1. Magnetism
Chapter 19

2. Introduction Applications involving magnetism Loudspeakers Meters
Electromagnets
Recording devices
Tapes
Computer disks
MRI

3. Important Concepts
All magnetic fields are produced by charges in motion.
Magnetic fields will affect charges moving through them.

4. Magnetic Poles
Magnets have poles.
North
South
191

5. The Law of Poles
Like poles repel, unlike poles attract.

6. Can a magnet have only one pole?No
Can a magnet have more than two poles?
Yes

7. Effects of Cutting?
If a magnet is cut in half, lengthwise, will the two halves attract or repel?
Repel
If a magnet is cut in half, across the width, will the two halves attract or repel?
Attract
36-2

8. Magnetic Induction
Magnetism may be induced in iron and other materials.
Stroking with a permanent magnet
Heating and then cooling in a strong magnetic field

9. Natural Magnets Magnetite (lodestone)
A naturally occurring magnetic material
Produced by heating and then cooling in the earth’s magnetic field

10. Soft Magnetic Materials
Iron
Easily magnetized
Weakens quickly

11. Hard Magnetic Materials
Cobalt and nickel
Difficult to magnetize
Stays strong longer

12. Magnetic Fields All charges are surrounded by an electric field.
Charges in motion are surrounded by a magnetic field.

13. Magnet Fields Magnitude (strength) Direction
Field descriptions must include:
Magnitude (strength)
Direction
The direction that an isolated north pole would move when placed in the field
19.2, 192

14. Magnetic Field Lines
Magnetic field lines can be shown by using small iron filings 79

15. Magnetic Field Of The Earth
North-seeking and south seeking poles
Compasses
Geographic poles vs. magnetic poles
Location (north magnetic is in Canada)
Dip angle
Magnetic declination
The angle between true north and magnetic north
19.4

16. What is believed to be the source of the earth’s magnetic field?
The earth’s magnetic field reverses every few million years
Evidence?

17. Animals and Magnetism
Some animals use the earth’s magnetic field for navigation.
Anaerobic bacteria
Lobsters
Birds

18. Magnetic Forces
A charge experiences a force when moving through a magnetic field.
Maximum force occurs when the motion is perpendicular to the field.
Zero force occurs when the motion is parallel or antiparallel to the field.
19.5

19. Factors Influencing Magnetic Force
Which factors will determine the magnitude of the force on a charge that is moving through a magnetic field?
Magnetic field strength
Quantity of charge
Velocity of the charge
Direction of the velocity

20. Magnetic Symbols New Symbols Formula Magnetic force (F)
Magnetic field strength (B)
Angle (q) (measured between v and B)
Formula

21. Magnetic Field Strength
Magnetic field strength units SI
Tesla (T) or (1 Wb/m2)

22. Magnetic Force
Formula
Right Hand Rule
19.6/19.7, 198

23. Magnetic Force On A Current Carrying-Conductor
A current-carrying wire will experience a force when placed in a magnetic field.
Symbols for current direction
Head
Tail 26

24. Magnetic Field Around a Conductor

25. Magnetic Force On A Straight Wire
Formula
There is no force when the current in the wire is in the same or opposite direction of the field.

26. Magnetic Force Applications
Stereo speaker
Voice coil
Electromagnetic pump
No moving parts
Possible heart replacement
Magnetohydrodynamic propulsion (MHD)
199

27. Torque On A Current Loop
A torque is exerted on a current-carrying loop when it is placed in a magnetic field
Applications
Galvanometers
Motors
Generators
19.15, 19.17, 200, 201

28. Torque formula Shape of coil is not important
Angle (q) is measured between B and the axis of the coil
Number of Turns (N)

29. Motion Of A Charged Particle In A Magnetic Field
If the velocity is perpendicular to the field, the magnetic force is always toward the center of the circular path.
Centripetal acceleration
No change in speed
Direction change
195, 80, 19.19

30. Curved Motion in a Magnetic Field
Radius of curvature formula:
197

31. QUESTIONS
1 – 8
Pg. 654

32. Electricity and Magnetism
Oersted lecture
An electric field deflected a nearby compass needle
This was the first realization of a link between electricity and magnetism

33. Magnetic Field Of A Long Straight Wire
Oersted demonstration
Compasses surrounding a current-carrying wire
The magnetic field lines form circles around the wire.
19.23a,b

34. Another Right-Hand Rule
The Right-Hand-Rule for a current
Indicates the directions for current, magnetic field , and force
19.23a

35. Magnetic Field Strength
Formula for the magnetic field strength due to a current in a long straight wire
mo is the permeability of free space
mo = 4p x 10-7 T.m/A

36. Magnetic Force Between Two Parallel Conductors
Two parallel current-carrying conductors will exert a force on each other. (very important)
19.28

37. Defining The Ampere Definition of the Ampere (1 A)
Two long parallel wires
One meter apart
Same current in each
Force/length = 2 x 10-7 N/m
Remember:
1 Ampere = 1 Coulomb/second

38. Magnetic Field Of A Current Loop
A current loop can increase the strength of the magnetic field.
Current loops have poles.
203, 19.28, 19.31

39. Magnetic Field Of A Solenoid
A solenoid is made up of several closely spaced loops.
Electromagnets are solenoids
Applications
Solenoids have poles
Field lines are nearly parallel within
204, 19.33, 19.32, 158, 159, 207

40. Formula for the magnetic field of a solenoid:
n = N/L

41. Causes of Magnetism Orbital motion produces magnetic fields.
These fields tend to cancel out.

42. Electron spin produces magnetic fields
In most substances, electron pairs have opposite spins which cancel.
Not in iron, nickel and cobalt

43. Magnetic Domains
Orientation
Alignment
Growth

44. Ferromagnetic Materials
Ferromagnetic materials are able to be magnetized. (permanent magnets)
Magnetic cores are used in electromagnets.
206

45. Questions ,
Pg. 655