UNIT FIVE: Electricity and Magnetism

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UNIT FIVE: Electricity and Magnetism. Chapter 16 Electricity Chapter 17 Magnetism. Chapter Seventeen: Magnetism. 17.1 Properties of Magnets 17.2 Electromagnets 17.3 Electric Motors and Generators 17.4 Generating Electricity. Chapter 17.1 Learning Goals.

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UNIT FIVE: Electricity and Magnetism
• Chapter 16 Electricity
• Chapter 17 Magnetism
Chapter Seventeen: Magnetism
• 17.1 Properties of Magnets
• 17.2 Electromagnets
• 17.3 Electric Motors and Generators
• 17.4 Generating Electricity
Chapter 17.1 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.
Key Question:

How do magnets and compasses work?

Investigation 17A

Magnetism

17.1 Properties of Magnets
• If 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.
17.1 Properties of Magnets
• All 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.
17.1 Properties of Magnets
• Whether the two magnets attract or repel depends on which poles face each other.
17.1 Properties of Magnets
• Magnetic forces can pass through many materials with no apparent decrease in strength.
17.1 Properties of Magnets
• Magnetic 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.
17.1 Magnetic fields
• The 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.
17.1 Magnetic fields
• A special kind of diagram is used to map the magnetic field.
• The force points away from the north pole and towards the south pole.
17.1 Magnetic fields
• You can actually see the pattern of the magnetic field lines by sprinkling magnetic iron filings on cardboard with a magnet underneath.
17.1 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.
17.1 Geographic and magnetic poles
• The planet Earth has a magnetic field that comes from the core of the planet itself.
17.1 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!

17.1 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 directlyto the geographic north pole.
• The difference between the direction a compass points and the direction of true north is called magnetic declination.
17.1 Declination and “true north”
• Magnetic declination is measured in degrees and is indicated on topographical maps.
17.1 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.
17.1 Earth’s magnetism
• Studies 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.
17.1 Earth’s magnetism
• The gaussis 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.
17.1 Earth’s magnetism
• Today, 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.