Take out a piece of paperTake out a piece of paper

Jot down what you know about Jot down what you know about magnets…magnets…• Where do we find magnets?Where do we find magnets?• What metals are magnetic?What metals are magnetic?• How can you make or destroy magnets?How can you make or destroy magnets?• How do we use magnets?How do we use magnets?

And…what would life be like without And…what would life be like without them?them?

MagnetsMagnets

Why do we discuss magnets along Why do we discuss magnets along with electricity?with electricity?

MAGMAGnetnetICIC Fields Fields

Why are we attracted to magnets?Why are we attracted to magnets?Or…Or…

Animal Magnetism: Myth or Animal Magnetism: Myth or MisunderstandingMisunderstanding

Discovery of MagnetsDiscovery of Magnets

Some debate – one of the great Some debate – one of the great controversies of our time!controversies of our time!

Story of sheep herder in Magnesia Story of sheep herder in Magnesia region of Greece.region of Greece.

Or did the Chinese discover it Or did the Chinese discover it thousands of years prior?thousands of years prior?

Magnetic Fields – OverheadMagnetic Fields – Overhead

Look at magnets on overhead with Look at magnets on overhead with iron filingsiron filings

Magnetic flux lines – flow out of the Magnetic flux lines – flow out of the north poles of magnets and into the north poles of magnets and into the south poles.south poles.• Closed loopsClosed loops• No overlap between 2 magnetic fieldsNo overlap between 2 magnetic fields

Remind you of anything?Remind you of anything?

Magnetic Fields – OverheadMagnetic Fields – Overhead

Paper clipsPaper clips• Soft vs hardSoft vs hard

How do you destroy a magnet?How do you destroy a magnet? Can you see the magnetic fields?Can you see the magnetic fields? Magnetic DomainsMagnetic Domains

Magnets interact with electricityMagnets interact with electricity

Ampere’s – the guy that current is Ampere’s – the guy that current is named for…named for…• Studied magnetism and electricityStudied magnetism and electricity

Observed current through a wireObserved current through a wire• In a magnetic fieldIn a magnetic field• Bounces or jumpsBounces or jumps

Current makes magnets?Current makes magnets?

Current causes a magnetic field Current causes a magnetic field around the wirearound the wire

Ampere: interaction between 2 Ampere: interaction between 2 magnetic fieldsmagnetic fields• One from a magnetOne from a magnet• One from electricityOne from electricity

Electricity can create an Electricity can create an electromagnetelectromagnet• Like at the junk yard or Like at the junk yard or

Magnetic Fields – now that Magnetic Fields – now that we’ve seen them…we’ve seen them…

The conventional way to think about The conventional way to think about magnetic fields…magnetic fields…

Direction of concentric field lines Direction of concentric field lines based on RHRbased on RHR• Grab the wire with your right handGrab the wire with your right hand• The current flow lines up with your The current flow lines up with your

thumbthumb• Your fingers curve in the direction of the Your fingers curve in the direction of the

magnetic fields.magnetic fields.

Magnetic field around a coilMagnetic field around a coil

Take one loop of wireTake one loop of wire Trace your right hand around the Trace your right hand around the

loop and look where your fingertips loop and look where your fingertips gogo

Resulting magnetic force inside the Resulting magnetic force inside the loop combines in one directionloop combines in one direction

What happens when you have 2 What happens when you have 2 loops?loops?

Magnetic fields – odds and endsMagnetic fields – odds and ends

Magnetic FieldsMagnetic Fields• Strength of the magnetic fieldStrength of the magnetic field• B (vector)B (vector)

Into or out of page (x or .)Into or out of page (x or .) ArrowsArrows

What next?What next?

Start Video?Start Video?

Magnetic ForceMagnetic Force

March 30/31, 2010March 30/31, 2010

POTDPOTD

Check homeworkCheck homework• Go over questionsGo over questions

Magnetic Force LectureMagnetic Force Lecture LabLab

HWHW

1.1. repel, attract, repel, attract, attractattract

2.2. twotwo3.3. a, ba, b4.4. parallel to the parallel to the

earth’s surface earth’s surface pointing towards pointing towards the geographic the geographic north polenorth pole

5.5. HardHard

1.1. Concentric circlesConcentric circles

2.2. Smaller region Smaller region insideinside

3.3. Opposite spin – Opposite spin – magnetic fields magnetic fields cancelcancel

Last time’s LabLast time’s Lab

Please turn it in now! ; )Please turn it in now! ; )

What’s the point?What’s the point?

