Electricity – Chapter 34 Day 1Day 1. Electrostatics… Wax has excess electrons and the wool is missing someWax has excess electrons and the wool is missing.

Electricity – Chapter 34Electricity – Chapter 34

• Day 1Day 1

Electrostatics…Electrostatics…

• Wax has excess Wax has excess electrons and the electrons and the wool is missing wool is missing somesome– Creates a force Creates a force

• The two objects The two objects attractattract– Unless…Unless…

Moving charges…Moving charges…

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

Potential Difference:Potential Difference:

• The difference in The difference in electrical potential electrical potential (voltage) …(voltage) …

• Provides “electrical Provides “electrical pressure”pressure”

• This is voltage!This is voltage!

VoltageVoltage

• No potential difference = No potential difference = no flow.no flow.

• Some difference = low Some difference = low flowflow

Electric Current:Electric Current:

• Current is the flow of electric Current is the flow of electric charge: “I”charge: “I”

• Measured in amperes (A).Measured in amperes (A).• 1 ampere – one coulomb of charge 1 ampere – one coulomb of charge

per secondper second

To keep the current going To keep the current going we need 2 things:we need 2 things:

1.1. A closed path (usually a wire).A closed path (usually a wire).

To keep the current going To keep the current going we need 2 things:we need 2 things:

• 2. A voltage source2. A voltage source– potential difference potential difference – or voltage “drop”or voltage “drop”

Water Water pump pump example:example:

Water Water pump pump example:example:

However…However…

• We use current to do work…We use current to do work…• We refer to this as the We refer to this as the loadload

– Or Or resistanceresistance

• Changes electrical energy to Changes electrical energy to another form of energy.another form of energy.

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

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

Resistance dependent on Resistance dependent on 4 things:4 things:

• 1. Length: (R) increases as it gets 1. Length: (R) increases as it gets longer.longer.

Resistance dependent on Resistance dependent on 4 things:4 things:

• 2. Width: (R) decreases when it is 2. Width: (R) decreases when it is wider.wider.

Resistance dependent on Resistance dependent on 4 things:4 things:

• 3. Material: (R) can increase or 3. Material: (R) can increase or decrease depending on decrease depending on composition.composition.

Resistance dependent on Resistance dependent on 4 things:4 things:

• 4. Temperature…4. Temperature…– As it heats up it has MORE resistance!As it heats up it has MORE resistance!

Quick Review…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 …?

Equation:Equation:

• Voltage = current x resistanceVoltage = current x resistance

V = iRV = iR

Ohm’s Law:Ohm’s Law:

• 1. The current 1. The current goes up when the goes up when the voltage goes up…voltage goes up…

Ohm’s Law:Ohm’s Law:

• 2. The current in a circuit gets smaller 2. The current in a circuit gets smaller as the resistance of the circuit increases.as the resistance of the circuit increases.

Same PE so no flow of water.

Increase PE at one end, so it will flow.

Why didn’t we die?Why didn’t we die?

• 250,000 volts!!!250,000 volts!!!– But not a lot of But not a lot of

charge…charge…

• Lots of potentialLots of potential– Small Potential Small Potential

energyenergy

• Like a rain drop…Like a rain drop…– Falling thousands Falling thousands

of feetof feet

ElectricityElectricity

Day 2 Day 2

February 24, 2010February 24, 2010

Today’s Plan:Today’s Plan:

• Grade Unit 8: Homework #1Grade Unit 8: Homework #1• Continue Electricity Lecture: More Continue Electricity Lecture: More

about Ohms Law and Powerabout Ohms Law and Power• Lab #1: Electricity…Lab #1: Electricity…• Homework: Unit 8:HW #2Homework: Unit 8:HW #2

Quick Review…Quick Review…

• Potential Difference Potential Difference is the…is the…

• It’s like the PE above It’s like the PE above ground per kgground per kg

• It’s called: _______It’s called: _______• It’s units are: _____It’s units are: _____• It looks like this on It looks like this on

an electric an electric schematic:schematic:

Review of ReeesistanceReview 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 What does it look like on a

schematic diagram?schematic diagram?

