1. Circuits Or Circles of Electricity

2. 3 Elements of a Circuit Voltage source – Charge pump – Battery or AC generator Complete path from positive to negative side of voltage source Load – Changes electrical PE into something useful – E.g. light bulb, motor, resistor

3. Mathematical Rules for Circuits Kirchoff’s voltage law: voltage used up in any complete path of circuit – Result of energy conservation Kirchoff’s current law: current into junction= current out of junction. – Current into and out of battery the same – Result of matter conservation Ohm’s Law I =  V/R Power equations P=I  V = I 2 R=  V 2 /R

4. Check Questions What is true about the current at A and B? a) I A >I B b) I A <I B c) I A =I B Which is true about the voltage drops of the resistors and the battery voltage? a) V bat =  V R1 +  V R2 b) V bat =  V R3 +  V R4 c) Both are true

5. Series vs. Parallel Circuits One complete path Voltage drops may differ for each component Current the same for each component Adding resistors increases resistance (R total = R 1 +R 2 +R 3 …) Current stops w/ any break Two or more complete paths Voltage drop for each branch is the same Current for each branch may differ Adding resistors reduces resistance 1/R total = 1/R 1 +1/R 2 +1/R 3 … or R total = (R 1 xR 2 )/(R 1 +R 2 ) Break in one branch has no effect on others

6. Check Question Which type of circuit uses up the energy of the battery faster? a)Parallel circuit b)Series circuit c)Same

7. House Wiring Parallel are used in house wiring – When one thing is turned off others stay on Overloading a circuit means it is carrying too much current. Overloads are avoided by – Splitting current among several circuits – Using fuses or circuit breakers at beginning or end of circuit with a smaller current rating than the wiring itself

8. The Evil Bulb Problem Series circuit 40 W brightest Current the same for all bulbs Voltage drop of bulbs determines the bulb’s power and brightness Voltage drop has a direct relationship with resistance The 40 W bulb has the thinnest filament and greatest resistance Parallel circuit 100 W brightest Voltage drop the same for all bulbs Current through bulb will determine the bulbs power and brightness Current has an inverse relationship with resistance The 100 W bulb has the thickest filament and thus the smallest resistance

9. Check Question Which pickles will light up first? a)The series pickles b)The parallel pickles c)The pickles closest to the plug d)All the pickles will light up at the same time