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.
Key Question: How do magnets and compasses work? Investigation 22A Magnetism
22.1 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.
22.1 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.
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 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.
22.1 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.
22.1 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.
22.1 Magnetic fields • You can actually see the pattern of the magnetic field lines by sprinkling magnetic iron filings on cardboard with a magnet underneath.
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 directlyto 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 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.
22.1 Earth’s magnetism • The 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 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.