Unit 11

1. Unit 11 Magnetism

2. Magnets • magnetism – force of attraction or repulsion • not all objects are affected by the force of magnetism • ex. wood, glass, paper, plastic • common metals affected by magnetism are iron, nickel, and cobalt

3. Magnets • poles – two ends of a magnet • every magnet has two poles • north (N) pole • south (S) pole • even if you break a magnet in half, each half will have a north pole and a south pole

4. Magnets • the north pole of a magnet points North • it is often called “North-seeking”

5. Properties of Magnets • if a north pole and a south pole are brought together, they will attract each other • opposites attract

6. Properties of Magnets • if the north pole of one magnet is brought near the north pole of another magnet, they will repel each other • if two south poles are brought together, they will repel each other

7. Properties of Magnets • like magnetic poles repel each other • unlike magnetic poles attract each other

8. Magnetic Fields • magnetic field – area around a magnet where magnetic forces can act • a magnetic field is made up of magnetic lines of force

9. Magnetic Fields • magnetic lines of force – lines that show the shape of a magnetic field • the magnetic lines of force are closest together at the poles of the magnet • this is where the magnet is strongest

10. Making a Magnet • some magnets occur in nature • these magnets are called natural magnets • ex. magnetite (also called lodestone)

11. Making a Magnet • materials that are not natural magnets can be magnetized

12. Making a Magnet • magnetic induction – process by which a material can be made into a magnet

13. Making a Magnet • some materials are easy to magnetize • ex. iron

14. Making a Magnet • a material that is easily magnetized tends to lose its magnetism quickly • a magnet made of this kind of material is called a temporary magnet

15. Making a Magnet • materials that are hard to magnetize will also stay magnetized for a long time • a magnet that is hard to magnetize but tends to keep its magnetism is called a permanent magnet

16. The Earth as a Magnet • William Gilbert • a British scientist • the Earth has north and south poles like a bar magnet

17. The Earth as a Magnet • the Earth has a North Magnetic Pole and a South Magnetic Pole • the North Magnetic Pole is located near the geographic North Pole • the South Magnetic Pole is located near the geographic South Pole

18. The Earth as a Magnet • the North Magnetic Pole is like the south pole of a bar magnet • the South Magnetic Pole is like the north pole of a bar magnet

19. The Earth as a Magnet • the Earth is surrounded by a magnetic field which extends far into space • magnetosphere – region of the Earth’s magnetic field

20. The Earth as a Magnet • the magnetosphere traps charged particles from the sun • when these particles enter the atmosphere, an aurora is formed • auroras are also called the northern and southern lights

21. Electricity and Magnetism • Hans Oersted • about 200 years ago • an electric current flowing in a wire causes a compass needle to move

22. Electricity and Magnetism • an electric current passing through a wire causes a magnetic field

23. Electricity and Magnetism • electromagnetism – relationship between electricity and magnetism

24. Electricity and Magnetism • Michael Faraday (British scientist) • Joseph Henry (American scientist) • when a wire is moved across a magnetic field, an electric current is induced in the wire

25. Electricity and Magnetism • electromagnetic induction – process by which an electric current is produced by moving a wire in a magnetic field

26. Electricity and Magnetism • a wire carrying an electric current always has a magnetic field around it • the magnetic field in a straight wire is not very strong • if the wire is wound into a coil, the magnetic field becomes much stronger as the individual magnetic fields overlap • the greater the number of coils, the stronger the magnetic field

27. Electromagnets • electromagnet – temporary magnet made by wrapping a current-carrying wire around an iron core • the center of an electromagnet is called the core • it is often made of iron

28. Electromagnets • as long as current is flowing, an electromagnet has a magnetic field • when current is turned off, there is no longer a magnetic field

29. Electromagnets • there are two ways to make an electromagnet stronger • increasing the number of coils • increasing the amount of current

30. Electromagnets • electromagnets are useful because they can be turned on and off • electromagnets have many important uses • ex. radios, telephones, computers

31. Electromagnets • changing the direction of an electric current causes the poles of an electromagnet to reverse • this feature is important in the production of electric motors

32. Transformers • transformer – device in which alternating current in one coil of wire induces a current in a second coil

33. Transformers • a transformer consists of two coils of wire around a magnet • the first coil, called the primary coil, is connected to the power source • the second coil, called the secondary coil, is connected to the load • a load is something that uses electricity, such as a light bulb or a motor

34. Transformers • a transformer only works with alternating current

35. Transformers • a step-up transformer increases voltage • it consists of more coils of wire in the secondary coil than the primary coil

36. Transformers • a step-down transformer decreases voltage • it consists of more coils of wire in the primary coil than the secondary coil

37. Electric Motors • electric motor – device that changes electrical energy into mechanical energy

38. Electric Motors • an electric motor is made up of an electromagnet and a permanent magnet

39. Electric Motors • the electromagnet is free to turn and is hooked up to a source of alternating current • changing the direction of an electric current causes the poles of an electromagnet to reverse

40. Electric Motors • attraction and repulsion between the electromagnet and the permanent magnet cause the electromagnet to spin

41. Electric Motors • an electric motor can use direct current if a special switch is used

42. Generators • generator – device that changes mechanical energy into electrical energy • a current can be induced in a loop of wire by spinning the loop inside a magnetic field • due to the changing direction of the wire, an alternating current is produced

43. Generators • an electric generator is made up of an insulated loop of wire and a U-shaped magnet • spinning the loop of wire in the magnetic field of the magnet produces an electric current

44. Generators • most of the energy we use every day comes from generators

45. Generators • mechanical energy for many generators is supplied by turbines • a turbine is a large wheel that is turned by moving steam or water