Electricity & Magnetism Lecture 18 - SFU
Transcript of Electricity & Magnetism Lecture 18 - SFU
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Electricity & MagnetismLecture 18
Today’sConcepts: A)Induc4on
B)RLCircuits
Electricity&Magne/smLecture18,Slide1
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ExtendeddeadlinefornextfewFlipItPhysicshomework:! 80%extendedbyoneweek.
SomeupcomingFlipItcheckpointsincludeques4onsonLabMaterials
ClassschedulenowincludesTiplerchapterreferences
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Tipler ConcordancePhysics 141 Spring 2017
Date Unit # Topics FlipItPhysics Unit Due Date
Wed, Jan 4 19 Electrostatics 01/11/2017 21-1,21-2
Fri, Jan 6 19 Coulomb's Law Coulomb's Law 21-3
Mon, Jan 9 19 Electric Fields Electric Fields
Wed, Jan 11 20 Electric Field Lines and Flux Electric Flux and Field Lines 01/18/2017 21-4,21-5
Fri, Jan 13 20 Gauss' Law Gauss' Law 22-1,22-2
Mon, Jan 16 20 Gauss' Law Calculations
Wed, Jan 18 21Electric Potential Energy 01/23/2017 22-3,
Fri, Jan 20 21 Electric Potential Electric Potential 23-1,23-2,23-3
Mon, Jan 23 22 Current and Resistance Electric Current 01/29/2016 23-4.23-5,23-6
Wed, Jan 25 22 Series and Parallel Circuits 24-1,24-2,24-3
Fri, Jan 27 23Kirchhoff's Rules 02/01/2017 25-1,25-2,25-3
Mon, Jan 30 23 Kirchhoff's Laws Conductors and Capacitors 25-4,25-4
Wed, Feb 1 24 Capacitance Capacitors 24-3
Fri, Feb 3 Midterm 1 25-6
Mon, Feb 6 24 RC Circuits RC Circuits 26-1,26-2
Wed, Feb 8 25 Magnetic Forces and Fields Magnetism 02/20/2017
Fri, Feb 10 25 Forces and Torques on Currents 26-3, (26-4)
Mon, Feb 13 Break 27-1,27-2
Wed, Feb 15 Break 27-4
Fri, Feb 17 Break 28-1,28-2,28-3,28-4
Mon, Feb 20 26s1 Magnetism from Electricity Ampere's Law, Biot-Savart Law 03/10/2017
Wed, Feb 22 26s2 Faraday's Law Motional EMF 28-6.28-7,28-8
Fri, Feb 24 27s1 Faraday's Law 03/10/2017 29-1,29-2
Mon, Feb 27 26s3 Induction and RL Circuits
Wed, Mar 1 27s2 AC Circuits LRC Circuits 29-3,29-4
Fri, Mar 3 Practice AC Circuits 29-5.29-6
Mon, Mar 6 Practice AC Circuits: Resonance and Power
Wed, Mar 8 practical exam 30-1,30-2
Fri, Mar 10 practical exam 30-3,30-4
Mon, Mar 13 28 Electromagnetic Waves Displacement Current and E-M Wa 03/20/2017 31-4,
Wed, Mar 15 28 Properties of Electromagnetic Waves
Fri, Mar 17 28 Polarization Polarization 31-1,31-2,31-3,31-5
Mon, Mar 20 29 Reflection and Images Reflection and Refraction 03/22/2017 32-1,32-2
Wed, Mar 22 30 Lenses, Mirrors 04/03/2017 32-4
Fri, Mar 24 Midterm 2
Mon, Mar 27 30 Optical Instruments
Wed, Mar 29 31 Interference 04/03/2017 33-1,33-2
Fri, Mar 31 31 Interference & Diffraction 33-3.33-4.33-8
Mon, Apr 3 32 Intro to Quantum Mechanics 04/07/2017
Wed, Apr 5 32 More Quantum 36-1,36-2
Fri, Apr 7 32 Review 36-3.36-4
Mon, Apr 10 Final Exam 8:30=11:30
Tipler 6th
Electric & Gravitational
Forces
Error Propagation, Digital
Meters
Magnetic Forces and Electric
Current
AC Circuits and the
OscilloscopeVerification of Faraday's Law
and Maxwell's Equations
Energy Transport and
Polarization
Refraction, Prisms and
Lenses
Lenses and Mirrors
Combination
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Stuff you said..
