Electricity & Magnetism Lecture 18 - SFU.ca Lecture 18 - Induction.pdf · Electricity &...

Electricity & MagnetismLecture 18

Today’sConcepts: A)Induc4on

B)RLCircuits

Electricity&Magne/smLecture18,Slide1

ExtendeddeadlinefornextfewFlipItPhysicshomework:! 80%extendedbyoneweek.

SomeupcomingFlipItcheckpointsincludeques4onsonLabMaterials

ClassschedulenowincludesTiplerchapterreferences

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

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

Comments

what'sthedifferencebetweenintegralbdotdaandintegralbdotds.Whendoweuseeachone! B•DAisforGauss’law! B•dsisforAmpere’slaw

Howisenergystoredinamagne4cfield,similartocapacitorsstoringenergyasanelectricfield?! Itjustis.Bothelectricfieldsandmagne4cfieldsstoreenergylikeaspringstoresenergywhencompressed

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,

Wrapawireintoacoiltomakean“inductor”…

dIdt

ε = −L

From the Prelecture: Self Inductance

Electricity&Magne/smLecture18,Slide2

currentI

L

dIdt

εL = −L

emfinducedacrossLtriestokeepIconstant.

Inductorspreventdiscon4nuouscurrentchanges!

It’slikeiner4a!

What this really means:

Electricity&Magne/smLecture18,Slide3

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

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

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

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

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

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

R

I

V = IRdIdtV = L L

+

−

−

+

Why is there Exponential Behavior?

VL

Electricity&Magne/smLecture18,Slide10

where

Prelecture:

RL

I

Lecture:

VLVBATT

Didwemessup?

No:Theresistanceissimplytwiceasbiginonecase.

Electricity&Magne/smLecture18,Slide11

ARerlong/meat0,movedto1 ARerlong/meat0,movedto2

A]erswitchmoved,whichcasehaslarger4meconstant?

A)Case1B)Case2C)Thesame

CheckPoint 8

Electricity&Magne/smLecture18,Slide12

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

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

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

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

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

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

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

Follow Up

Theswitchinthecircuitshownhasbeenclosedforalong4me.

WhatisI2,thecurrentthroughR2?

(Posi4vevaluesindicatecurrentflowstotheright)

R1

LV

R2

R3

A)B)C)D)

Electricity&Magne/smLecture18,Slide20

Theswitchinthecircuitshownhasbeenclosedforalong4meatwhichpoint,theswitchisopened.

WhatisI2,thecurrentthroughR2immediatelya]er

switchisopened?(Posi4vevaluesindicatecurrentflowstotheright)

Follow Up 2R1

LV

R2

R3

I2

Electricity&Magne/smLecture18,Slide21

A)B)C)D)E)