Last time:Last time:• Magnetism from ElectricityMagnetism from Electricity

We can see what happens!We can see what happens!• Must be a force acting on it…Must be a force acting on it…

TodayToday• Calculate the FORCE on a charge in a Calculate the FORCE on a charge in a

magnetic fieldmagnetic field

Magnetic ForceMagnetic Force

Consider a charged particleConsider a charged particle• Like an electron or a proton…Like an electron or a proton…• In the presence of a magnetic fieldIn the presence of a magnetic field

The force felt by the chargeThe force felt by the charge• F = qvBF = qvB

q= charge (Coulombs)q= charge (Coulombs) V = velocity (m/s)V = velocity (m/s) B = strength of magnetic field (Tesla)B = strength of magnetic field (Tesla)

Strength of BStrength of B

The strength of a typical lab magnetThe strength of a typical lab magnet• 1.5 T1.5 T

The strength of the earth’s magnetic The strength of the earth’s magnetic fieldfield• Is it stronger or weaker than a lab Is it stronger or weaker than a lab

magnet?magnet? Yes!! Nice call!!Yes!! Nice call!!

• Earth’s is about 50 Earth’s is about 50 μμTT• Or… 5.0 x 10Or… 5.0 x 10-5-5 T T

Force as in VECTORForce as in VECTOR

What direction is What direction is the force?the force?

Assume B is a Assume B is a uniform magnetic uniform magnetic fieldfield

The particle moves The particle moves at a velocity of vat a velocity of v

Right hand rule #2Right hand rule #2• Force comes out of Force comes out of

wall at youwall at you

Force as in VECTORForce as in VECTOR

Right hand rule #2Right hand rule #2• Based on a Based on a

positive chargepositive charge If it’s If it’s negativenegative……

• It’s the other It’s the other directiondirection

ExampleExample

• A charged particleA charged particle• Charge = 1.7 x 10Charge = 1.7 x 10-6-6 C C

• Is traveling north at 47 m/sIs traveling north at 47 m/s• Through a uniform magnetic fieldThrough a uniform magnetic field

• BB = 1.5 T straight up = 1.5 T straight up• What is F?What is F?• FF = q = qvBvB = (1.7 x 10 = (1.7 x 10-6-6 C) x (47 m/s) C) x (47 m/s)

x 1.5 Tx 1.5 T• F = 1.2 x 10F = 1.2 x 10-4-4 N (east) N (east)

Uniform Magnetic FieldUniform Magnetic Field

B is directed into the wallB is directed into the wall A particle travels perpendicular to BA particle travels perpendicular to B The force on the particle moves itThe force on the particle moves it

• Perpendicular to travel directionPerpendicular to travel direction What is the path of the particle?What is the path of the particle?

Is the particle + or - ?Is the particle + or - ?

Wire in a Uniform Magnetic FieldWire in a Uniform Magnetic Field

Similar to a charge…except…Similar to a charge…except… Thumb along current’s pathThumb along current’s path Fingers up magnetic fieldFingers up magnetic field RHRRHR

The calculation…The calculation…

Force = qvBForce = qvB• qv = i x lengthqv = i x length

Force = BilForce = Bil• B – magnetic field strengthB – magnetic field strength• i – currenti – current• l – length of wire in fieldl – length of wire in field

ExampleExample

A 6.0 m wire carries a current of 7.0 A 6.0 m wire carries a current of 7.0 A in the x direction.A in the x direction.

A magnetic force = 7.0 x 10A magnetic force = 7.0 x 10-6-6 N in N in the –y directionthe –y direction

What is the direction and magnitude What is the direction and magnitude of the magnetic field?of the magnetic field?

7.0 x 107.0 x 10-6-6 N = B x 7.0 A x 6.0 m N = B x 7.0 A x 6.0 m• B = F/(i l) B = F/(i l) • B = 1.7 x 10B = 1.7 x 10-7-7 T T

Can you see it?Can you see it?

What happens when you have 2 What happens when you have 2 wires carrying current in the same wires carrying current in the same direction.direction.

