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Electricity – Chapter 34 Day 1Day 1

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Electrostatics… Wax has excess electrons and the wool is missing someWax has excess electrons and the wool is missing some –Creates a force The two objects attractThe two objects attract –Unless…

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Moving charges… You put a conductor between the two…You put a conductor between the two… –The force pushes the electrons –until the charge in that area neutralizes

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Potential Difference: The difference in electrical potential (voltage) …The difference in electrical potential (voltage) … Provides electrical pressureProvides electrical pressure This is voltage!This is voltage!

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Voltage No potential difference = no flow.No potential difference = no flow. Some difference = low flowSome difference = low flow

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Electric Current: Current is the flow of electric charge: ICurrent is the flow of electric charge: I Measured in amperes (A).Measured in amperes (A). 1 ampere – one coulomb of charge per second1 ampere – one coulomb of charge per second

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To keep the current going we need 2 things: 1.A closed path (usually a wire).

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To keep the current going we need 2 things: 2. A voltage source2. A voltage source –potential difference –or voltage drop

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Water pump example:

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However… We use current to do work…We use current to do work… We refer to this as the loadWe refer to this as the load –Or resistance Changes electrical energy to another form of energy.Changes electrical energy to another form of energy.

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Resistance is measured in ohms ( ).Resistance is measured in ohms ( ).

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The larger the resistance ( ), the harder it is to push current through the conductor.The larger the resistance ( ), the harder it is to push current through the conductor.

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Resistance dependent on 4 things: 1. Length: (R) increases as it gets longer.1. Length: (R) increases as it gets longer.

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Resistance dependent on 4 things: 2. Width: (R) decreases when it is wider.2. Width: (R) decreases when it is wider.

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Resistance dependent on 4 things: 3. Material: (R) can increase or decrease depending on composition.3. Material: (R) can increase or decrease depending on composition.

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Resistance dependent on 4 things: 4. Temperature…4. Temperature… –As it heats up it has MORE resistance!

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Quick Review… What do you need for a circuit?What do you need for a circuit? How do we use electricity?How do we use electricity? Resistance depends on …?Resistance depends on …?

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Equation: Voltage = current x resistanceVoltage = current x resistance V = iR

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Ohms Law: 1. The current goes up when the voltage goes up…1. The current goes up when the voltage goes up…

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Ohms Law: 2. The current in a circuit gets smaller as the resistance of the circuit increases.2. The current in a circuit gets smaller as the resistance of the circuit increases.

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Same PE so no flow of water. Increase PE at one end, so it will flow.

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Why didnt we die? 250,000 volts!!!250,000 volts!!! –But not a lot of charge… Lots of potentialLots of potential –Small Potential energy Like a rain drop…Like a rain drop… –Falling thousands of feet

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Electricity Day 2 February 24, 2010

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Todays Plan: Grade Unit 8: Homework #1Grade Unit 8: Homework #1 Continue Electricity Lecture: More about Ohms Law and PowerContinue Electricity Lecture: More about Ohms Law and Power Lab #1: Electricity…Lab #1: Electricity… Homework: Unit 8:HW #2Homework: Unit 8:HW #2

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Quick Review… Potential Difference is the…Potential Difference is the… Its like the PE above ground per kgIts like the PE above ground per kg Its called: _______Its called: _______ Its units are: _____Its units are: _____ It looks like this on an electric schematic:It looks like this on an electric schematic:

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Review of Reeesistance What is the purpose of resistance?What is the purpose of resistance? What are the units?What are the units? What does it look like on a schematic diagram?What does it look like on a schematic diagram?

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Current flows ____________ a circuit.

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Voltage – like height… Voltage is appliedVoltage is applied _________ a circuit. It doesnt flow – it is what causes current to flow.

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Voltage How much potential difference is there between the two prongs on an outlet?How much potential difference is there between the two prongs on an outlet? 120 volts = 120 Joules of energy per coulomb of charge.

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Voltage is the pressure

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In order for current to flow… You need…You need… A closed circuitA closed circuit –A loop for current to flow through An applied voltageAn applied voltage –Some pressure to get it moving

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Ohms Law: Voltage = current x resistanceVoltage = current x resistance V = i RV = i R Volts = amps x ohmsVolts = amps x ohms V = A Ω A 12 volt battery is hooked up to a light bulb that has 4 ohms of resistance.A 12 volt battery is hooked up to a light bulb that has 4 ohms of resistance. –What is the current?

