magnetism an attractive subject… march 29, 2010. let’s start off easy… what do we know about...
TRANSCRIPT
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Magnetism
An attractive subject…March 29, 2010
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Let’s start off easy…
What do we know about magnets?
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Magnetic field lines
What do they look like?
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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.
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How are magnets different?
Magnetic poles cannot be isolated, charges can.When a magnet is broken, it still have two poles.
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Magnetic Field Lines
Remember these?
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Magnetic Field Lines:
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Magnetism is created by moving electric charge.
Moving electrons are current. The charge in motion creates a magnetic field!
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Why isn’t aluminum magnetic?
Electrons spinning in opposite directions.The magnetic fields cancel each otherBut not in some metals:
Iron, Nickel, Cobalt
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Domains…
Little areas (domains) have magnetic propertiesMagnets occur when these line up
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Temporary magnets
Place iron next to a magnet
The domains line up.
Take magnet away and they go back to being random.
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Step on a crack… break yo’ momma’s back
What happens when you break a magnet in half?
Does it stop being a magnet?
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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.
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De-magnetize?
Drop it or Heat itWill cause the domains to become random again.
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Electromagnets
Current passing through a wire causes a magnetic field.
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A coil of wire…
The magnetic field becomes bunched Everything points in the same direction.You get an electromagnet!!
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A core of iron…
The domains of the iron align with the electro magnetYou get a stronger magnet!
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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
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Magnetism & Electricity
March 31, 2010
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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
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So, who can tell me…
What is magnetism?
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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…
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Getting loopy
What happens…If you wrap that wire into a loop?
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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
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Electro-magnetism?
Watch and see…Current through a wire In a magnetic field…
What do you see?
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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
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This is how our meters work!
Idea #1 – MAGNETIC FIELD AROUND A CURRENT CARRYING WIRECoil of wire and a magnetic needle
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Motors?
IDEA #2 – CURRENT IN A MAGNETIC FIELD CREATES A FORCEYes – the same concept is at work in a motor…
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Another look…in the opposite direction
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Motors: Take electrical energy and convert it to mechanical energy.
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Generators: Take mechanical energy and convert it to electrical energy.
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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…
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Magnetism & Electricity
April 2, 2010
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Plan of the Day
Review homework and labGenerators and MotorsLab
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Electro-magnetism - review
Current through a wire
In a magnetic field…
Force acts on itInteraction between 2 magnets!
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If the current is reversed…
The force pushes in the opposite direction.
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What is the difference…
Between a motor andA generator?
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Motors:
Take electrical energy Convert it into mechanical energy.
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Generators:
Take mechanical energy Convert it to electrical energy.
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Motors?
Current flows through the wire…A force acts on itOne side goes up
The other down
What has to happen at the bottom?
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Generator or Motor?
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Lab
Build a motor…(or was it a generator???)
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Transformers: Robots in Disguise
April 6, 2010
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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.
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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.
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Mechanical energy – water, wind, steam, etc.
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Steam is the driving force for most electrical power plants.
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Electricity is not a source of energy…
It is a form of energy that must have a source.
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Transformers:
A device for changing the voltage Through electromagnetic induction.
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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
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Relationship
Primary voltage = secondary voltage# of turns # of turns
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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.
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Power lines: high V, low i.
Stepped down to…
Electrical outlets: low V, high i.
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How does the bell work?
Hmmm…
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The Grid
April 8, 2010
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The Grid
How does electricity get from the generator
Like Bonneville Dam…
To your house or our school?
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The Grid
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The Power Plant
Start with some means of turning
HydroWindSteam turbine
Create electricity6,000 V range
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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
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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
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At your house
A transformer steps down the power
From 2,200 volts (typical)To 240 volts
And through the meter
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Got the picture?
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Review
What causes magnetism?What do we know about magnets?Which is stronger:
Gravity or magnetism?
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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?
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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?
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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?