Check your answer with a friend…Check your answer with a friend… What about if they are in the What about if they are in the

opposite direction?opposite direction? Same thing…Same thing…

Think about a loop…Think about a loop…

In a magnetic field…In a magnetic field…

Lab introLab intro

Discovery labDiscovery lab A magnetA magnet A solenoidA solenoid A galvanometerA galvanometer

• To measure current (To measure current (μA)μA) Determine as much as you canDetermine as much as you can Your grade will be based on your Your grade will be based on your

discoveriesdiscoveries• Don’t skimpDon’t skimp

Induced CurrentInduced Current

March 31 & April 1, 2010March 31 & April 1, 2010

Turn in …Turn in …

LabLab Get homework outGet homework out

1.1. 0.081 T0.081 T

2.2. UpUp

3.3. 0.75 N0.75 N

4.4. AttractsAttracts

5.5. Out of pageOut of page

Today’s Learning GoalsToday’s Learning Goals

What are the factors that affect What are the factors that affect induced current?induced current?

Understand that induced currentUnderstand that induced current generates its own generates its own magnetic fieldmagnetic field

Calculate the magnitude of an Calculate the magnitude of an induced voltageinduced voltage

Right hand rulesRight hand rules

What are they? What are they? Check your understanding with a Check your understanding with a

neighborneighbor

Induced currentInduced current

Relative motion between a wire and Relative motion between a wire and a magnetic field…a magnetic field…• Creates a potential difference in the Creates a potential difference in the

wirewire• Current flows – if the circuit is complete!Current flows – if the circuit is complete!

Recall: a charged particle moving in Recall: a charged particle moving in a magnetic field experience a force.a magnetic field experience a force.• F = qvBF = qvB

F = Bil: Consider a wireF = Bil: Consider a wire

Relative motion is required Relative motion is required • due to the forcedue to the force

Pull it through a magnetic fieldPull it through a magnetic field• Perpendicular to the fieldPerpendicular to the field

The magnetic force causes the The magnetic force causes the positive ones to movepositive ones to move• According to the RHRAccording to the RHR

And the negative charges to moveAnd the negative charges to move• According to the “left” hand ruleAccording to the “left” hand rule

F = qvB: Consider a wireF = qvB: Consider a wire

Separation of chargesSeparation of charges• Like a potential difference or voltageLike a potential difference or voltage

If there is a completed circuitIf there is a completed circuit• Then current flowsThen current flows

Motion is essentialMotion is essential

The voltage (and current) is The voltage (and current) is maintainedmaintained• As long as the wire is moving relative to As long as the wire is moving relative to

the magnetic field.the magnetic field. The faster it moves – the greater the The faster it moves – the greater the

currentcurrent The greater the magnetic field, the The greater the magnetic field, the

greater the currentgreater the current

Orientation mattersOrientation matters

In a complete circuit/loop of wire In a complete circuit/loop of wire • more current flows if motion is more current flows if motion is

perpendicular to the magnetic field.perpendicular to the magnetic field. The number of field lines that you The number of field lines that you

“cut” as it moves…“cut” as it moves…• Increase that and you increase current!Increase that and you increase current!

This means you can also rotate the This means you can also rotate the coil in a magnetic field.coil in a magnetic field.

Faraday’s Law of InductionFaraday’s Law of Induction

Factors that affect the induced Factors that affect the induced voltage:voltage:• The number of coils in the circuit (N)The number of coils in the circuit (N)• The angle between B and the coil (The angle between B and the coil (θθ))• Area of the coil (mArea of the coil (m22))• B = magnetic field strength (T)B = magnetic field strength (T)

emf = -N emf = -N ΔΔ(AB(cos (AB(cos θθ))/ ))/ ΔΔtt

Faraday’s Law of InductionFaraday’s Law of Induction

emf = -N emf = -N ΔΔ(AB(cos (AB(cos θθ))/ ))/ ΔΔtt• If you vary the: If you vary the:

cross sectional areacross sectional area Magnetic fieldMagnetic field The angle of orientationThe angle of orientation

• With respect to a change in timeWith respect to a change in time• THEN you will see an induced emfTHEN you will see an induced emf

ExampleExample

A coil with 25 turns of wireA coil with 25 turns of wire Cross sectional area of 1.8 mCross sectional area of 1.8 m22

Magnetic field applied at a right Magnetic field applied at a right angle to the plane of the coilangle to the plane of the coil

The field is increased from 0.00 T to The field is increased from 0.00 T to 0.55 T in 0.85 seconds0.55 T in 0.85 seconds

What is the magnitude of the What is the magnitude of the induced emf?induced emf?

