ELECTRICITY AND MAGNETISM

1. ELECTRICITY AND MAGNETISM

2. Chapter Twenty-Two: Electricity and Magnetism • 22.1 Properties of Magnets • 22.2 Electromagnets • 22.3 Electric Motors

3. 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.

4. Key Question: How do magnets and compasses work? Investigation 22A Magnetism

5. 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.

6. 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.

7. 22.1 Properties of Magnets • Whether the two magnets attract or repel depends on which poles face each other.

9. 22.1 Properties of Magnets • Magnetic forces can pass through many materials with no apparent decrease in strength.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 22.1 Geographic and magnetic poles • The planet Earth has a magnetic field that comes from the core of the planet itself.

16. 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!

18. 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.

19. 22.1 Declination and “true north” • Magnetic declination is measured in degrees and is indicated on topographical maps.

20. 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.

21. 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. 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.

23. 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.

24. Chapter Twenty-Two: Electricity and Magnetism • 22.1 Properties of Magnets • 22.2 Electromagnets • 22.3 Electric Motors

25. Chapter 22.2 Learning Goals • Define electromagnet. • Build a simple electromagnet. • Compare permanent magnets and electromagnets.

26. Key Question: How are electricity and magnetism related? Investigation 22B Electromagnets

27. 22.2 Electomagnets • Electromagnets 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.

28. 22.2 Right hand rule • To 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.

30. 22.2 Electromagnets in Toasters • By 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 is pulled down by an electromagnet while bread is toasting.

31. 22.2 Doorbells • A doorbell contains an electromagnet. • When the button of the bell is pushed, it sends current through the electromagnet.

32. 22.2 Building an electromagnet • You 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.

33. 22.2 Building an electromagnet • There are two ways to increase the current in a simple electromagnet: • Apply more voltage by adding a second battery. • Add more turns of wire around the nail. Why do these two techniques work?

34. 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.

35. 22.2 Magnetic materials • Atoms act like tiny magnets. Permanent magnets have their atoms aligned, creating the magnetic forces we observe.

36. 22.2 Magnetic materials • In iron, the atoms are free to rotate and easily align their individual north and south poles.

37. 22.2 Nonmagnetic materials • The atoms in non-magnetic materials, like plastic, are not free to move or change their magnetic orientation.

39. Chapter Twenty-Two: Electricity and Magnetism • 22.1 Properties of Magnets • 22.2 Electromagnets • 22.3 Electric Motors

40. 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.

41. 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.

42. 22.3 Electric motors • Around the edge of a disk are several magnets, their alternating north and south poles facing out.

43. 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.

44. 22.3 Electric motors • In 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.

45. 22.3 Electric motors • As the rotor spins, a commutator reverses the direction of the current in the electromagnet.

46. 22.3 Electric motors Motors have three parts: • A rotor with magnets that alternate. • One or more fixed magnets around the rotor. • A commutator that switches the direction of current to keep the rotor spinning.

48. 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.

49. 22.3 Battery run electric motors • A simple battery powered motor has three electromagnets.

50. 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.