Current flows ____________ Current flows ____________ a circuit.a circuit.

Voltage – like height…Voltage – like height…

• Voltage is applied Voltage is applied

_________ a circuit. _________ a circuit. It doesn’t flow – it It doesn’t flow – it is what causes is what causes current to flow.current to flow.

VoltageVoltage

• How much potential How much potential difference is there difference is there between the two between the two prongs on an outlet?prongs on an outlet?

• 120 volts = 120 Joules of energy per coulomb of charge.

Voltage is the “pressure”Voltage is the “pressure”

In order for current to In order for current to flow…flow…

• You need…You need…• A closed circuitA closed circuit

– A loop for current to flow throughA loop for current to flow through

• An applied voltageAn applied voltage– Some “pressure” to get it movingSome “pressure” to get it moving

Ohm’s Law:Ohm’s Law:

• Voltage = current x resistanceVoltage = current x resistance• V = i RV = i R• Volts = amps x ohmsVolts = amps x ohms

V = A V = A ΩΩ• A 12 volt battery is hooked up to a A 12 volt battery is hooked up to a

light bulb that has 4 ohms of light bulb that has 4 ohms of resistance.resistance.– What is the current?What is the current?

One more…One more…

• A circuit has 4 amps of current A circuit has 4 amps of current flowing through it. flowing through it.

• There is a toaster in the circuit that There is a toaster in the circuit that has a resistance of 5 ohms.has a resistance of 5 ohms.– What is the voltage applied to the What is the voltage applied to the

circuit?circuit?

Resistance values Resistance values dependent upon…dependent upon…

• 11

• 22

• 33

ResistorsResistors

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

Current – how much Current – how much matters…matters…

• Remember: current = volts / Remember: current = volts / resistanceresistance

• If you increase volts If you increase volts – and resistance stays the same and resistance stays the same – more current must flow.more current must flow.

Current – how much Current – how much matters?matters?

• .001 amps = felt.001 amps = felt• .005 amps = painful.005 amps = painful• .010 amps = involuntary muscle .010 amps = involuntary muscle

contractioncontraction• .015 amps = loss of muscle control.015 amps = loss of muscle control• .070 amps = through heart serious .070 amps = through heart serious

damage, last for more than a second damage, last for more than a second usually fatalusually fatal

Power Rangers?Power Rangers?

•Power Power – How fast How fast

you use you use energy!energy!

Watt’s up with Power?Watt’s up with Power?

• Electrical power = Electrical power = current x voltagecurrent x voltage

• Units = wattsUnits = watts

Volts or ampsVolts or amps

• Voltmeters “hug” a resistorVoltmeters “hug” a resistor– Measure voltage drop Measure voltage drop acrossacross a resistor a resistor

• Ammeters hold hands INSIDE the Ammeters hold hands INSIDE the circuit.circuit.– Measure current flowing Measure current flowing throughthrough……

LabLab

• Using voltmeter and ammeterUsing voltmeter and ammeter• Vary resistance – constant voltageVary resistance – constant voltage

– Measure current and voltageMeasure current and voltage

• Vary voltage – constant resistanceVary voltage – constant resistance– Measure current and voltageMeasure current and voltage

Today’s PlanToday’s Plan

• Grade Homework – show me both Grade Homework – show me both Ohm and Power practice problemsOhm and Power practice problems

• Series Circuits LectureSeries Circuits Lecture• Series Circuits LabSeries Circuits Lab• Homework: Unit 8: HW #3Homework: Unit 8: HW #3

Electricity Day 3Electricity Day 3

February 26, 2010February 26, 2010

Series CircuitsSeries Circuits

AC DCAC DC

• Alternating current – in our wallsAlternating current – in our walls– Alternates direction 60 x’s a second!Alternates direction 60 x’s a second!– That’s 60 “hertz”That’s 60 “hertz”

• Compared to Direct Current…Compared to Direct Current…

What is moving?What is moving?