Willtherebemorelabac4vi4esinvolvingoscilloscopes?Unlikelabac4vi4esinvolvingmagnets,theoscilloscopesareforeign,new,andcuriouslyinteres4ngtoplay/learnwithduetotheabove?
WhataretheexactdifferencesbetweentheCapacitorandInductor.Inwhichsitua4onwouldIusetheCapacitorinsteadoftheInductororviceversa?
http://www.sfu.ca/phys/141/1154/Schedule141.pdf
Capacitors are yin — Inductors are yang
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Comments
what'sthedifferencebetweenintegralbdotdaandintegralbdotds.Whendoweuseeachone! B•DAisforGauss’law! B•dsisforAmpere’slaw
Howisenergystoredinamagne4cfield,similartocapacitorsstoringenergyasanelectricfield?! Itjustis.Bothelectricfieldsandmagne4cfieldsstoreenergylikeaspringstoresenergywhencompressed
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E = ��B
dt= �NA
dBdt
Today’s Experiment
Electricity&Magne/smLecture18,Slide2
Loop area A
E = ��B
dt= �A
dBdt
If there are more turns N in the loop,
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Wrapawireintoacoiltomakean“inductor”…
dIdt
ε = −L
From the Prelecture: Self Inductance
Electricity&Magne/smLecture18,Slide2
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currentI
L
dIdt
εL = −L
emfinducedacrossLtriestokeepIconstant.
Inductorspreventdiscon4nuouscurrentchanges!
It’slikeiner4a!
What this really means:
Electricity&Magne/smLecture18,Slide3
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Twosolenoidsaremadewiththesamecrosssec/onalareaandtotalnumberofturns.InductorBistwiceaslongasinductorA
ComparetheinductanceofthetwosolenoidsA)LA = 4 LB B)LA = 2 LB C)LA = LB D)LA = (1/2) LB E)LA = (1/4) LB
(1/2)2 2
CheckPoint 2
Electricity&Magne/smLecture18,Slide4
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RL
At t = 0:I = 0VL = VBATTVR = 0(L islikeagiantresistor)
VBATT
I = 0
At t >> L/R:VL = 0 VR = VBATTI = VBATT/R(Lislikeashortcircuit)
R
I = V/R
L
VBATT
Whennocurrentisflowingini4ally:
How to think about RL circuits Episode 1:
VL
Electricity&Magne/smLecture18,Slide5
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WhatisthecurrentIthroughthever/calresistorimmediatelyaRertheswitchisclosed?
(+isinthedirec/onofthearrow)
A)I = V/R B)I = V/2R C)I = 0 D)I = −V/2R E)I = −V/R
Inthecircuit,theswitchhasbeenopenforalong/me,andthecurrentiszeroeverywhere.
At/met=0theswitchisclosed.
IL = 0
CheckPoint 4
Electricity&Magne/smLecture18,Slide6
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WhatisthecurrentIthroughthever/calresistoraRertheswitchhasbeenclosedforalong/me?
(+isinthedirec/onofthearrow)
A)I=V/R B) I =V/2RC)I= 0D)I=−V/2R E)I= −V/R
RL Circuit (Long Time)
Electricity&Magne/smLecture18,Slide7
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RL
VBATT
I = V/R
At t >> L/R:I = 0VL = 0VR = 0
R
I = 0
LR
I = VBATT/RVR = IR VL = VR
At t = 0:
When steady current is flowing initially:
How to Think about RL Circuits Episode 2:
VL
Electricity&Magne/smLecture18,Slide8
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WhatisthecurrentIthroughthever/calresistorimmediatelyaRertheswitchisopened?
(+isinthedirec/onofthearrow)
A)I=V/RB)I=V/2RC)I=0 D)I= −V/2RE)I= −V/R
AReralong/me,theswitchisopened,abruptlydisconnec/ngthebaXeryfromthecircuit.
CheckPoint 6
Electricity&Magne/smLecture18,Slide9
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R
I
V = IRdIdtV = L L
+
−
−
+
Why is there Exponential Behavior?
VL
Electricity&Magne/smLecture18,Slide10
where
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Prelecture:
RL
I
Lecture:
VLVBATT
Didwemessup?
