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)R1R2R3I1I2I3Kirchoff’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 circuitWhat 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 capacitorKirchoff’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 dischargeRC time constant€ τ =RC€ Q = Qoe−t /τ€ I = Ioe−t /τThur. Oct. 16, 2008 Physics 208 Lecture 14 6Charging a capacitorAgain 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 capacitorsCell membrane:‘Empty space’ separating charged fluids (conductors)~ 7 - 8 nm thickIn 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 dipolesMagnetic charges (monopoles) have never been observed.Magnetic dipole characterized by dipole momentTorque 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 15MagnetsClearly magnets interact with each otherSometimes attracting, sometimes repellingBut 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 PoleAre inseparableThur. Oct. 16, 2008 Physics 208 Lecture 14 17Magnetic fieldSimilar in spirit to electric fieldExerts torque on a mangetic dipoleMagnetic field exerts a torque on compass needle Aligns it with magnetic field lines.Magnetic field lines indicate direction of local magnetic fieldField lines leave magnet at N poleenter 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 LinesSimilaritiesDensity gives strengthArrow gives directionLeave +, NorthEnter -, SouthDifferencesStart/Stop on electric chargeNo 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 magnetsNorth Pole and South PoleThis is the elementarymagnetic particleCalled 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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