Magnetism

An attractive subject…March 29, 2010

Let’s start off easy…

What do we know about magnets?

Magnetic field lines

What do they look like?

How are magnets like electrical charges?

Attract and repel without touching

Force fieldsStrength depends on the distance of separation Like poles repel, opposites attract.

How are magnets different?

Magnetic poles cannot be isolated, charges can.When a magnet is broken, it still have two poles.

Magnetic Field Lines

Remember these?

Magnetic Field Lines:

Magnetism is created by moving electric charge.

Moving electrons are current. The charge in motion creates a magnetic field!

Why isn’t aluminum magnetic?

Electrons spinning in opposite directions.The magnetic fields cancel each otherBut not in some metals:

Iron, Nickel, Cobalt

Domains…

Little areas (domains) have magnetic propertiesMagnets occur when these line up

Temporary magnets

Place iron next to a magnet

The domains line up.

Take magnet away and they go back to being random.

Step on a crack… break yo’ momma’s back

What happens when you break a magnet in half?

Does it stop being a magnet?

How do we magnetize an object?

Place object in a strong magnetic field.

Allow domains to line up.Tap softly to encourage orientation.

And/orRub with metal with magnet.

De-magnetize?

Drop it or Heat itWill cause the domains to become random again.

Electromagnets

Current passing through a wire causes a magnetic field.

A coil of wire…

The magnetic field becomes bunched Everything points in the same direction.You get an electromagnet!!

A core of iron…

The domains of the iron align with the electro magnetYou get a stronger magnet!

Lab…M from E

2 parts:Magnetic field around a wireMagnetic field around a solenoid (coil of wire)

Your goal is to see if you can determine the shape of each field using small compasses

Magnetism & Electricity

March 31, 2010

Plan of the Day

Extra credit – by tomorrow in my boxAny missed assignments?

In my hands by Thursday.And I’m not here Wednesday after school

HomeworkElectric currents and magnetic fieldsLab!Review test

So, who can tell me…

What is magnetism?

Magnetic Fields around wires

If you run current through a wire…You create a magnetic field around the wire

Raise your right hand and repeat after me…

Getting loopy

What happens…If you wrap that wire into a loop?

A charged particle…

If we take a charged particleLike an electron…

And shoot it into a magnetic field…A force acts on it.RHR #2We should all be grateful that this is the case:

Cosmic radiation

Electro-magnetism?

Watch and see…Current through a wire In a magnetic field…

What do you see?

Back to the wire…

Direction of force based on current and magnetic field!RHR #2The root cause is the movement of charge in a magnetic field

This is how our meters work!

Idea #1 – MAGNETIC FIELD AROUND A CURRENT CARRYING WIRECoil of wire and a magnetic needle

Motors?

IDEA #2 – CURRENT IN A MAGNETIC FIELD CREATES A FORCEYes – the same concept is at work in a motor…

Another look…in the opposite direction

Motors: Take electrical energy and convert it to mechanical energy.

Generators: Take mechanical energy and convert it to electrical energy.

Lab – E from M

You’re going to explore how to create electricity from a magnet.

Generator or motor?Open-ended lab

The more you discover The better your grade…

Magnetism & Electricity

April 2, 2010

Plan of the Day

Review homework and labGenerators and MotorsLab

Electro-magnetism - review

Current through a wire

In a magnetic field…

Force acts on itInteraction between 2 magnets!

If the current is reversed…

The force pushes in the opposite direction.

What is the difference…

Between a motor andA generator?

Motors:

Take electrical energy Convert it into mechanical energy.

Generators:

Take mechanical energy Convert it to electrical energy.

Motors?

Current flows through the wire…A force acts on itOne side goes up

The other downWhat has to happen at the bottom?

Generator or Motor?

Lab

Build a motor…(or was it a generator???)

Transformers: Robots in Disguise

April 6, 2010

Think back to the labs…

How about the first lab…What did you observe?

Lab 2: What did you do to create current in the coil?

Lab 3:Explain to a friend how you used these ideas to create a motor.

SpeakersCurrent is quickly reversed back and forth in a wireCauses the coil to move and vibrate the cone. Sends out sound waves into the air.

Mechanical energy – water, wind, steam, etc.

Steam is the driving force for most electrical power plants.

Electricity is not a source of energy…

It is a form of energy that must have a source.

Transformers:

A device for changing the voltage Through electromagnetic induction.

Transformer

Uses an electromagnet to do the same thing!Wrap coils of wire around an iron core

Current in coilWrap another coil of wire around the other side

RelationshipPrimary voltage = secondary voltage# of turns # of turns

The power remains constant (conservation of energy)

If voltage goes up, then current must go down.If voltage goes down, then current must go up.

Power lines: high V, low i.

Stepped down to…

Electrical outlets: low V, high i.

How does the bell work?

Hmmm…

The Grid

April 8, 2010

The Grid

How does electricity get from the generator

Like Bonneville Dam…To your house or our school?

The Grid

The Power Plant

Start with some means of turning

HydroWindSteam turbine

Create electricity6,000 V range

Transmission

Electricity is stepped up at a transmission substation~6,000 V stepped up to 120,000 V (and higher)Reduces power losses

Feeds transmission linesTypical distances are 300 miles

Distribution Grid

Once it’s where it’s needed…It needs to be “stepped-down” to be usefulFirst stop is the substation

Stepped down to about 2,200 VSplit into different paths

At your house

A transformer steps down the power

From 2,200 volts (typical)To 240 volts

And through the meter

Got the picture?

Review

What causes magnetism?What do we know about magnets?Which is stronger:

Gravity or magnetism?

Review

What is the difference between a permanent magnet and an electromagnet?How do we increase the strength of an electromagnet?What happens when charged particles move near a magnetic field?

Review

How do you determine the direction of the force acting on charged particles?How does a speaker work?How does an electric motor work?What makes up the electrical grid?How do transformers make it practical to transmit electricity long distances?

Lab review

Lab M from EWhat shape do magnetic fields have around a wire?…around a coil of wire?

Lab E from MWhat do you need in addition to a magnet to generate electricity?

Door bellWas the clapper attracted to the bell?Why did it ring repeatedly?