The answer, please:The answer, please:

emf = -N emf = -N ΔΔ(AB(cos (AB(cos θθ))/ ))/ ΔΔtt emf = - 25 (1.8memf = - 25 (1.8m22)cos0 ()cos0 (ΔΔB/B/ΔΔt)t) emf = - 45 (0.55 – 0.00T/0.85 s)emf = - 45 (0.55 – 0.00T/0.85 s) emf = - 29 Vemf = - 29 V

Note that all factors are included in Note that all factors are included in the answer. the answer.

But something has to change!But something has to change!

Lenz’s LawLenz’s Law

If you induce a current in a wireIf you induce a current in a wire• That current creates a magnetic field of That current creates a magnetic field of

its own!its own! Lenz tells us that the induced current Lenz tells us that the induced current

creates a magnetic field that creates a magnetic field that opposes the applied magnetic field.opposes the applied magnetic field.

The induced current tries to keep the The induced current tries to keep the field strength constant.field strength constant.

Today’s GoalsToday’s Goals

Factors that affect induced Factors that affect induced voltage/currentvoltage/current

Induced current generates its own Induced current generates its own _________________?_________________?

Calculate the magnitude of an Calculate the magnitude of an induced voltageinduced voltage

Lab!Lab!

Today’s lab DOES NOT connect Today’s lab DOES NOT connect directly with the lesson!!directly with the lesson!!

It does relate to magnetic fields It does relate to magnetic fields generated by current carrying wire…generated by current carrying wire…

Have fun!Have fun!

TransformersTransformers

April 5/8, 2010April 5/8, 2010

Transformers: Saving the PlanetTransformers: Saving the Planet We generate electricity at Bonneville We generate electricity at Bonneville

• Alternating currentAlternating current We use it in our homesWe use it in our homes

• 40 miles away40 miles away Is that a problem?Is that a problem? Line losses…Line losses…

• Big “i”… big losses…Big “i”… big losses… So how do we change the current??So how do we change the current??

How indeed…How indeed…

Remember the door bell example…Remember the door bell example… The bell only works when there is a The bell only works when there is a

change in electrical current.change in electrical current. The system is DCThe system is DC

• When connected, a constant current When connected, a constant current flows.flows.

What would happen if it was AC?What would happen if it was AC?

Same idea…Same idea…

2 coils connected with a soft metal 2 coils connected with a soft metal corecore

The current in the first coil creates a The current in the first coil creates a magnetic field.magnetic field.

AC circuit - current changesAC circuit - current changes• 60 times a second (Hertz) 60 times a second (Hertz)

The changing magnetic field induces The changing magnetic field induces a current in the secondary coila current in the secondary coil

Primary and secondaryPrimary and secondary

The coil that is “hot” is “primary”The coil that is “hot” is “primary” The coil that receives an induced The coil that receives an induced

current is “secondary”current is “secondary” VV11/N/N11 = V = V22/N/N22

• V – voltage V – voltage • N – number of turns on the coilN – number of turns on the coil

Step up or step down?Step up or step down?

VV11 = 110 V = 110 V NN11 = 75 turns = 75 turns VV22 = 2400 V = 2400 V NN22 = ? = ? N2 = (NN2 = (N11xVxV22)/V)/V11

• NN22= (75x2400)/110= (75x2400)/110

• NN22= 1600 turns= 1600 turns

If you want more…If you want more…

If you want the voltage to increase…If you want the voltage to increase…• Will you have more primary or Will you have more primary or

secondary windings?secondary windings?

The holy grail?The holy grail?

No, there is no free lunch…No, there is no free lunch… Power in on one sidePower in on one side

• Equals power out on the other…Equals power out on the other…• Vi = ViVi = Vi

And that is the key to reducing line And that is the key to reducing line losses!losses!

Another exampleAnother example

A 120 volt system has 2 amps of current A 120 volt system has 2 amps of current flowingflowing

What is the power in and the power out?What is the power in and the power out? If you want 60 volts on the secondary sideIf you want 60 volts on the secondary side

• How much current will flow?How much current will flow? If the secondary has 20 wraps of wire…If the secondary has 20 wraps of wire…

• How many will the primary have?How many will the primary have?

