Or Circles of Electricity Circuits Or Circles of Electricity 5a Students know how to predict the voltage or current in simple direct current electric circuits constructed from batteries, wires, resistors and capacitors. 5b Students know how to solve problems using Ohm’s law. 5c Students know any resistive element in a DC circuit dissipates energy, which heats the resistor. Students can calculate the power in any resistive element using the formula Power = I xI x R.

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 What are the three things needed for a circuit? List two types of voltage source and several types of loads. Activities: State that we would like current do useful things for us. To do this we construct electrical circuits. Circuits require three things to work. A voltage source which pumps charge around the circuit, a complete path from one end of the voltage source to the other and a load which changes the electrical PE into useful energy. List some voltage sources: batteries and AC generators List some loads: light bulb, resistor or heater, motor. Point out energy changes that each is making.

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 = DV/R Power equations P=IDV = I2R=DV2/R Questions: State Kirchoff’s voltage law. What conservation law does this come from? State Kirchoff’s current law. What conservation law is this a restatement? What other mathematical relationships exists for every circuit? Activities: 1) Write Kirchoff’s voltage and current laws on board while students copy previous slide. Discuss what Kirchoff’s voltage law says and that this is nothing more than conservation of energy for electricity. Discuss what Kirchoff’s current law says and how electrons are small pieces of matter and cannot be destroyed according to the law of conservation of matter. 4) Add Ohm’s law and the power equations to the board and state that these hold for all circuits.

Check Questions What is true about the current at A and B? a) IA>IB b) IA<IB c) IA=IB Which is true about the voltage drops of the resistors and the battery voltage? a) V bat =DVR1 +DVR2 b) V bat = DVR3 +DVR4 c) Both are true

Series vs. Parallel Circuits One complete path Voltage drops may differ for each component Current the same for each component Adding resistors increases resistance (Rtotal = R1 +R2 +R3 …) 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/Rtotal = 1/R1 +1/R2 +1/R3 … or Rtotal= (R1 xR2)/(R1+R2) Break in one branch has no effect on others Questions: How do series and parallel circuits compare in terms of a) number of current paths b) voltage drops c) current splits d) effect of adding resistors to total resistance e) current changes when circuit is broken. Activities: Draw three bulb series and parallel circuit schematics on board while students copy down previous slide. Refer to series circuit diagram and point out features listed on slide. Demo: Construct three bulb series circuit with battery and bulbs. Demonstrate the features on slide. Be sure to show how current and voltage drops occur in circuit. Project 1st half of slide and allow students to copy it down. Repeat this process with the parallel schematic and then construct it with bulbs and battery

Check Question Which type of circuit uses up the energy of the battery faster? Parallel circuit Series circuit Same

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 Questions: What type of circuit is used in wiring a house? How do you know? How do contractors avoid overloading house wiring? Is an overload an excess of voltage or current? What is the purpose of circuit breakers and fuses and where should they be located in a circuit? Activities: Draw a house circuit with several outlets between two extended terminals while students are copying previous slide. Discuss how this is a parallel circuit and that we know houses use this type of circuit because you can turn off one appliance or light without everything else going out. Discuss purpose and placement of circuit breakers to avoid overloads. State that overloads occur when too much current is in a wire. Discuss how multiple circuits help to distribute current load over several separate wires and reduce the current any one circuit uses.

The Evil Bulb Problem Parallel circuit 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 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 Question: A 40, 60 and 100 watt bulb are connected in parallel. Which bulb is the brightest and why? If they were connected in series which bulb would be the brightest and why? Activites: Do parallel experiment and go through the math. Make chart as go through logic Do the series experiment and go through the math Do and discuss the pickle experiment.

Check Question Which pickles will light up first? The series pickles The parallel pickles The pickles closest to the plug All the pickles will light up at the same time