1. Chapter 20: Magnetism
Purpose: To describe magnetic field around a permanent magnet.
Objectives:
Describe a magnetic poles
Describe magnetic field.
Magnetic field lines.

2. Warm-up Where and how do we use magnets?
What is an electro magnet and where is it used?

3. Describing a magnet
magnet is a material or object that produces a magnetic field.
A bar magnet
Horse shoe magnet

4. Magnetic poles
Magnetic poles are regions that produce magnetic forces s N
North pole
South pole

5. Like poles repel, unlike poles attract.
South repels south, North Repels North, and South attracts North.

6. Breaking the magnet Magnetic poles cannot be isolated.
When you break a magnet into two you end up with two magnets.
Keep breaking further and further, you still get the same results.

7. Magnetic field
The magnetic field (usually denoted B) is a space around a magnet where magnetic force is exerted.
The SI units of magnetic field (B) is Tesla (T)
Iron fillings trace out a pattern of magnetic field lines in the space around the magnet.

8. Magnetic field is represented by magnetic field lines
where lines are closer together, the field strength is greater.
The direction of magnetic field is, from North pole to south pole.

9. Nature of magnetic field
What is in a magnet that makes it a magnet?
Permanent magnets, like all other substances are composed of atoms that contain electrons. These electrons are in a continuous motion.
Electrons have two types of motion;
a) spinning on their positions and
b) Orbiting around the nucleus.
Magnetic Materials

10. source of all magnetism
The source of all magnetism is moving electric charge (motion of electrons)
Every spinning and orbiting electron is a tiny magnet that creates a magnetic field around an atom.
Question;
What is the source of magnetic field on a bar magnet?_____________________________

11. Magnetic domains Magnetic domains are large cluster of aligned atom.
Each domain contains dipoles (magnetized atoms) that are aligned.
Irregular shaped domains with aligned dipoles

12. Magnetic and non magnetic
Magnetic materials have electrons spinning in the same direction. This leads to aligned atoms with a strong magnetic field around them.
Examples include Iron, Nickel, and Cobalt
For non magnets, the electron spins opposite to one another causing their magnetic fields to cancel each other. Therefore, no net magnetic field exists

13. EARTHS MAGNETIC FIELD

14. Electron Vs Earth

15. Magnetic declination
Magnetic declination is the discrepancy between the orientation of a compass and true North.
That means that compasses do not point to true Geographic North.
Magnetic declination

16. Homework
Using double bubble Differentiate between magnetic poles from electric charges.
Textbook Pg 576 # 1 -6

17. Warm-up
What is the source of all magnetic field?

18. Beam of electrons
Since moving electrons create magnetism, a beam of electrons will have a magnetic field around it.

19. Electric currents produce Magnetic fields
Straight conductors (wires) will have magnetic fields around them. The magnetic field form complete loops around the conductors.

20. Direction of magnetic field
First Right hand rule
The thumb indicate the direction of current.
The four fingers indicate direction of magnetic field.

21. Right hand rule
First Right hand rule
Electric current
Magnetic field

22. Question
Consider a beam of electrons moving from west to east, what is the direction of magnetic field at point G? N E W e-
G • S

23. Question
What is the direction of a magnetic field at the center of a current carrying loop with a counter-clockwise flow of current?

24. answer
Out of the page

25. Magnetic Field Produced by a Coil
This is how magnetic field lines looks like in and out of an electromagnet
Counter-clockwise current----North pole
Clockwise current flow---South Pole.

26. North and south poles of an electromagnet.
Second Right hand rule.
Wrap the fingers around the coil in the direction of current.
The thump will indicate the North pole.

27. question
What is the direction of the magnetic field inside the coil shown on the right?

28. Class work
Textbook Pg 576 # 7-16

29. Warm-up How come a magnet can attract a nail?
What kind of charge, negative or positive, attracts a neutral object?
What happens to the electrons within a iron nail when it gets magnetized?

