UNIT FIVE: Electricity and Magnetism

2. UNIT FIVE: Electricity and Magnetism • Chapter 16 Electricity • Chapter 17 Magnetism

4. Chapter Seventeen: Magnetism • 17.1 Properties of Magnets • 17.2 Electromagnets • 17.3 Electric Motors and Generators • 17.4 Generating Electricity

5. 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.

6. Key Question: How do magnets and compasses work? Investigation 17A Magnetism

7. 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.

8. 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.

9. 17.1 Properties of Magnets • Whether the two magnets attract or repel depends on which poles face each other.

11. 17.1 Properties of Magnets • Magnetic forces can pass through many materials with no apparent decrease in strength.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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. 17.1 Geographic and magnetic poles • The planet Earth has a magnetic field that comes from the core of the planet itself.

18. 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!

20. 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.

21. 17.1 Declination and “true north” • Magnetic declination is measured in degrees and is indicated on topographical maps.

22. 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.

23. 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.

24. 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.

25. 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.