Magnetism
Magnets
Magnetic Poles
• Magnets named from Magnesia, Greece
• Lodestones found there almost 3000 years ago
Magnets have a North Pole and a South Pole
– Lodestones are naturally occurring rocks based on the mineral magnetite, Fe3O4 or iron(II,III) oxide
Opposite poles attract
Like poles repel
Poles cannot be separated. Breaking a magnet in half makes two smaller magnets.
Lodestone
→
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Magnetic Domains
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Magnetic Domains
Domains are small magnetized regions in a material. They are formed from groups of atoms that are lined up in the same way.
Magnet strength comes from more domains lined up
Magnet strength comes from more domains lined up. • Domains are very small – so, a small sample of iron has a huge number of domains.
• In a magnet the domains are always lined up.
• A magnetic field can force atoms to line up in other substances.
• A shock (physical or electrical) and high temperatures will cause demagnetization.
Magnetic
Not Magnetic
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Magnetization
“Permanent” Magnets are materials that can have their own magnetic field. They can be made of • iron
• cobalt
• nickel
i k l
A magnet can lose its field or have it weakened by:
• Heating the to high temperatures.
• Hammering or hard impacts. (Don’t drop them!)
• Storing the magnets with repelling poles.
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Magnetism
Magnet Types
Magnetic Field
Hard Magnets take a long time to magnetize because the domains do not move easily. Once magnetized, they do not lose their strength easily. Hard magnets are used in permanent magnets.
Magnetic Fields are where magnetic forces can be felt.
Soft Magnets can be magnetized quickly. They can lose their magnetic field easily.
Soft magnets are used for electromagnets so the field can be turned on and off.
Magnetic fields can be created by:
1 Magnetic domains (electron spins of an atom)
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Magnetic domains (electron spins of an atom)
2. Moving charges, such as an electric current
Magnetic fields get weaker at a greater distance from the magnet 7
Magnetic Field Lines
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Viewing a Magnetic Field • Lines represent where magnetic force is felt
• Closed loops leaving north pole to south pole
• More lines = more strength
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Opposites Attract
Bar Magnet
Wire with Current
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A compass can be used to find field lines
Likes Repel
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Bar Magnet
Wire with Current
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Magnetism
Earth is a Magnet
The Earth as a magnet
Geographic North Pole
Magnetic South Pole
S
N
• The North magnetic pole is in Antarctica
• The South magnetic pole is in Northern Canada
We know this because the North part of a compass will seek a south pole
Magnetic poles move as much as 15 km in a year
The poles have shifted sides many times. This occurs at an average of 300,000 years
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Electromagnetism
Geographic South Pole
Magnetic North Pole
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Electromagnetic Field
Electromagnetism is the magnetic field created from an electric current traveling through a wire
In 1820, Hans Christian Oersted, a Danish scientist, discovered that moving electric charges in a wire create magnetic fields. By using a compass, Oersted found the magnetic field runs counterclockwise to the direction of current.
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The Right Hand Rule
The Right Hand Rule can be used to help remember the direction of the magnetic Field.
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Viewing an Electromagnetic Field Wrap your right hand with your thump pointing up.
The thumb points in the
The thumb points in the direction of current
Fingers point the direction of the magnetic field
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Compasses
Metal Filings
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Magnetism
Solenoid
Solenoid
Current in
Current out
A Solenoid is a coil of wire carrying an electric current and will have a similar magnetic field to a bar magnet
A solenoid can increase its magnetic field by:
1. Adding more loops of wire
2. Increasing the electric current
S
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Electromagnet
N
The Poles depend on the direction of current and the direction of the wire wrap
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Electromagnet
An Electromagnet is a solenoid with a soft iron core in the middle. By adding the iron core, the magnetic field is much stronger by thousands of times.
The more loops of wire, the stronger the magnet.
A great advantage of an electromagnet is the ability to turn it on or off magnetism as needed. Switching the battery around will change poles
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Right Hand Rule 2
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Forces on Currents in Magnetic Fields
I = Current
F = I L B
Use the Third Right Hand Rule for Direction!
F = Force (N)
I = Current (A)
Curl your fingers in the direction of current
Your thumb will point North
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L = Length of Wire (m)
B = Magnetic Field (T or N / A∙m)
T is for a Tesla
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Magnetism
Right Hand Rule 3
Current Carrying Conductors
F = Force B = Magnetic Field I = Current
Your fingers point in the direction of the magnetic field.
Your thumb points in the direction of current.
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Your palm will point in the direction of force.
Basic DC Electric Motor
Current in Same Direction Attracts
Current in Opposite Direction Repels
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DC Electric Motor
An Electric Motor converts electrical energy into mechanical energy: Electricity → Mo on
Motors use a commutator and brushes to work.
p
Commutator – part of a motor used to make the current change direction every half turn. This allows a motor to make complete turns.
Brushes – part of a motor that connects wires to the commutator
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DC Electric Motor
How a Motor Works
Armature
Brush
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With power, the Armature becomes a magnet. The left side of the armature is pushed away from the left magnet and drawn toward the right. This causes half of a rotation.
Commutator
Armature– part of a motor that that is wrapped in wire and becomes magnetized by the electric current
The commutator reverses the direction of current and the magnetic field. The process repeats.
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Magnetism
Electromagnetic Induction
Electromagnetic Induction
Electromagnetic Induction: the process of creating a current in a circuit by changing a magnetic field. Moving a magnet into and out of a coil of wire causes charges in the wire to move.
Faraday’s law states: An electric current can be produced in a circuit by a changing magnetic field crossing the circuit.
p
g
As the loop moves in and out of the magnetic field of the magnet, a current is induced in the circuit. Rotating the circuit or changing the strength of the magnetic field will also induce a current in the circuit.
Electromagnetic induction obeys conservation of energy. Pushing a loop through a magnetic field requires work.
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Electromagnetic Induction
Across Magnetic Field: Maximum Current
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Generator
With Magnetic Field: No 33
Current Flow
Generators
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Alternating Current
generator: a machine that converts mechanical energy to electrical energy
AC generators produce the electrical energy you use in your home. The current operates at 60Hz.
alternating current (AC): an electric current that changes direction at regular intervals For each
changes direction at regular intervals. For each half rotation of the loop, the current produced by the generator reverses direction.
This means the wall outlet will change current from 120 volts to ‐120
120 volts to 120 volts 60 times every second. volts 60 times every second
The cycle in your home looks like:
120 volts → Off → ‐120 volts → Off → Repeat
Current flow in an AC circuit constantly moves back and forth rather than completely through the circuit. 35
The lights in your house flicker off 120 time a second.
This happens so fast that we cannot notice flickering.
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Magnetism
Mechanical Energy Sources
A variety of mechanical energy sources are used to generate electricity for homes. Each of these involve turning a turbine to generate electricity.
Some examples include:
Gas power plants
Gas power plants
Coal Power Plants
Nuclear Power Plants
Wind Power
Hydroelectric power (Dams)
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Transformers
Solar Energy uses a different method and does not use a generator.
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Step Up Transformer
Transformers can step up or step down the voltage. This is based on the ratio of turns between the primary and secondary coils.
On a telephone pole
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Transformers
Step Down Transformer
Np
Ns
N = Number of turns
N
= Number of turns
V = voltage
I = Current
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
Vp
Vs

Is
Ip
p = primary coil
p
primary coil
s = secondary coil
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