensiveAmmeter
Analogdevicesthatcanmeasurelowcurrents Noshuntresistor
Defleconofthepointercomesasaresponsetocurrentflowingthroughthecoil(RC)inamagnecfield
Toconvertagalvanometerintoanammeter,connectashuntresistorinparalleltothegalvanometer
Thevalueoftheshuntresistanceconnectedwilldetermineyournewrangeofdeflecon
AlowershuntresistorwillallowmoreofthecurrentthroughthatcomponentofthecircuitsolesscurrentwillpassthroughtheRc
oftheGalvanometer,therebyincreasingitsrange7
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ConvertingAmmeters
GalvanometerscanalsobeconvertedintoVoltmetersbyaddingalarge(mulplier)resistanceinserieswiththe
galvanometer
TheintroduconofthemulplierresistorintothecircuitINCREASEDthepotenaldifferencebetweenAandBbecause
thepotenaldropswhencurrentflowsthrougharesistor
GI
Vfsd=IRcRcG
IRcRs
A B
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Measuringthee.m.fofa
batterywithapotentiometer
ewanttomeasureEtest
Connectacellofknowne.m.fandfindthebalancepoint
Recordthebalancelength Getk(thep.d.perunitlength)
FlicktheswitchandmeasurethenewbalancelengthLBD
kisknownsosolveforEtest
Etest=kLBD=(Estand/LBC)LBD
+
-
A
B
CEdriver
+
-
Etest+
-
Estand
switchV
LBC
+
-
A
B
DEdriver
+
-
Etest+
-
Estand
switchV
LBD 9
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Measuringinternalresistance
Tomeasuretheinternalresistancerofthetestcell(Etest)
useasimilararrangement
GetEtestwiththepreviousmethod
Replacethestandardcellbyaknownresistance
Completethecircuitandmeasurenewbalancelength
ithRconnectedVacrossthetestcell=kLBK=(EtestR)/(R+r)
+
-
A
B
KEdriver
+
-
Etest
switchV
LBK Rr
kLBK
=
kLBDR
R+ r
r = R(L
BDL
BK)
LBK
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ToDo
Fromchapter22readsecon22.5 Fromchapter23readsecons23.1,23.2
HomeworkAssignmentwk3:quesons23.1,23.3,23.4,23.5,23.6
Handitinnolaterthan:00pmnextWednesday-LATEWORKWILLNOTBEACCEPTED
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