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ELECTRICITY AND MAGNETISM
Chapter Twenty-Two: Electricity and Magnetism
22.1 Properties of Magnets
22.2 Electromagnets
22.3 Electric Motors
Chapter 22.1 Learning Goals
Identify properties of magnetic materials and use interactions between magnets to explain attraction and repulsion.
Describe the source of Earth’s magnetism.
Explain how a compass works.
Investigation 22A
Key Question:How do magnets and compasses work?
Magnetism
22.1 Properties of MagnetsIf a material is magnetic, it has the
ability to exert forces on magnets or other magnetic materials nearby.
A permanent magnet is a material that keeps its magnetic properties.
22.1 Properties of MagnetsAll magnets have
two opposite magnetic poles, called the north pole and south pole.
If a magnet is cut in half, each half will have its own north and south poles.
22.1 Properties of Magnets
Whether the two magnets attract or repel depends on which poles face each other.
22.1 Properties of Magnets
Magnetic forces can pass through many materials with no apparent decrease in strength.
22.1 Properties of MagnetsMagnetic forces are used
in many applications because they are relatively easy to create and can be very strong.
Large magnets create forces strong enough to lift a car or a moving train.
22.1 Magnetic fieldsThe force from a magnet gets weaker as it gets farther away.
Separating a pair of magnets by twice the distance reduces the force by 8 times or more.
22.1 Magnetic fieldsA special kind of
diagram is used to map the magnetic field.
The force points away from the north pole and towards the south pole.
22.1 Magnetic fieldsYou can actually see the pattern of the magnetic field lines by sprinkling magnetic iron filings on cardboard with a magnet underneath.
22.1 Magnetic field lines
A compass needle is a magnet that is free to spin.
Because the needle aligns with the local magnetic field, a compass is a great way to “see” magnetic field lines.
22.1 Geographic and magnetic poles
The planet Earth has a magnetic field that comes from the core of the planet itself.
22.1 Geographic and magnetic poles
The names of Earth’s poles were decided long before people understood how a compass needle worked.
The compass needle’s “north” end is actually attracted to Earth’s “south” magnetic pole!
22.1 Declination and “true north”Because Earth’s geographic north
pole (true north) and magnetic south pole are not located at the exact same place, a compass will not point directly to the geographic north pole.
The difference between the direction a compass points and the direction of true north is called magnetic declination.
22.1 Declination and “true north”Magnetic declination is measured
in degrees and is indicated on topographical maps.
22.1 Declination and “true north”Magnetic declination is
measured in degrees and is indicated on topographical maps.
Most good compasses contain an adjustable ring with a degree scale used compensate for declination.
22.1 Earth’s magnetismStudies of earthquake waves reveal that the Earth’s core is made of hot, dense molten metals.
Huge electric currents flowing in the molten iron produce the Earth’s magnetic field.
22.1 Earth’s magnetismThe gauss is a unit used to measure the strength of a magnetic field.
The magnetic field of Earth (.5 G) is weak compared to the field near the ceramic magnets you have in your classroom. (300- 1,000 G).
For this reason you cannot trust a compass to point north if any other magnets are close by.
22.1 Earth’s magnetismToday, Earth’s
magnetic field is losing approximately 7 percent of its strength every 100 years.
If this trend continues, the magnetic poles will reverse sometime in the next 2,000 years.
Chapter Twenty-Two: Electricity and Magnetism
22.1 Properties of Magnets
22.2 Electromagnets
22.3 Electric Motors
Chapter 22.2 Learning Goals
Define electromagnet.
Build a simple electromagnet.
Compare permanent magnets and electromagnets.
Investigation 22B
Key Question:How are electricity and magnetism related?
Electromagnets
22.2 ElectomagnetsElectromagnets are magnets that are created when there is electric current flowing in a wire.
The simplest electromagnet uses a coil of wire wrapped around some iron.
22.2 Right hand ruleTo find the north pole
of an electromagnet, use the right hand rule.
When the fingers of your right hand curl in the direction of the wire, your thumb points toward the magnet’s north pole.
22.2 Electromagnets in ToastersBy changing the
amount of current, you can easily change the strength of an electromagnet or even turn its magnetism on and off.
A toaster tray ispulled down by an electromagnet whilebread is toasting.
22.2 Doorbells
A doorbell contains an electromagnet.
When the button of the bell is pushed, it sends current through the electromagnet.
22.2 Building an electromagnetYou can easily
build an electromagnet from wire and a piece of iron, such as a nail.
Wrap the wire in many turns around the nail and connect a battery.
22.2 Building an electromagnet There are two ways to
increase the current in a simple electromagnet:
1. Apply more voltage by adding a second battery.
2. Add more turns of wire around the nail.
Why do these two techniques work?
22.2 Similarities in permanent and electromagnets The charged electrons in atoms
behave like small loops of current.
Electric current through loops of wire creates an electromagnet.
Atomic-scale electric currents create a permanent magnet.
22.2 Magnetic materials
Atoms act like tiny magnets.
Permanent magnets have their atoms aligned, creating the magnetic forces we observe.
22.2 Magnetic materials In iron, the atoms are free to
rotate and easily align their individual north and south poles.
22.2 Nonmagnetic materials
The atoms in non-magnetic materials, like plastic, are not free to move or change their magnetic orientation.
Chapter Twenty-Two: Electricity and Magnetism
22.1 Properties of Magnets
22.2 Electromagnets
22.3 Electric Motors
Chapter 22.3 Learning Goals
Describe the role of electromagnetism in the function of electric motors and generators.
Contrast energy transformations in electric motors and generators.
Explain the principle of electromagnetic induction.
22.3 Electric motors and generators
Permanent magnets and electromagnets work together to make electric motors and generators.
The secret is in the ability of an electromagnet to reverse its north and south poles.
22.3 Electric motors
Around the edge of a disk are several magnets, their alternating north and south poles facing out.
22.3 Electric motors
To make the disk spin, you bring a permanent magnet close to its edge.
The free magnet attracts one of the magnets in the disk and repels the next one. The disk is a “rotor”
because it rotates.
22.3 Electric motorsIn a working electric
motor, an electromagnet replaces the magnet you reversed with your fingers.
The electromagnet switches its poles to make the rotor keep turning.
22.3 Electric motors
As the rotor spins, a commutator reverses the direction of the current in the electromagnet.
22.3 Electric motorsMotors have three
parts:1. A rotor with magnets
that alternate.
2. One or more fixed magnets around the rotor.
3. A commutator that switches the direction of current to keep the rotor spinning.
22.3 Battery run electric motors An electric motor that runs from
batteries has the same three parts.
The permanent magnets are on the outside, and the electromagnets turn in the rotor.
22.3 Battery run electric motors
A simple battery powered motor has three electromagnets.
22.3 Electromagnetic induction Motors transform electrical energy
into mechanical energy.
Electric generators do the opposite.
They transform mechanical energy into electrical energy.
The process of using a moving magnet to create electric current is called electromagnetic induction.
22.3 Electromagnetic induction A moving magnet produces a
current in a coil of wire.
22.3 Generating electricity A generator converts mechanical
energy into electrical energy using the law of induction.
As long as the disk is spinning, there is a changing magnetic field through the coil and electric current is created.
Investigation 22C
Key Question:How does an electric motor work?
Electromagnetic Forces
Under the SeaHave you ever wondered
what it would be like to take a “walk” on the seafloor? Engineer Jim Varnum gets to go “down under” regularly, because he is a pilot of an extremely complex robot called Jason II. This robot dives deep into the ocean and is used to take pictures and collect data from the seafloor.
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