magnetism an attractive subject… march 29, 2010. let’s start off easy… what do we know about...
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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 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 labThe 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 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 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 coil
Wrap another coil of wire around the other side
Relationship
Primary 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?