Change of pace?Change of pace?

A worksheet to consider…A worksheet to consider… Nike scholarship?Nike scholarship? Homework clarification…Homework clarification…

The GridThe Grid

The Power PlantThe Power Plant

Start with some Start with some means of turningmeans of turning• HydroHydro• WindWind• Steam turbineSteam turbine

Create electricityCreate electricity• Typically about Typically about

5,000 V 5,000 V

TransmissionTransmission

Electricity is stepped up at a substationElectricity is stepped up at a substation•~5,000 V stepped up to ~500,000 V~5,000 V stepped up to ~500,000 V•Reduces power lossesReduces power losses

Feeds high voltage transmission linesFeeds high voltage transmission lines Typical distances are 300 milesTypical distances are 300 miles

Distribution GridDistribution Grid

Once it’s where it’s Once it’s where it’s needed…needed…

It needs to be It needs to be “stepped-down” to “stepped-down” to be usefulbe useful

First stop is the First stop is the substationsubstation• Stepped down to Stepped down to

about 7,200 Vabout 7,200 V• Split into different Split into different

pathspaths

At your houseAt your house

A transformer A transformer steps down the steps down the power power • From 7,200 volts From 7,200 volts

(typical)(typical)• To 240 voltsTo 240 volts

And through the And through the metermeter

Last stop…Last stop…

Into your circuit Into your circuit breaker panelbreaker panel

And into the houseAnd into the house• As 240 V (washer, As 240 V (washer,

range, etc)range, etc)• And 120 V And 120 V

(everything else)(everything else)

Got the picture?Got the picture?

Grid FactsGrid Facts

Grid FactsGrid Facts

30% of energy 30% of energy consumed in the US consumed in the US is in the form of is in the form of electricityelectricity

Average thermal Average thermal efficiency of a power efficiency of a power plant is 30%plant is 30%

157,000 miles of 157,000 miles of high voltage lines in high voltage lines in the USthe US

Unit 9 ReviewUnit 9 Review

ElectromagnetismElectromagnetism

HomeworkHomework1.1. 55 turns55 turns

2.2. 3.5 x 103.5 x 1044 turns turns

3.3. 25 turns25 turns

4.4. 156:1156:1

Magnets – what are they good Magnets – what are they good for?for?

Spend a minute writing down what Spend a minute writing down what you know about magnets…you know about magnets…

You should know…You should know…• How many polesHow many poles• How to destroy themHow to destroy them• What happens if you break themWhat happens if you break them• What metals can be magnetizedWhat metals can be magnetized• What allows magnetism to occurWhat allows magnetism to occur

Faraday had a dreamFaraday had a dream What were the 3 right hand rules?What were the 3 right hand rules? They explain…They explain…

• The direction of magnetic field lines The direction of magnetic field lines around a wirearound a wire

• Interaction between electrical charges Interaction between electrical charges and magnetismand magnetism

• Interaction between current and Interaction between current and magnetismmagnetism

Deep thoughtsDeep thoughts F = BilF = Bil F = qvBF = qvB Faraday’s lawFaraday’s law Lenz’s lawLenz’s law

Induced emfInduced emf We can cause current to flow through We can cause current to flow through

a circuit…a circuit…• Interaction between magnetic field and Interaction between magnetic field and

circuitcircuit It explains a lotIt explains a lot

• How generators and motors workHow generators and motors work• Doorbell (DC)Doorbell (DC)• Transformers (AC)Transformers (AC)

TransformersTransformers V1/N1 = V2/N2V1/N1 = V2/N2 Power in = Power outPower in = Power out

Vi (in) = Vi (out)Vi (in) = Vi (out)

LabsLabs Magnetic field around a wire and a coilMagnetic field around a wire and a coil

• Which is stronger? Which is stronger? • Recall the shapes and direction of the field Recall the shapes and direction of the field

lineslines Electrical current generated by waving a Electrical current generated by waving a

magnet around…magnet around…• How could you change the direction?How could you change the direction?• How could you change the magnitude?How could you change the magnitude?

Doorbell Lab – electromagnetismDoorbell Lab – electromagnetism