Magnetism

1. Magnetism

2. Magnets have been known for centuries. The Chinese and Greeks knew about the “magical” properties of magnets. The ancient Greeks used a stone substance called “magnetite.” They discovered that the stone always pointed in the same direction. Later, stones of magnetite called lodestones” were used in navigation.

3. Lodestone A naturally occurring magnetic rock, composed of iron based material called magnetite 1

4. What is Magnetism? Magnetism is the force of attraction or repulsion of a magnetic material due to the arrangement of its atoms, particularly its electrons. 2

6. Properties of Magnets

7. They attract magnetic materials • A material that shows strong magnetic effects is said to be a ferromagnetic material. • Common ferromagnetic materials: • • Iron • • Nickel • • Cobalt You can make anything that's not magnetized -- but made out of iron, nickel, or cobalt -- a magnet. 3a,3b

8. They have2 magnetic poles (dipole); • Each magnet, no matter what its shape has 2 poles so they are called magnetic dipoles. • One pole of the magnet will always point north and is labeled the north pole. The other end is labeled the south pole. • The poles of the magnet are where the magnetic effect is greatest. • The poles are found very near (but not at) the ends of the magnet. 4a, 4b, 4c, 4d

9. 3. Like poles repel, Unlike poles attract Like repels like… Opposites attract! 5

10. If you cut a magnet in half, you get 2 magnets! No matter how many times a magnet is cut in half, each piece retains its magnetic properties. 6

11. Magnetic Fields

12. Magnetic Fields The region where the magnetic forces act and can be detected is called the magnetic field. Themagnetic field is a vector. 7

13. Magnetic field lines are imaginary lines of force. They are also called Fluxlines. • Magnetic field lines spread out from one pole, curve around a magnet, and return to the other pole. • Flux lines always point N to S outside a magnet. • The lines form complete loops from pole to pole and never cross. • The closer the flux lines, the stronger the field. • Magnetic fields act in three dimensions. • You can NOT see magnetic fields, but you can see the effects. 7a,7b,7c,7d,7e,7f,7g

15. The magnetic field around a magnetic configuration may be mapped using field lines in the same way an electric field is mapped. 8

16. A compass is simply a thin magnet or magnetized iron needle balanced on a pivot. It can be used to detect magnetic fields. The needle will rotate to point toward the opposite pole of a magnet. It can be very sensitive to small magnetic fields. 9

17. The magnetic field of a bar magnet can be traced with a compass. The north pole of a compass points in the direction of the magnetic field from the magnet’s north pole to its south pole. 10

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19. Magnetic Phenomena

20. All magnetic results from forces betweenmoving electrical charges. 11

21. The magnetic properties of a material depend on the structure of its atoms. 12

22. Most materials have paired up electrons spinning in opposite directions 13

23. The magnetic fields of two electrons with opposite spins cancel one another; there is no net magnetic field for the pair. 14

24. Some materials like Iron, Nickel, Cobalt and Neodymium • Have a single electron or paired up electrons spinning in the same direction. This creates a small magnetic field. • The magnetic fields created by one does not cancel the other. • An atomic sized magnet is created. 15

26. In certain materials, the magnetic fields of the spinning electrons of many atoms are aligned with one another. Groups of billions of atoms that all have magnetic fields that are lined upin thesamedirection are known asmagnetic domainswithin a sample of a material. 16

27. Domain Domain When an unmagnetized substance isplaced in a magnetic field,the substance can become magnetized. When the spinningelectronsin the magnetic domains line up in the same direction, a magnet is created. 17

28. An unmagnetized substance looks like this… While a magnetized substance looks like this…

30. Ferromagnetic iron particles (3-10 micron in diameter) are suspended in a carrier liquid.

31. Magnetic Flux Lines In A Motor

32. Some metals, such as the ordinary steel that paper clips are made of, are easy to magnetize but lose their magnetism quickly. Materials that lose their magnetism quickly are called temporarymagnets. 18

33. Harder metals are more difficult to magnetize but tend to stay magnetized. A magnet made up of material that keeps its magnetism is called a permanent magnet. In a permanent magnet, the domain alignment persists after the external magnetic field is removed. In a permanent magnet, the magnetic field comes from the motion of the electrons inside the material, or more precisely, the electron spin. 19

34. Magnetic forces get weaker with distance. 20

35. When you move a magnet, the magnetic field spreads out around the magnet at the speed of light. The speed of light is 300 million km per second. 21

36. How does a magnet attract another object? The magnet’s field causes the object to become a magnet.

37. How to break a magnet: 1. Drop it Causes the domains to be knocked out of alignment. 2. Heat it They vibrate faster and more making it more difficult for the domains randomly thus to staying alignment. 22

38. The Earth Is A Magnet

39. In 1600 Sir William Gilbert confirmed that a compass always points north but no one knew why. Gilbert suggested that a compass always points north because the Earth acts as a magnet. 23

40. While the Earth’s core is magnetic, we know it is not a solid permanent magnet. Studies of the Earth’s core reveal that it is made of hot, dense molten iron, nickel and possibly other metals that slowly circulate around a solid inner core. Huge electric currents flowing through the circulating molten metal (iron and nickel) produce the Earth’s magnetic field. 24

41. The earth’s magnetic field is immense. The Earth’s magnetic field is strongest near the North and South magnetic poles. 25

42. Earth’s magnetic poles are not the same as the geographic poles. The magnetic north pole (in northern Canada) is about 1,250 kilometers from the geographic north pole. Geographic North is sometime called true North. The magnetic south pole is near the coast of Antarctica. 26, 27

44. We use the Earth’s magnetic field to find direction. Magnetic north is where a compass needle points, and it is actually Earth’s magnetic south pole. 29, 30

45. Magnetic Declination-the angle between geographic north and the north to which a compass needle points. Magnetic declination changes over time because the magnetic poles move slowly. Between 1580 and 1820, the direction of magnetic north in London changed by 35 degrees. 28

46. Magnetic North Pole Sometimes, the Earth’s magnetic poles flip. This happens every half-million years or so. Magnetic South Pole

47. Reversals happen on average only about once every 500,000 years, and they take hundreds if not thousands of years to complete. The last reversal was over 750,000 years ago, so the Earth is overdue for another switch of the plant’s north and south poles. The sun's magnetic shield appears to reverse its polarity approximately every 11 years. http://www.pbs.org/wgbh/nova/magnetic/timeline.html

48. The Magnetosphere

49. Earth’s magnetic field extends into space, which contains electrically charged particles. The Earth’s magnetic field affects the movements of these charged particles. In turn, the charged particles also affect the Earth’s magnetic field.

50. Solar wind - a stream of electrically charged particles flowing at high speeds from the sun. The solar wind pushes against Earth’s magnetic field, and surrounds the field. 31