ELECTRICITY AND MAGNETISM 22.1
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Transcript of ELECTRICITY AND MAGNETISM 22.1
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ELECTRICITY AND MAGNETISM 22.1
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Chapter Twenty-Two: Electricity and Magnetism
22.1 Properties of Magnets
22.2 Electromagnets
22.3 Electric Motors
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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.
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Investigation 22A
Key Question:How do magnets and compasses work?
Magnetism
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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.
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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.
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22.1 Properties of Magnets
Whether the two magnets attract or repel depends on which poles face each other.
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22.1 Properties of Magnets
Magnetic forces can pass through many materials with no apparent decrease in strength.
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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.
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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.
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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.
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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.
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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.
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22.1 Geographic and magnetic poles
The planet Earth has a magnetic field that comes from the core of the planet itself.
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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!
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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.
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22.1 Declination and “true north”Magnetic declination is measured
in degrees and is indicated on topographical maps.
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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.
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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.
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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.
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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.