Chapter 18 Direct Current Circuits DC current goes only in one direction Most intuitive similar to water flow The baby of Thomas Edison 18 1 Sources of EMF Most common batteries mostly chemical processes DC converted from AC adapters Not so obvious solar cells Historically they are called sources of electromotive force EMF clearly wrong because it is a source of voltage not force 18 2 Resistors in Series one resister Effectively adding lengths Req L1 L2 A Since R is proportional to L add resistance Req R1 R2 R 2R R Current and Voltage Resistors connected end to end If charge goes through one resistor it must go through other I1 I2 Ieq Both have voltage drops V1 V2 Veq R1 Req R2 18 3 Resistors in Parallel We know you will be going down the tube But look we have two tubes for you to choose from R R R 2 Both ends of resistors are connected Current is split between two wires I1 I2 Ieq Voltage is same across each V1 V2 V R1 R2 Req Christmas light Example How much power comes out of a 100 W light bulb Answer not enough information to determine Factory rating one battery and one device e g light bulb P IV P V2 R P I 2R R100 V2 P 1202 100 144 How about a 60 W light bulb R60 V2 P 1202 60 240 Which light bulb has the thicker filament Which light bulb takes more current P IV Example Example Mixed circuits For HW very tricky Try it when finding the power or current for a given resistor R1 R2 R3 Calculate current through each resistor R1 10 R2 20 R3 30 44 V R1 Simplify R2 and R3 are in parallel 1 R23 1 R2 1 R3 V23 V2 V3 I23 I2 I3 R23 12 R23 R123 Simplify R1 and R23 are in series R123 R1 R23 V123 V1 V23 I123 I1 I23 Ibattery R123 22 I123 44 V 22 2 A Power delivered by battery P IV 2 44 88W cont Calculate current through each resistor R1 10 R2 20 R3 30 44 V R123 R1 Expand R1 and R23 are in series R123 R1 R23 V123 V1 V23 I123 I1 I23 Ibattery I23 2 A V23 I23 R23 24 V Expand R2 and R3 are in parallel 1 R23 1 R2 1 R3 V23 V2 V3 I2 V2 R2 24 20 1 2A I23 I2 I3 I3 V3 R3 24 30 0 8A R23 R1 R2 Internal Resistance related EMF R3 18 4 Kirchhoff s Rules and Complex DC circuits Confusing before you understand it and amusing after you get it 0 5 equations 99 5 concepts Break down circuits to loops and junctions i e intersections Junction rule what goes in has to come out Loop rule Potential drops over a closed loop has to be 0 whether you count CW or CCW How do you count voltage changes Look at the current If you go with the current you drop If you go against the current you rise 18 5 RC Circuits C circuits Charging is done instantly q CV Not good can ruin the battery Add some resistance to slow it down RC Circuits For HW find charge current and voltage at a given time especially at t 0 and infinity
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