1. Chapter 21
MAGNETISM

2. Magnetic Levitation Train (Shanghai)
Maglev uses powerful magnets to hold a train a few millimeters from the track and propel it with little noise or vibration.

3. Earth’s Magnetic Field

4. Earth’s Magnetic Field

5. Properties of Magnets
1. North and South Poles

6. Properties of Magnets
2. Law of magnetic poles
Unlike poles attract

7. Properties of Magnets
2. Law of magnetic poles
Like poles repel

8. LAWS OF ATTRACTION LIKE POLES REPEL N S S N N S N S
UNLIKE POLES ATTRACT
N S
S N
N S
N S

9. LAWS OF ATTRACTION
LIKE POLES REPEL

10. Properties of Magnets Testing a magnet
1. Bring one end of an unknown object to a suspended magnet.
2. If repulsion occurs, then the unknown object is a magnet since repulsion occurs between like poles.

11. Magnetic Induction
Magnetic Induction is the process of inducing magnetism in an unmagnetised ferromagnetic material without any contact with the magnet.

12. Induced Magnetism
Making of magnetic materials (e.g. steel and Iron) into Magnets
Inducing magnetism in an ordinary piece of magnetic material
Induced magnetism is a temporary process

13. Induced Magnetism
The two iron nails become induced magnets and show a repulsion between the far ends

14. Induced Magnetism
Repulsion between the two iron nails increases as a bar magnet is brought towards the nails

15. MAGNETIC POLES A magnet is a dipole (two pole)
There is no monopole (single pole)

16. MAGNETIC POLES A magnet is a dipole (two pole)
There is no monopole (single pole)

17. Magnetic Materials Most materials are NOT magnetic
Naturally occurring magnetic materials are called FERROMAGNETIC (Iron-Like)
FERROMAGNETIC MATERIALS are
Iron, Steel
Cobalt
Nickel
Alloys of above metals

18. Magnetic Domains These are small regions in a FERROMAGNETIC
Material where all the magnetic fields of the atoms are aligned

19. Magnetization S N
If the domains are aligned, the material is polarized and becomes magnetized

20. Magnetization
A magnetised bar
Unmagnetised bar

21. Methods of Magnetisation & Demagnetisation
Making a magnet by stroking
Single Touch
Divided Touch

22. Methods of Magnetisation & Demagnetisation
Making a magnet by stroking
Single Touch

23. Methods of Magnetisation & Demagnetisation
Making a magnet by stroking
Single Touch

24. Methods of Magnetisation & Demagnetisation
Making a magnet by electrically

25. Methods of Magnetisation & Demagnetisation
Making a magnet by electrically

26. Methods of Magnetisation & Demagnetisation
Polarity of the magnet determined by:
(i) Right-Hand Grip Rule
Polarity of the
magnetised specimen

27. Methods of Magnetisation & Demagnetisation
Methods of demagnetising magnets
Heating
- misalignment of “tiny” magnets, increase vibration
of atoms
2. Hammering

28. Methods of Magnetisation & Demagnetisation
Methods of demagnetising magnets
3. Alternating Current

29. Methods of Magnetisation & Demagnetisation
Methods of demagnetising magnets
3. Alternating Current

30. Magnetic Fields & the Plotting Compass
A magnetic field is the region where a magnetic force is exerted on any magnetic object placed within the influence of the field.

31. Magnetic Fields & the
Plotting Compass

32. Magnetic Fields & the
Plotting Compass

33. Magnetic Fields & the
Plotting Compass

34. Magnetic Fields & the
Plotting Compass
X: neutral point

35. Magnetic Fields & the
Plotting Compass
X: neutral point

36. Magnetic Fields & the
Plotting Compass

37. Earth’s Magnetic Field

38. Earth’s Magnetic Field

39. Magnetic Properties of Iron & Steel
Question:
Comment on the magnetic properties of Iron & Steel

40. Magnetic Properties of Iron & Steel

41. Magnetic Properties of Iron & Steel
(soft Magnet)
Steel
(Hard Magnet)
Easy to magnetize
Can be magnetized by a weak magnetic field
Harder to magnetize
Required a strong magnetic field to magnetise
Easy to demagnetize
Harder to demagnetize
Used in Electromagnets, transformer cores and magnetic shields
Good in making permanent magnets and bar magnets

42. Magnetic Field Near A Wire

43. Magnetic Field Near A Wire

44. Magnetic Field Near A Wire
Right-Hand Grip Rule

45. Magnetic Field Near A Wire

46. Magnetic Field Near A Wire

47. Magnetic Field Near A Wire

48. Magnetic Field Near A Wire
Iron Filings around a current-carrying wire

49. Magnetic Field Near A Wire

50. Magnetic Field Near A Loop
Iron Filings around a current-carrying loop

51. Magnetic Field due to a solenoid

52. Magnetic Field due to a solenoid
Iron Filings around a current-carrying coils of loops

53. Magnetic Field due to a solenoid

54. Magnetic Field due to a solenoid
The strength of the magnetic field can be
increased by:
Increasing the current
Increasing the number of turns per unit length of the solenoid
Using a soft-iron core within the solenoid

55. Magnetic Field due to a solenoid
Electromagnets are far more useful than permanent magnets because:
They can be switched on and off.
2. The strength of the magnetic field can be changed, by altering the current.
3. They can easily be made into a variety of shapes and are less expensive to make

56. Uses of electromagnets
The electric Bell

57. Uses of electromagnets
The electric Bell
-The current flows in the coil
making an electromagnet
-The armature is attracted to
the core
-As it moves, the circuit is
broken at the contact screw.
-The electromagnet is switched
off, so the armature moves back.
The armature keeps vibrating
back and forth making the
hammer strike the gong.

58. Uses of electromagnets
Magnetic Relay
Relays are used as safety devices.
A large current circuit can be switched on by a small current circuit
When the small current/voltage (that is safe) in the input circuit is switched on, the electromagnet becomes magnetic and attracts the iron armature.
The armature rotates towards the electromagnet, pushing the contacts together. This switches on the large current/voltage (that is dangerous) in the output circuit.
This type of relay circuit is used in the ignition of a car.

59. Uses of electromagnets
Magnetic Relay

60. Uses of electromagnets
Magnetic Relay

61. Uses of electromagnets
Magnetic Relay

62. Uses of electromagnets
Magnetic Relay