• How long does it take to get the How long does it take to get the lights to come on?lights to come on?

• Imagine you are an electron in a Imagine you are an electron in a wire…wire…

• Where do the electrons that heat Where do the electrons that heat your toaster element come from?your toaster element come from?

Drift SpeedDrift Speed

• In a DC Circuit In a DC Circuit – There is a steady pushThere is a steady push

• Even though the electrons aren’t Even though the electrons aren’t flowing so very fastflowing so very fast– They are movingThey are moving– About 1 meter per hourAbout 1 meter per hour

Now, on to Ohm….Now, on to Ohm….

Or the disgraced high school Or the disgraced high school teacherteacher

Life and timesLife and times

• Georg Simon Ohm: Georg Simon Ohm: – Bavaria in 1787 Bavaria in 1787

• Defined relationship between voltage, Defined relationship between voltage, current, and resistance. current, and resistance.

• Dismissed by his colleagues. Dismissed by his colleagues. – Ohm resigns from his high-school teaching Ohm resigns from his high-school teaching

position position – Lived in poverty and shame. Lived in poverty and shame.

• And now…the inside story:And now…the inside story:

Ohm was a cleaver ladOhm was a cleaver lad

• Had a small grain Had a small grain millmill

• Powered by a Powered by a waterwheelwaterwheel

• Ohm pondered Ohm pondered the relationship of the relationship of electricity in his electricity in his Volta BatteryVolta Battery

• Then one day…Then one day…

The series connectionThe series connection

• A series circuit is like holding handsA series circuit is like holding hands– Electricity passes through each personElectricity passes through each person

• One at a time One at a time • Until it reaches the other side of the voltage Until it reaches the other side of the voltage

sourcesource

• Total voltage of a Total voltage of a seriesseries system system– V = iRV = iReqeq

• RReqeq = R = R11 + R + R22 + R + R33 … …– For however many there areFor however many there are

What’s that mean?What’s that mean?

• Current only has one pathCurrent only has one path– Must have same value through entire Must have same value through entire

circuitcircuit• The resistors have to share The resistors have to share

“voltage drop”“voltage drop”– Larger resistors use more voltage Larger resistors use more voltage

dropdrop– Total voltage drop = Total voltage drop = ΣΣV for all V for all

resistorsresistors

Let’s look at one:Let’s look at one:

• 100 volt system100 volt system• 4 resistors (5 4 resistors (5 ΩΩ, , 10 10 ΩΩ, 15 , 15 ΩΩ, , 20 20 ΩΩ))• What is the total resistance?What is the total resistance?

– RReqeq = ??? = ???

• And the current?And the current?• The voltage drop across each The voltage drop across each

resistor?resistor?

Series Circuits Rules:Series Circuits Rules:

• Cut a wire anywhere and the Cut a wire anywhere and the current stops current stops – Only 1 pathway.Only 1 pathway.– Need a complete circuitNeed a complete circuit

• Current is same everywhere in Current is same everywhere in circuit circuit – Charge is conserved.Charge is conserved.

Series Circuits Rules:Series Circuits Rules:

• Add all individual resistors to get Add all individual resistors to get total resistance.total resistance.

• RReqeq = R1 + R2 + R3 + … = R1 + R2 + R3 + …

The BIG problems with The BIG problems with series circuits…series circuits…

• One device is faulty – they all go One device is faulty – they all go out!!out!!

The Series steps!!The Series steps!!

1.1. Add resistances to get RAdd resistances to get Reqeq

2.2. Calculate current: i=V/RCalculate current: i=V/Reqeq

3.3. Calculate voltage drop across Calculate voltage drop across each resistor. each resistor.

• V=IRV=IR

4.4. Check answer Check answer • Add individual voltage drops to get Add individual voltage drops to get

total voltage.total voltage.

Example Problem:Example Problem:

R1 = 2

R2 = 3

R3 = 10

Voltage = 30 V

Lab – did Ohm get it right?Lab – did Ohm get it right?

• To the table!To the table!

