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UW-Madison PHYSICS 208 - Physics 208 Lecture 14 Notes

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Last time…Exam 2 is Tuesday Oct. 28Resistor-capacitor circuitDischarging the capacitorRC dischargeCharging a capacitorDischarging a capacitorQuestionSlide 9Human capacitorsCell Membrane RC circuitMagnetismMagnets and magnetic fieldsMagnetic dipolesMagnetsLet’s Break A Magnet!Magnetic fieldMagnetic Field LinesElectric vs Magnetic Field LinesMagnetism: Permanent magnetsThe Earth is a magnetThur. Oct. 16, 2008 Physics 208 Lecture 14 1I1I2I3I1=I2+I3Last time…Kirchoff’s junction lawEquivalent resistance(parallel, series)R1R2R3I1I2I3Kirchoff’s loop lawThur. Oct. 16, 2008 Physics 208 Lecture 14 2Exam 2 is Tuesday Oct. 28Students w / scheduled academic conflict please stay after class Tues. Oct. 21 to arrange alternate time.5:30-7 pm, 2103 Ch (here)Covers: all material since exam 2.Bring: CalculatorOne (double-sided) 8 1/2 x 11 note sheetExam review: Thursday, Oct. 23, in classThur. Oct. 16, 2008 Physics 208 Lecture 14 3Resistor-capacitor circuitWhat happens to charges on the capacitor after switch is closed?Why does the charge on the capacitor change in time?Why does the charge flow through the resistor?Thur. Oct. 16, 2008 Physics 208 Lecture 14 4Discharging the capacitorKirchoff’s loop lawABCD€ VB−VA( )+ VD−VC( )= 0€ ΔVc= Qc/C€ −IR€ ⇒ I =QcRCCharges in the current I come from capacitor:€ I = −dQcdtThur. Oct. 16, 2008 Physics 208 Lecture 14 5RC dischargeRC time constant€ τ =RC€ Q = Qoe−t /τ€ I = Ioe−t /τThur. Oct. 16, 2008 Physics 208 Lecture 14 6Charging a capacitorAgain Kirchoff’s loop law:€ ε−IR − QC/C = 0Time t = 0: € Qc= 0 ⇒ I =ε/R€ ⇒ I =ε/R − QC/RCLooks like resistor & battery: uncharged cap acts like short circuitt increases: € Qc> 0 ⇒ I <ε/RVC increases, so VR decreasesTime t = : € VC=ε ⇒ VR= 0 ⇒ I = 0Fully charged capacitor acts like open circuitThur. Oct. 16, 2008 Physics 208 Lecture 14 7Discharging a capacitor€ Q = Qmax1− e−t /τ( )Thur. Oct. 16, 2008 Physics 208 Lecture 14 8QuestionThe circuit contains three identical light bulbs and a fully-charged capacitor. Which is brightest?QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.A. AB. BC. CD. A & BE. All equally brightThur. Oct. 16, 2008 Physics 208 Lecture 14 9QuestionThe circuit contains three identical light bulbs and an uncharged capacitor. Which is brightest?QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.A. AB. BC. CD. A & BE. All equally brightThur. Oct. 16, 2008 Physics 208 Lecture 14 10Human capacitorsCell membrane:‘Empty space’ separating charged fluids (conductors)~ 7 - 8 nm thickIn combination w/fluids, acts as parallel-plate capacitorCytoplasmExtracellular fluidPlasma membrane100 µmThur. Oct. 16, 2008 Physics 208 Lecture 14 11Cell Membrane RC circuitQuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.Nerve signal is an action potential that propagates down RC cell-membrane networkThur. Oct. 16, 2008 Physics 208 Lecture 14 12MagnetismQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Thur. Oct. 16, 2008 Physics 208 Lecture 14 13Magnets and magnetic fieldsQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Thur. Oct. 16, 2008 Physics 208 Lecture 14 14Magnetic dipolesMagnetic charges (monopoles) have never been observed.Magnetic dipole characterized by dipole momentTorque on magnetic dipole € r μ € r τ =r μ ×r B ( Compare electric dipole: ) € r τ =r p ×r E Torque tends to aligns magnetic dipole with magnetic fieldThur. Oct. 16, 2008 Physics 208 Lecture 14 15MagnetsClearly magnets interact with each otherSometimes attracting, sometimes repellingBut the magnetic particles are sort of a ‘composite’ positive and negative ‘magnetic charge’.Visualized as a bar with positive pole (North) at one end and negative pole (South) at other. These ‘magnetic charges’ cannot be broken apart — always appear in N-S pairs.NSThur. Oct. 16, 2008 Physics 208 Lecture 14 16Let’s Break A Magnet!North Pole and South PoleAre inseparableThur. Oct. 16, 2008 Physics 208 Lecture 14 17Magnetic fieldSimilar in spirit to electric fieldExerts torque on a mangetic dipoleMagnetic field exerts a torque on compass needle Aligns it with magnetic field lines.Magnetic field lines indicate direction of local magnetic fieldField lines leave magnet at N poleenter magnet at S poleNSThur. Oct. 16, 2008 Physics 208 Lecture 14 18Magnetic Field LinesNS+-•Magnetic Field Lines–Arrows give direction –Density gives strength–Looks like dipoleThur. Oct. 16, 2008 Physics 208 Lecture 14 19Electric vs Magnetic Field LinesSimilaritiesDensity gives strengthArrow gives directionLeave +, NorthEnter -, SouthDifferencesStart/Stop on electric chargeNo Magnetic Charge, lines are continuous!Convention for 3-D situations:x x x x x x x INTO Page••••••••••••• OUT of PageThur. Oct. 16, 2008 Physics 208 Lecture 14 20Magnetism: Permanent magnetsNorth Pole and South PoleThis is the elementarymagnetic particleCalled magnetic dipole(North poleand South pole)Poles interact with each other similar to charges.NSNSNSSNLikes repelOpposites attractThur. Oct. 16, 2008 Physics 208 Lecture 14 21The Earth is a magnetEarth is a bar magnet.North magnetic pole ~ at south geographic poleA compass is a bar magnetCompass needle aligns with local Earth


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UW-Madison PHYSICS 208 - Physics 208 Lecture 14 Notes

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