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One more… A circuit has 4 amps of current flowing through it.A circuit has 4 amps of current flowing through it. There is a toaster in the circuit that has a resistance of 5 ohms.There is a toaster in the circuit that has a resistance of 5 ohms. –What is the voltage applied to the circuit?

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Resistance values dependent upon… 1 2 3

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Resistors Resistors often regulate the amount of current passing through the appliance.Resistors often regulate the amount of current passing through the appliance.

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Current – how much matters… Remember: current = volts / resistanceRemember: current = volts / resistance If you increase voltsIf you increase volts –and resistance stays the same –more current must flow.

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Current – how much matters?.001 amps = felt.001 amps = felt.005 amps = painful.005 amps = painful.010 amps = involuntary muscle contraction.010 amps = involuntary muscle contraction.015 amps = loss of muscle control.015 amps = loss of muscle control.070 amps = through heart serious damage, last for more than a second usually fatal.070 amps = through heart serious damage, last for more than a second usually fatal

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Power Rangers? PowerPower –How fast you use energy!

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Watts up with Power? Electrical power = current x voltageElectrical power = current x voltage Units = wattsUnits = watts

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Volts or amps Voltmeters hug a resistorVoltmeters hug a resistor –Measure voltage drop across a resistor Ammeters hold hands INSIDE the circuit.Ammeters hold hands INSIDE the circuit. –Measure current flowing through…

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Lab Using voltmeter and ammeterUsing voltmeter and ammeter Vary resistance – constant voltageVary resistance – constant voltage –Measure current and voltage Vary voltage – constant resistanceVary voltage – constant resistance –Measure current and voltage

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Todays Plan Grade Homework – show me both Ohm and Power practice problemsGrade Homework – show me both Ohm and Power practice problems Series Circuits LectureSeries Circuits Lecture Series Circuits LabSeries Circuits Lab Homework: Unit 8: HW #3Homework: Unit 8: HW #3

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Electricity Day 3 February 26, 2010 Series Circuits

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AC DC Alternating current – in our wallsAlternating current – in our walls –Alternates direction 60 xs a second! –Thats 60 hertz Compared to Direct Current…Compared to Direct Current…

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What is moving? How long does it take to get the lights to come on?How long does it take to get the lights to come on? Imagine you are an electron in a wire…Imagine you are an electron in a wire… Where do the electrons that heat your toaster element come from?Where do the electrons that heat your toaster element come from?

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Drift Speed In a DC CircuitIn a DC Circuit –There is a steady push Even though the electrons arent flowing so very fastEven though the electrons arent flowing so very fast –They are moving –About 1 meter per hour

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Now, on to Ohm…. Or the disgraced high school teacher

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Life and times Georg Simon Ohm:Georg Simon Ohm: –Bavaria in 1787 Defined relationship between voltage, current, and resistance.Defined relationship between voltage, current, and resistance. Dismissed by his colleagues.Dismissed by his colleagues. –Ohm resigns from his high-school teaching position –Lived in poverty and shame. And now…the inside story:And now…the inside story:

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Ohm was a cleaver lad Had a small grain millHad a small grain mill Powered by a waterwheelPowered by a waterwheel Ohm pondered the relationship of electricity in his Volta BatteryOhm pondered the relationship of electricity in his Volta Battery Then one day…Then one day…

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The series connection A series circuit is like holding handsA series circuit is like holding hands –Electricity passes through each person One at a timeOne at a time Until it reaches the other side of the voltage sourceUntil it reaches the other side of the voltage source Total voltage of a series systemTotal voltage of a series system –V = iR eq R eq = R 1 + R 2 + R 3 …R eq = R 1 + R 2 + R 3 … –For however many there are

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Whats that mean? Current only has one pathCurrent only has one path –Must have same value through entire circuit The resistors have to share voltage dropThe resistors have to share voltage drop –Larger resistors use more voltage drop –Total voltage drop = ΣV for all resistors

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Lets look at one: 100 volt system100 volt system 4 resistors (5 Ω, 10 Ω, 15 Ω, 20 Ω)4 resistors (5 Ω, 10 Ω, 15 Ω, 20 Ω) What is the total resistance?What is the total resistance? –R eq = ??? And the current?And the current? The voltage drop across each resistor?The voltage drop across each resistor?