Electricity Day 4Electricity Day 4

March 2, 2010March 2, 2010

Parallel CircuitsParallel Circuits

Today’s Plan:Today’s Plan:

1.1. HomeworkHomework

2.2. Hand in Series Circuit LabHand in Series Circuit Lab

3.3. Parallel Circuits Parallel Circuits

4.4. Parallel Circuit LabParallel Circuit Lab

Back to OhmBack to Ohm

• Ohm figured out the series circuitsOhm figured out the series circuits– Current is the same throughoutCurrent is the same throughout– Add the resistances to calculate..Add the resistances to calculate..

• THE “Req”THE “Req”• Equivalent resistanceEquivalent resistance

• Didn’t seem to work for parallel Didn’t seem to work for parallel circuits…circuits…

• Meanwhile, out at the mill…Meanwhile, out at the mill…

Parallel CircuitsParallel Circuits

Parallel Circuit Rules:Parallel Circuit Rules:

• Cut it on the main Cut it on the main branch and all branch and all current stops.current stops.

• Cut it on a side Cut it on a side branch branch – only affects that only affects that

branchbranch

Parallel Circuit Rules:Parallel Circuit Rules:

• The voltage drop The voltage drop across each across each resistor resistor – is the same as the is the same as the

total voltagetotal voltage

VVtt = V = V11 = V = V22 = V = V33

Parallel Circuit Rules:Parallel Circuit Rules:

• Current flows to Current flows to each resistor as if each resistor as if it is the only oneit is the only one

• Ohm’s Law Ohm’s Law applies to each applies to each branch.branch.

Parallel Circuit Rules:Parallel Circuit Rules:

• More current goes More current goes through low through low resistant resistant – It is easier. It is easier.

Parallel Circuit Rules:Parallel Circuit Rules:

• As the number of As the number of branches increasebranches increase– Req decreases. Req decreases.

• There are more There are more pathways for the pathways for the current to flow. current to flow.

Example Problem:Example Problem:

V = 12 Volts

R1 = 1

R2 = 4

R3 = 4

Ohm’s Law for Parallel Ohm’s Law for Parallel CircuitsCircuits

• Find individual branch currents Find individual branch currents using Ohm’s Lawusing Ohm’s Law

• Add branch currents to get total Add branch currents to get total current.current.

• Determine total resistant valueDetermine total resistant value

Your turn... Your turn...

• V = 12 VoltsV = 12 Volts

• R1 = 6 R1 = 6

• R2 = 3 R2 = 3

Measuring CurrentMeasuring Current

• The hard part is figuring out where The hard part is figuring out where to put it.to put it.

Today’s Today’s Plan:Plan:

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

Networking complex Networking complex circuitscircuits

• Adding parallel and series circuits Adding parallel and series circuits together.together.

Use parallel and series Use parallel and series circuit rules to determine circuit rules to determine everything.everything.

1. Convert to series 1. Convert to series circuit.circuit.

1 Ώ

6 Ώ

3 Ώ

4 Ώ

4 Ώ

10 V

2. Determine resistance 2. Determine resistance for each resistor series. for each resistor series.

2 Ώ

1 Ώ

2 Ώ

10 V

3. Determine total 3. Determine total resistance value.resistance value.

5 Ώ

10 V

4. Determine total current4. Determine total current

• V = iRV = iR• i = V/Ri = V/R• i = 2 Ai = 2 A

5 Ώ

10 V

5. Drop in volts over each 5. Drop in volts over each resistorresistor

1 1 = = 2 2 = = 2 2 = =

1 Ώ

2 Ώ

10 V2 Ώ

6. Determine current on 6. Determine current on each branch. (I = V/R)each branch. (I = V/R)

• 6 6 = =• 3 3 = =• 4 4 = =• 4 4 = =

2 Ώ

6 Ώ3 Ώ

4 Ώ

4 Ώ

10 V

7. Add currents to check 7. Add currents to check total current.total current.

1 Ώ

6 Ώ

3 Ώ

4 Ώ

4 Ώ

10 V

Your turn!Your turn!

Today’s 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