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 fields Strength 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 other But not in some metals: Iron, Nickel, Cobalt

Domains… Little areas (domains) have magnetic properties Magnets 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/or Rub with metal with magnet.

De-magnetize? Drop it or Heat it Will 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 magnet You get a stronger magnet!

Lab…M from E 2 parts: Magnetic field around a wire Magnetic 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 box Any missed assignments? In my hands by Thursday. And I’m not here Wednesday after school Homework Electric currents and magnetic fields Lab! 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 particle Like an electron… And shoot it into a magnetic field… A force acts on it. RHR #2 We 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 #2 The 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 WIRE Coil of wire and a magnetic needle

Motors? IDEA #2 – CURRENT IN A MAGNETIC FIELD CREATES A FORCE Yes – 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 lab Generators and Motors Lab

Electro-magnetism - review Current through a wire In a magnetic field… Force acts on it Interaction between 2 magnets!

If the current is reversed… The force pushes in the opposite direction.

What is the difference… Between a motor and A 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 it One side goes up The other down What 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.

Speakers Current is quickly reversed back and forth in a wire Causes 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 coil Wrap another coil of wire around the other side

Relationship Primary voltage = secondary voltage # 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 Hydro Wind Steam turbine Create electricity 6,000 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 lines Typical distances are 300 miles

Distribution Grid Once it’s where it’s needed… It needs to be “stepped-down” to be useful First stop is the substation Stepped down to about 2,200 V Split 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 E What shape do magnetic fields have around a wire? …around a coil of wire? Lab E from M What do you need in addition to a magnet to generate electricity? Door bell Was the clapper attracted to the bell? Why did it ring repeatedly?