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Series Circuits Rules: Cut a wire anywhere and the current stopsCut a wire anywhere and the current stops –Only 1 pathway. –Need a complete circuit Current is same everywhere in circuitCurrent is same everywhere in circuit –Charge is conserved.

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Series Circuits Rules: Add all individual resistors to get total resistance.Add all individual resistors to get total resistance. R eq = R1 + R2 + R3 + …R eq = R1 + R2 + R3 + …

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The BIG problems with series circuits… One device is faulty – they all go out!!One device is faulty – they all go out!!

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The Series steps!! 1.Add resistances to get R eq 2.Calculate current: i=V/R eq 3.Calculate voltage drop across each resistor. V=IRV=IR 4.Check answer Add individual voltage drops to get total voltage.Add individual voltage drops to get total voltage.

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Example Problem: R1 = 2 R2 = 3 R3 = 10 Voltage = 30 V

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Lab – did Ohm get it right? To the table!To the table!

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Electricity Day 4 March 2, 2010 Parallel Circuits

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Todays Plan: 1.Homework 2.Hand in Series Circuit Lab 3.Parallel Circuits 4.Parallel Circuit Lab

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Back to Ohm Ohm figured out the series circuitsOhm figured out the series circuits –Current is the same throughout –Add the resistances to calculate.. THE ReqTHE Req Equivalent resistanceEquivalent resistance Didnt seem to work for parallel circuits…Didnt seem to work for parallel circuits… Meanwhile, out at the mill…Meanwhile, out at the mill…

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Parallel Circuits

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Parallel Circuit Rules: Cut it on the main branch and all current stops.Cut it on the main branch and all current stops. Cut it on a side branchCut it on a side branch –only affects that branch

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Parallel Circuit Rules: The voltage drop across each resistorThe voltage drop across each resistor –is the same as the total voltage V t = V 1 = V 2 = V 3

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Parallel Circuit Rules: Current flows to each resistor as if it is the only oneCurrent flows to each resistor as if it is the only one Ohms Law applies to each branch.Ohms Law applies to each branch.

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Parallel Circuit Rules: More current goes through low resistantMore current goes through low resistant –It is easier.

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Parallel Circuit Rules: As the number of branches increaseAs the number of branches increase –Req decreases. There are more pathways for the current to flow.There are more pathways for the current to flow.

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Example Problem: V = 12 Volts R1 = 1 R2 = 4 R3 = 4

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Ohms Law for Parallel Circuits Find individual branch currents using Ohms LawFind individual branch currents using Ohms Law Add branch currents to get total current.Add branch currents to get total current. Determine total resistant valueDetermine total resistant value

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Your turn... V = 12 Volts R1 = 6 R2 = 3

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Measuring Current The hard part is figuring out where to put it.The hard part is figuring out where to put it.

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Todays Plan: Finish Lab 3 – parallel circuitsFinish Lab 3 – parallel circuits Homework reviewHomework review Complex CircuitsComplex Circuits Lab design and executionLab design and execution

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Networking complex circuits Adding parallel and series circuits together.Adding parallel and series circuits together.

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Use parallel and series circuit rules to determine everything.

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1. Convert to series circuit. 1 Ώ 6 Ώ 3 Ώ 4 Ώ 10 V

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2. Determine resistance for each resistor series. 2 Ώ 1 Ώ 2 Ώ 10 V

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3. Determine total resistance value. 5 Ώ 10 V

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4. Determine total current V = iRV = iR i = V/Ri = V/R i = 2 Ai = 2 A 5 Ώ 10 V

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5. Drop in volts over each resistor 1 = 2 = 1 Ώ 2 Ώ 10 V 2 Ώ

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6. Determine current on each branch. (I = V/R) 6 =6 = 3 =3 = 4 =4 = 2 Ώ 6 Ώ 3 Ώ 4 Ώ 10 V

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7. Add currents to check total current. 1 Ώ 6 Ώ 3 Ώ 4 Ώ 10 V

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Your turn!

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Todays Lab: Design a Network Circuit – construct two nice clean diagrams of circuit. One will be used for the expected results (using 3 volt) and the other will be the actual results (measured) Collect all necessary data

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Electric Current, Voltage, and Resistance

Electric Current, Voltage, and Resistance

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