Last time…ResistorsKirchoff’s junction lawQuick QuizWork and energy in a circuitLight bulbs and powerTwo different bulbsResistors in SeriesSlide 9Resistors in ParallelSlide 11QuestionPowerPoint PresentationMore complicated circuitsSlide 15Electrical measurementsKirchoff’s loop lawResistor-capacitor circuitRC dischargeTue. Oct. 14, 2008 Physics 208 Lecture 13 1Last time…Combining capacitorsBegin circuitsResistor circuitsStart resistor-capacitor circuitsToday…Tue. Oct. 14, 2008 Physics 208 Lecture 13 2ResistorsSchematic layoutCircuitsPhysical layoutTue. Oct. 14, 2008 Physics 208 Lecture 13 3Kirchoff’s junction lawCharge conservationIinIoutIout = IinI1I2I3I1=I2+I3I2I3I1I1+I2=I3Tue. Oct. 14, 2008 Physics 208 Lecture 13 4Quick QuizWhich bulb is brighter?A. AB. BC. Both the sameCurrent through each must be sameConservation of current (Kirchoff’s current law)Charge that goes in must come outIIIITue. Oct. 14, 2008 Physics 208 Lecture 13 5Work and energy in a circuitWhat happens as charge q moves?Uqc=qVcUqd=qVd Ohm’s law: Charge has lost potential energyEnergy lost Elost = qIREnergy dissipated in resistor as Heat (& light in bulb)Power dissipated in resistor = € ⇒ ΔUq= q Vd−Vc( )€ Vd−Vc( )= −IR ⇒€ dElostdt=dqdtIR = I2RJoules / s = WattsTue. Oct. 14, 2008 Physics 208 Lecture 13 6Light bulbs and powerHousehold voltage is 120V Cost24 hours on requires MG&E ~ 13¢ / kWatt-hour 60 Watt€ 60W = 60J /s = I2R = I IR( )= VI€ I = 60W /120V = 0.5A€ 60J /s( )24hour( )3600s /hour( )= 5,184,000J€ 1kW − ho u r = 1000J /s( )3600s/hour( )= 3,600,000J€ R = V /I = 120V /0.5A = 240Ω19¢ / dayTue. Oct. 14, 2008 Physics 208 Lecture 13 7Two different bulbsCurrent same through eachPower dissipated differentBrightness differentR1R2abcdeIIII€ P1= I2R1€ P2= I2R2Tue. Oct. 14, 2008 Physics 208 Lecture 13 8Resistors in SeriesI1 = I2 = IPotentials add V =V1 + V2 = IR1 + IR2 = = I (R1+R2)The equivalent resistance Req = R1+R2 RR=2R2 resistors in series:R LLike summing lengths€ R = ρLATue. Oct. 14, 2008 Physics 208 Lecture 13 9Quick QuizWhat happens to the brightness of the bulb B when the switch is closed?A. Gets dimmerB. Gets brighterC. Stays sameD. Something else Battery is constant voltage,V across bulb B doesn’t changeso I through bulb B doesn’t changeTue. Oct. 14, 2008 Physics 208 Lecture 13 10Resistors in ParallelV = V1 = V2I = I 1 + I 2 (lower resistance path has higher current)Equivalent ResistanceR/2 RRAdd areas€ R = ρLATue. Oct. 14, 2008 Physics 208 Lecture 13 11Quick QuizWhat happens to the brightness of the bulb A when the switch is closed?A. Gets dimmerB. Gets brighterC. Stays sameD. Something elseTue. Oct. 14, 2008 Physics 208 Lecture 13 12QuestionAs more and more resistors are added to the parallel resistor circuit shown here the total current flowing I… ….R1R2R3R4IA. Increases if each Ri getting biggerB. Increases if each Ri getting smallerC. Always increasesD. Always decreasesE. Stays the sameEach resistor added adds V/Ri to the total current ITue. Oct. 14, 2008 Physics 208 Lecture 13 13You use one power strip to plug in your toaster, coffee pot, microwave. Toaster Coffee Pot Microwave10 A 5 A 12 AEverything works great until you plug in your space heater, then you smell smoke. This is becauseQuestionA. The resistance of the circuit is too highB. The voltage in the circuit is too highC. The current in the circuit is too highTue. Oct. 14, 2008 Physics 208 Lecture 13 14More complicated circuitsBoth series & parallelDetermine equivalent resistanceReplace combinations with equivalent resistanceTue. Oct. 14, 2008 Physics 208 Lecture 13 15Quick QuizThe circuit below contains three 100W light bulbs. The emf = 110 V. Which light bulb(s) is(are) brightest ?A. AB. BC. CD. B and CE. All three are equally bright.Tue. Oct. 14, 2008 Physics 208 Lecture 13 16Electrical measurementsA multimeter can measure currents (as an ammeter), potential difference (as a voltmeter)Electrical measuring devices must have minimal impact in the circuitRVVoltmeterThe internal resistance of the ammeter must be very smallI = IA= V+VA = RI + rAI RIfor rA 0VAARAmmeterIIAVThe internal resistance of the voltmeter must be very largeI = Iv+IR VV = VVIVIRI€ I =εrV+εRrV→∞ ⏐ → ⏐ ⏐εRTue. Oct. 14, 2008 Physics 208 Lecture 13 17Kirchoff’s loop lawConservation of energyR1R2R3I1I2I3Tue. Oct. 14, 2008 Physics 208 Lecture 13 18Resistor-capacitor circuitWhy does QC (charge on capacitor) depend on time after switch is closed?R increases: does Qc change faster, slower, same?C increases: does Qc change faster, slower, same?Qc decreases: does Qc change faster, slower, same?Tue. Oct. 14, 2008 Physics 208 Lecture 13 19RC dischargeRC time constanttime t€ τ
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