30. Warm-up
What happens when you put a small compass in a magnetic field say of a straight conductor?

31. Lab
Lab- magnetic field

32. Magnetic force on moving charge
What happens when a charge/current moves near a magnetic field? B N
Magnetic field S A
Current carrying conductor

33. Direction of current and magnetic field
Maximum force is achieved when the current is flowing perpendicular to the magnetic field.
If the current is flowing parallel to the magnetic field, there will be no interaction and therefore no force

34. Direction of Force on a conductor
The conductor is forced to move in a certain direction.
The Third right hand rule indicates the direction of the force.
Concepts in Motion

35. What is the direction of the magnetic force on the current in each of the six cases below

36. Magnitude force on a current carrying wire.

37. Magnetic force
Magnetic force formula is just the magnetic field formula rearranged as shown;
When Current (I) and the length (L) of conductor is given we use the 2nd formula.

38. The magnetic field formula
Where;
B = magnetic field,
V = speed of the charged particle, and
q = charge of the particle,
F = force of the field

39. Example
A proton speeding at 3.0 x107 m/s experiences a magnetic field of 4.0 Teslas. What is the magnetic force pulling on the proton?

40. Class work
Textbook page 577, # 1-7

41. Path of a charged particle
The path of a charged particle in a uniform magnetic field is a circle. (Just as the force of gravity causes the moon to move in a circle around the earth).
Force on the particle, and particle’s motion are perpendicular to each other.

42. Path of a charged particle
Thus, a charged particle moves in a circular path with constant centripetal acceleration.
Therefore;

43. Example;
An electron travels at 2.0 x 107 m/s in a plane perpendicular to a uniform 0.01 T magnetic field. Describe the motion of the electron quantitatively.

44. Helical path of a charged particle
If the velocity of the charged particle is not perpendicular to the magnetic field, the path of the charged particle becomes Helical.

45. Why Helical path
The velocity component perpendicular to the field results in circular motion about the field lines. However, the velocity component parallel to the field lines results in no force. The particle continues to move in the direction of that vector.

46. Direction of charge
Electrons (negative charges) move in the opposite direction of positive charges and current.

47. Magnetic field at distance r from the wire
Magnetic field around a straight conductor is directly proportional to the current through the wire and inversely proportional to the radius or ( the perpendicular distance from the wire)

48. Magnetic field at distance r from the wire

49. Example
An electric wire in the wall of a building carries a DC current of 25 A vertically upward. What is the magnetic field due to this current at point p 10 cm due north of the wire?

50. Exercise
Textbook pg 577 # 8-18

51. Warm-up:
Can a stationary electron be set into motion with a magnetic field? With an electric field? Explain.

52. Class work
Workbook problems
Pg 226; #s 1- 4 and
Pg 231; #s A1- A5

53. Worksheet- Magnetism

54. Force between two parallel wires
Two parallel wires carrying currents I1 and I2 separated by a distance r, exerts a force F on each other given by the formula:

55. Wire 1
consider the field produced by wire 1 and the force it exerts on wire 2 (call the force F 2   ). The field due to I 1   at a distance r  is given to be;

56. Force exerted of wire 2
This field is uniform along wire 2 and perpendicular to it, and so the force F 2  it exerts on wire 2 is given by;

57. We get the formula for the force F2 as;
Force exerted on wire 2
Substituting for B1 from;
We get the formula for the force F2 as;

58. Direction of force
When the currents in the two wire is in the same direction, the force is attraction.
When the currents are in opposite direction, the force is repulsion.

59. Class work
Textbook pg 578 # 26-32

60. Putting electricity to work
This force can then be used for daily work such as;
sharpen pencils
Cut down trees,
Driving cars,
In other words, we transform electrical energy to mechanical energy in order to do work.

61. Simple Galvanometer
This is a meter that detects small electric currents.
When electricity is present in the coil, each loop produces its own effect on the needle causing it to deflect.

62. Electric motor
This is the device used to transform electrical energy to mechanical energy
Concepts in Motion

63. Direct Current Motors
Direct Current Motors

64. Loud speakers
Loud speakers convert electric energy to mechanical energy just as Motors do.
Changing current in a magnetic field creates varying forces on the conductor, and varying motions of the paper cone which is attached to the conductor.

65. Research Homework Explain how a motor works
Explain how a speaker works