Chapters 25.4 and 26 to 26.3How current flowsSlide 3A simple circuitEMF DevicesBatteriesInternal ResistanceSlide 8Terminal Voltage (Effective Voltage)Loop RuleFrom an electrical perspectiveResistance ruleEMF RulePutting these ideas into practice i.e changing a circuit into an equationResistors in SeriesReducing Networks, If You Can, then DO SO!How to find a potential differenceOne last word on internal resistanceJunction RuleResistors in ParallelResistor Color CodesGuidelines for Problem Solving1Chapters 25.4 and 26 to 26.32How current flows1 2 3x3EMF—Electromotive ForceAny chemical, solar, mechanical, heat method of creating a potential differenceBatteriesAlternator, dynamoSolarcell, photovoltaicThermocoupleSymbol EUnits: volts4A simple circuiti5EMF DevicesIdeal EMF device– no internal resistanceReal EMF device– some internal resistance6BatteriesSeveral Different Types of Batteries (called cells)Wet Cell (left)Car BatteriesHigh Current AppsDry Cells (Zn-Cu paste)Dry Cell RatingsAAA 20 mAAA 25 mAC 80 mAD 150 mAAkaline CellsAAA 200 mAAA 300 mAC 500 mAD 600 mAEMF=1.5 VCells are constant voltage sources: 1.5 3 6 9 12 15 24 48 VPbO2PbO2PbO2PbPbPbH2SO4PbSO4e-e-PbSO47Internal ResistanceThe battery itself can have some resistance to current flowCould be terminalsCould be plates or pasteCould be combinationPbO2PbO2PbO2PbPbPbH2SO48Internal ResistanceWe treat the internal resistance as if an external resistor had been added to the circuit just ahead of the positive terminal9Terminal Voltage (Effective Voltage)EMF RvavbVab=Va-Vb=EMF-iR10Loop RuleThe algebraic sum of changes in potential encountered in a complete traversal of the circuit must be zero.AKA Kirchoff’s Loop RuleConsiderNashvillePanama CityDr. WombleTotal Elevation Change = 011From an electrical perspective12Resistance ruleFor a move through a resistor in the direction of current, the change in potential is –iRIf the move opposes the current then the change in potential is +iR.imoveVaVbVa-Vb= -iRVa-Vb= +iR13EMF RuleFor a move from the negative terminal to the positive terminal then the change in potential is +EMFFor a move from “+” to “-” then the change in potential is -EMFmove+EMF-EMF14Putting these ideas into practice i.e changing a circuit into an equationR=65 EMF=5 V1. Pick a direction for the current.2. Pick a direction of circuit traversal3. Sum the potentials as you traverse the circuitiMy moveXFrom X, +iR+EMF=0EMF=-iR5=-65ii=-0.076 A or -76 mAThe negative sign means that we guessed the wrong direction for the current.15Resistors in SeriesConnected resistances are said to be in series when a potential difference that is applied across their combination is the sum of the resulting potential differences across all the resistances.eqRiEMF R1R2R3EE-iR1-iR2-iR3=0ReqE321RRRiEMFSo Req=R1+R2+R316Reducing Networks, If You Can, then DO SO!Always try to reduce the total number of variables by using the equivalent resistance.For N resistors in series, the equivalent resistance is Req=R1+R2+….+RN17How to find a potential differenceTo find the potential difference between any two points1. Start at one point2. Traverse the circuit following any path3. Add algebraically the changes in potentialR1R2R3EPoint APoint BBlue—Va –iR3-iR2=VbVa-Vb=i(R3+R2)iRed—Va-E+iR1=VbVa-Vb=E-iR118One last word on internal resistanceRecall Power, P=ViErE-ir=0Va+ir-E=VbVa-Vb=E-irP=iV <-(Va-Vb)P=i(E-ir)P=Ei-i2rXXabiPower of EMF DeviceThermal dissipation (losses)19Junction RuleSum of all currents entering a junction must equal the sum of all currents leaving the junction.i1i3i2i1i3i2i1i3i2i1+i3=i2i1+i2=i3i1+i2+i3=0IN = OUT20Resistors in ParallelConnected resistances are said to be in parallel when a potential difference that is applied across their combination results in that same potential difference across each resistance.ER1R2R3ReqEEi1i2i3iii=i1+i2+i332132132111111111RRRRRRRRRVRVRVRVeqeqeq21Resistor Color Codes Color CodesColor Value ToleranceBlack 0Brown 1 1%Red 2 2%Orange 3 3%Yellow 4 4%Green 5Blue 6Violet 7Gray 8White 9Gold 5%Silver 10%No Color 20%1 2 3 TBand1*10+Band2 x 10^ Band 3+/- Band T22Guidelines for Problem Solving1. Replace network of resistors with their equivalents (if possible)2. If you can’t simplify to a single loop, then use the junction rule and the loop rule to set up a series of equations. Be sure to:1. Pick a direction of current (sign is a mathematical convention)1. If you traverse a resistor against the current then +iR else –iR2. If you traverse an EMF source from low potential to high potential then the EMF is positive, else negative.3. You have the following arbitrary choices1. Directions of currents2. Which loops to use3. Direction of traversal of each loop4. Starting point and ending point4. ABOVE ALL, REMEMBER:1. TRAVERSE THE LOOP COMPLETELY2. ONCE YOU HAVE CHOSEN A DIRECTION OF THE CURRENT YOU MUST STICK WITH THIS DIRECTION UNTIL YOU HAVE FINISHED THE