Chapter 2 Electromagnetism. Chapter 2 Bellringers Friday 9/11/09 What do you know about magnets?...
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Transcript of Chapter 2 Electromagnetism. Chapter 2 Bellringers Friday 9/11/09 What do you know about magnets?...
Chapter 2
Electromagnetism
Chapter 2 Bellringers
Friday 9/11/09
What do you know about magnets?
North and south poles
North and south poles attract
Like poles repel each other
Bar magnets and horseshoe magnets
Section 2-1: Magnets and Magnetism (Pgs. 40-46)
Properties of Magnets
Magnet- any material that attracts iron or things made of iron.
Properties of magnets
1. All have a south and north pole
2. Exert forces on each other
3. Surrounded by magnetic field
Magnetic Poles
A magnet’s magnetic forces are NOT the same throughout the magnet.
Strongest forces are near the ends of a magnet.
Each end is a magnetic pole
Magnetic poles- points on a magnet that have opposite magnetic qualities.
North and South
End of the magnet that points North is the magnet’s north pole
End of the magnet that points South is the magnet’s south pole
Poles always occur in pairs
Never find a magnet with just a south pole or just a north pole
Magnetic Forces
Magnetic force- attraction or repulsion created by spinning electric charges in the magnet.
Forces are always present in a magnet
Like poles repel each other
Opposite poles attract each other
Magnetic Fields
Magnetic field- region around a magnet’s pole in which magnetic forces act.
Shape of field can be shown with lines
Magnetic field lines- map the shape and strength of magnetic force on a magnet
Closer the lines, stronger the force
Lines closest at magnet’s poles
Monday 9/14/09
Name one property that all magnets have.
All have a north and south poleAll exert forces on each otherAll have magnetic fields
What creates the magnetic forces in a magnet?
Spinning electric charges from the electrons in the magnet’s atoms
Cause of Magnetism?
Whether or not a material is magnetic, depends on the material’s atoms
An atom’s moving electrons cause magnetism
As electrons move, it creates a magnetic field around the atom
Each atom becomes like a tiny magnet
Materials that are magnetic have their atoms lined up in domains
Domains- groups of atoms in small areas
Arrangement of domains determines whether an object is magnetic
North and south poles of all the atoms in a domain line up
Material is then magnetized
If poles do not line up, magnetic fields of all the atoms cancel out, and material is not magnetized
Losing Alignment
Magnets domains can move, making the magnet demagnetized
Domains can move by:
1. Dropping a magnet
2. Putting magnet in an opposite magnetic field
3. Increasing temperature
Higher temps cause atoms to vibrate faster causing them not to be lined up
Making a magnet
Item must have domains that move
Examples: iron, nickel, and cobalt
Rub one pole of a magnet in one direction on the item
Item’s domains become aligned with magnetic field of the magnet
Since domains are aligned, the item is magnetized
What can be picked up by a magnet?
Anything that will allow it’s domains to move
Example: paper clip
Bring magnet close to paper clip
Magnet’s magnetic field causes paper clip’s domains to align – causes paper clip to become temporary magnet
Paper clip’s north pole will attract to magnet’s south pole and vice versa
Tuesday 9/15/09
What must atoms do in order for an item to be magnetic?
Atoms must line up their north and south poles
Name three things that can demagnetize a magnet?
Drop itPut it in an opposite magnetic field
Increase the temperature
Cutting a magnet
End up with two separate magnets
Each piece has a north and south pole
REMEMBER Properties of Magnets
1. All have a south and north pole
Kinds of magnets
1. Ferromagnets - are made of iron, nickel, or cobalt
2. Electromagnets - are made by electric current
3. Temporary magnets – easy to magnetize, but lose magnetism quickly
4. Permanent magnets – difficult to magnetize, but keep magnetism longer
Earth as a Magnet
William Gilbert suggested Earth was a giant magnet.
Earth’s magnetic poles are near the geological poles
Magnetic south is in geological north
In order for a magnet’s north pole to point geological north, it must be attracted to a magnetic south
Magnetic north is in geological south
Why Earth is magnetic?
There is NO bar magnet in the Earth
Earth’s magnetic field is created by electric charges that exist in the Earth’s core
Outer core is liquid iron and cobalt and moves as Earth rotates.
As a result, electric charges also rotate creating a magnetic field
Magnetic Light Show
Aurora – beautiful display of lights that appear in the sky above the poles
Formed when charged particles from the sun hit oxygen & nitrogen atoms in the air
Earth’s magnetic field blocks charged particles from the sun
Earth’s Magnetic Field
So why are auroras seen at poles?
Earth’s magnetic field bends in at the poles
Charged particles able to enter the atmosphere near the poles
Northern lights (aurora borealis) appear at north pole
Southern lights (aurora australis) appear at south pole
Section 2-2 Magnetism from Electricity
Discovery of Electromagnetism
Discovered by Hans Christian Oersted
Held a compass near a wire carrying an electric current
Compass needle did not point north
Only thing that can cause this is another magnetic field
Oersted concluded that electric current produces a magnetic field
Also found that the direction of the magnetic field depended on direction of the current.
Run current one direction, compass needles turn in a clockwise direction
Run current the other direction, compass needles turn counterclockwise direction
Electromagnetism – the interaction between electricity and magnetism
Thursday 9/17/09How did Oersted discover that electric current produces a magnetic field?
Held a compass near a current carrying wire and compass did not point North
What determines the direction of a magnetic field in a current carrying wire?