No:Theresistanceissimplytwiceasbiginonecase.
Electricity&Magne/smLecture18,Slide11
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ARerlong/meat0,movedto1 ARerlong/meat0,movedto2
A]erswitchmoved,whichcasehaslarger4meconstant?
A)Case1B)Case2C)Thesame
CheckPoint 8
Electricity&Magne/smLecture18,Slide12
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A)Case1B)Case2C)Thesame
CheckPoint 10ARerlong/meat0,movedto1 ARerlong/meat0,movedto2
Immediatelya]erswitchmoved,inwhichcaseisthevoltageacrosstheinductorlarger?
Electricity&Magne/smLecture18,Slide13
I0 is V/R in both casesA) VL(0) = I0*2RB) VL(0) = I0*3R
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A)Case1B)Case2C)Thesame
CheckPoint 12ARerlong/meat0,movedto1 ARerlong/meat0,movedto2
A]erswitchmovedforfinite4me,inwhichcaseisthe
currentthroughtheinductorlarger?
Electricity&Magne/smLecture18,Slide14
A) τ1 = L/2RB) τ2 = L/3R
VL
τ1τ2
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ConceptualAnalysisOnceswitchisclosed,currentswillflowthroughthis2-loopcircuit.KVRandKCRcanbeusedtodeterminecurrentsasafunc4onof4me.
StrategicAnalysisDeterminecurrentsimmediatelya]erswitchisclosed.Determinevoltageacrossinductorimmediatelya]erswitchisclosed.DeterminedIL/dtimmediatelya]erswitchisclosed.
R1
LV
R2
R3
Calculation
Theswitchinthecircuitshownhasbeenopenforalong4me.Att = 0,theswitchisclosed.
WhatisdIL/dt,the4merateofchangeofthecurrentthroughtheinductorimmediatelya]erswitchisclosed
Electricity&Magne/smLecture18,Slide15
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WhatisIL,thecurrentintheinductor,immediatelyaRertheswitchisclosed?
A)IL = V/R1 up B)IL = V/R1 down C)IL = 0
IL = 0
Calculation
Theswitchinthecircuitshownhasbeenopenforalong4me.Att = 0,theswitchisclosed.
R1
LV
R2
R3
Electricity&Magne/smLecture18,Slide16
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A)B)C)D)
Calculation
IL(t = 0 +) = 0
Theswitchinthecircuitshownhasbeenopenforalong4me.Att = 0,theswitchisclosed.
R1
LV
R2
R3
WhatisthemagnitudeofI2,thecurrentinR2,immediatelya]ertheswitchisclosed?
Electricity&Magne/smLecture18,Slide17
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IL(t = 0 +) = 0 I2(t = 0 +) = V/(R1+ R2+ R3)
Calculation
Theswitchinthecircuitshownhasbeenopenforalong4me.Att = 0,theswitchisclosed.
R1
LV
R2
R3
I2
A)B)C)D)E)
WhatisthemagnitudeofVL,thevoltageacrosstheinductor,immediatelya]ertheswitchisclosed?
Electricity&Magne/smLecture18,Slide18
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A)B)C)D)
VL(t = 0 +) = V(R2+ R3)/(R1+ R2+ R3)
Calculation
Theswitchinthecircuitshownhasbeenopenforalong4me.Att = 0,theswitchisclosed.
WhatisdIL/dt,the4merateofchangeofthecurrentthroughtheinductorimmediatelya]erswitchisclosed
R1
LV
R2
R3
Electricity&Magne/smLecture18,Slide19
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Follow Up
Theswitchinthecircuitshownhasbeenclosedforalong4me.
WhatisI2,thecurrentthroughR2?
(Posi4vevaluesindicatecurrentflowstotheright)
R1
LV
R2
R3
A)B)C)D)
Electricity&Magne/smLecture18,Slide20
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Theswitchinthecircuitshownhasbeenclosedforalong4meatwhichpoint,theswitchisopened.
WhatisI2,thecurrentthroughR2immediatelya]er
switchisopened?(Posi4vevaluesindicatecurrentflowstotheright)
Follow Up 2R1
LV
R2
R3
I2
Electricity&Magne/smLecture18,Slide21
A)B)C)D)E)