The direction of the current
Using Electromagnetism
Magnetic field generated by an electric current is weak.
Solenoids and electromagnets strengthen this magnetic field
Maglev (magnetic levitation) trains are pulled down the track by an electromagnet
Solenoids
Solenoid- coil of wire that produces a magnetic field when carrying electric current.
Magnetic field produced by a solenoid can be increased two ways
1. Increase the numbers of loops in the solenoid
2. Increase the current running through the wire
Electromagnets
Electromagnet- a solenoid wrapped around an iron rod.
Magnetic field of the solenoid makes the atoms in the iron line up.
Both the solenoid and the iron now have a magnetic field
The two magnetic fields combine to create a strong magnetic field for the electromagnet
Magnetic field of an electromagnet can be made even stronger
Same ways as a solenoid
1. Increase the number of loops in the solenoid
2. Increase the current running through the solenoid
Turning Electromagnets On and Off
Solenoid only has magnetic field if electric current is in the wire
Turn on current, solenoid has a magnetic field and the iron also has a magnetic field
Turn off current, magnetic field is no longer present and iron is not magnetic
What is a coil of wire with an electric current?
Solenoid
What do we add to a solenoid to make it an electromagnet?
Iron
What two things increase the magnetic fields of solenoids and electromagnets?
Increase number of loops
Increase current
Applications of Electromagnetism
A current carrying wire can cause a compass needle to move
If a current carrying wire can cause a magnet to move, can a magnet cause a current carrying wire to move?
Yes, and this is useful in electric motors
Electric Motors
Electric Motors - device that changes electrical energy into mechanical energy.
All have a loop of wire that can rotate
Located between the poles of a permanent magnet
Loop of wire is attached to a battery
Wire now has a current running through it so it also has a magnetic field
Since it is in between the poles of a permanent magnet, the permanent magnet pulls on the loop of wireand rotates it
Galvanometers
Galvanometers- uses an electromagnet to measure electric current
Has electromagnet placed between poles of a permanent magnet
Electromagnet also attached to a needle
When current runs through, electromagnet turns on and is pulled by permanent magnet moving needle
Section 2-3 Electricity from Magnetism (Pgs. 54-59)
Electric Current from a Changing Magnetic Field
Oersted discovered that an electric current could produce a magnetic field.
If an electric current can produce a magnetic field, can a magnetic field produce an electric current?
Michael Faraday’s experiments answer this question
Faraday’s Experiment
Wrapped a wire attached to a battery around an iron ring creating an electromagnet
Wrapped a second wire around the other side of the ring and attached it to a Galvanometer.
Galvanometer measured no electric current in the second wire
Would appear magnetism cannot create an electric current, until Faraday discovered something.
Faraday’s Discovery
The instant he connected or disconnected the wire to the battery, the galvanometer pointer moved.
As long as battery was fully connected, no electric current was measured by the
galvanometer.
Electric current is ONLY made when magnetic field is changing.
Electromagnetic Induction- the process by which an electric current is made by
changing a magnetic field.
Inducing Electric Current
Move either the wires or the magnet
Move a magnet through the loops of a wire
Move a wire between the poles of a magnet
Either way, magnetic field is changing and electric current is produced in the wires.
Charges in a wire only move when they pass through a magnetic field.
How to increase the electric current
1. Move magnet through the loops faster
2. Add more loops to the wire
3. Move wire faster between the poles of the magnet
Wednesday 9/23/09
In order for a magnetic field to produce an electric current, what must happen?
Magnetic field must be changing
What can you do to increase the current produced by a magnetic field.
Move the magnet or wires faster
Add more loops of wire
Electric Generators
Electric Generator- uses electromagnetic induction to change mechanical energy into electrical energy
A wire sits between the poles of a permanent magnet.
A crank turns the wire.
Because the wire keeps moving through the magnetic field, a current is produced in the wire.
Generators Produce Alternating current
Every half turn, the wire crosses the magnetic field in a different direction
Current goes opposite direction
As a result, the electric current produced is an alternating current
Alternating current MUST be produced in order for transformers to work.
Power Plant Generators
Use many coils of wire instead of one
Rotate magnets around the wire instead of rotating wire inside of the magnet
Use many sources to turn the turbines that turn the magnets
1. Steam from nuclear reactions
2. Water
3. Wind
Transformers
Transformers- increases or decreases the voltage of
alternating current.
Two separate coils of wire wrapped around an iron ring
Primary (first) coil receives an alternating current from the generator
Current produces a magnetic field in the iron ring
Because current is alternating, magnetic field of iron ring is constantly changing direction.
That changing magnetic field allows an electric current to be produced in the
secondary (second) wire.
If current was not alternating, no current would be produced in second wire.
Thursday 9/24/09
What type of current do generators produce?
Alternating current
What increases and decreases the voltage of alternating current?
Transformers
Step-Up, Step-Down
Number of loops in primary and secondary coils determines whether voltage is
increased or decreased.
Step-up Transformer- increases the voltage and decreases current
More loops in secondary coil causing voltage to increase
Step-Down Transformers – decreases voltage and increases current
Less loops in the secondary coil causes voltage to decrease
Amount of power (W) stays constant
Power = current (I) x voltage (V)
So if voltage goes down, current must go up and visa versa.
Why Increase and Decrease Voltage?
Voltage must be increased after leaving power plant in order to travel long
distances on power lines
Voltage is stepped down before it enters your house to keep appliances from
blowing up