1. Magnetic Forces and Fields

2. Magnetic Forces and Fields
Law of Magnetism:
Like magnetic poles repel and unlike magnetic poles attract
• Magnets must have 2 poles – break a magnet in half ……
• First magnetic mineral called lodestone
• Gilbert (know his name) – Earth is a giant piece of lodestone
• Gilbert responsible for early field ideas

3. Magnetic Forces and Fields
Magnetic Field strengths – unit Tesla (T)
You’ll learn more conversions later
Relative Magnetic Field strengths: (table 12.1)

4. Magnetic Forces and Fields
Concept Check, page 586
Magnetic Fields Gravitational Fields Electric Fields
attraction and repulsion attraction only attraction and repulsion
Towards or away from towards centre of towards or away poles on source source from centre of source
act on other magnets act on all act on charged and ferromagnetic masses masses metals (Fe, Ni, Co)

5. Magnetic Forces and Fields
Direction of magnetic field (by definition):
The direction of force on the north-seeking pole of a compass or
bar magnet
N S
N S
N S
N S
N S

6. Magnetic Forces and Fields
The pole marked “N” on a bar magnet is not a pole like the Earth’s north pole – it is the opposite.
It is North-seeking.
If the Earth was a
bar magnet, our north
pole would be stamped
S!!!!!!!

7. Magnetic Forces and Fields
Field around a bar magnet:
To visualize, picture what a compass needle (or bar magnet) would do if placed near the bar magnet

8. Magnetic Forces and Fields
Oersted, 1820 – magnetic field near a current-carrying wire:
Demo
Symbol: wire carrying current out of page
wire carrying current into page
How to remember:
Think arrow

9. Magnetic Forces and Fields
Using the first left hand rule given on page 588 (below) you determine the field around wires with electron flow out of and into the page

10. Magnetic Forces and Fields
Field around an electromagnetic coil or solenoid given by second left hand rule.
Another way of looking at it – do cross-sectional cut-away of coil.
Like a bar magnet!

11. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Second left hand rule:
magnetic field line 00

12. Magnetic Forces and Fields
Applications of electromagnets:
relay switches, motors, alarm bells, door bells, speakers
What causes magnets to be magnets?
Domain theory: tiny magnetic fields generated in ferromagnetic materials by electron movement, add up to produce small regions of magnetic fields called domains.
Domains are tiny (< 1 mm across)

13. Magnetic Forces and Fields
Domains are scattered in the absence of an external magnetic field
In the presence of an external field they line up, and the ferromagnetic material is attracted to a magnet, and behaves, itself, as a magnet
Remove external field: they scatter again –
if…………………..

14. Magnetic Forces and Fields
Read Magnetism in Nature, page 590 and
Then, Now, and Future, page 591

15. Magnetic Forces and Fields
Check and Reflect, page 592
Discuss questions 4, 6, 7, 9, and 12

16. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Experiments with Cathode Ray Tubes (CRT) – late 1800’s, led to many changes in atomic physics (last unit of Physics 30)
A very simple cathode ray tube could be constructed as shown below:
Other CRT’s – next page
glass tube at near vacuum
anode
cathode
+ -

17. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Thomson’s e/m

18. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Beam seemed to come from the (-) terminal of the tube (cathode) – later substantiated
Interaction of a moving charge with magnetic field:
Don’t copy – just for explanation

19. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Magnetic force is always perpendicular to the charged particle’s velocity
In this course magnetic field also always perpendicular to velocity (except in descriptive situations) so force will be perpendicular to both velocity and magnetic field direction
The 3rd Left Hand Rule can be used to determine the direction of the force on free charges and currents in a magnetic field

20. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
(or free negative charge)
Go back to previous diagram to see that they are consistent

21. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields

22. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields

23. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Use left hand for negative particles like electrons or anions; right hand for positive particles like protons, alpha particles, and cations
Examples: Skills Practice, page 595
Cathode Ray Tube Demo
a) Out of page
b) Into page
c) Into page (RH)

24. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
front of arc
Figure 12.19, page 597

25. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Northern (and Southern) Lights – Aurora Borealis and Aurora Australis
Are the particles shown in the picture (+) or (-)?

26. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Magnetic force calculations:
Examples: Practice Problems 1, 3 page 599

27. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Practice Problem 1, page 599
Magnitude only

28. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Practice Problem 3, page 599
(fingers) direction at equator
(thumb)
(palm) down towards earth’s surface)

29. Magnetic Forces and Fields
12.2 Moving Charges and Magnetic Fields
Do Check and Reflect, page 601, questions 3 and 5
Do SNAP Problems, page 120, questions 1, 3, 4, 6, and 10

30. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Electric Current, I
Unit Ampere (A) – commonly called amp

31. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Force on a wire in a magnetic field similar to force on a free charge in a magnetic field
Same hand rule
Formula almost the same
Free charge: Current in Wire:
Where I = current (A) and = length of perpendicular wire in field (m)

32. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
New breakdown for 1 T,
Earlier:
Now:
Don’t need to memorize these – show why not – but should be able to show that they’re equal

33. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Example: Practice Problem 1, page 605

34. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Example: Practice Problem 2, page 605
To find the answer you need a force equal and opposite to the force of gravity
Do SNAP page 120, questions 2, 8
Review question 12

35. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
The magnetic field due to coil is either (2nd LHR)
Since only wires carrying current perpendicular to field are affected, this means only section XY is involved or
What direction is current in the coil? Hint: 2nd and 3rd LHR’s

36. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Force between two current carrying wires:
attraction
N S
repulsion
N S
Principle: same direction – attract opposite direction - repel

37. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
SI definition of the Ampere – current in each of 2 parallel wires, 1 m long, 1 m apart, that results in a force of 2.00 x 10-7 N
Why isn’t 1 C/s good enough for defining the amp?

38. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Concept Check page 608 – discuss
Motors
Use 3rd left hand rule to try to understand why armature moves and why commutator is necessary

39. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Current must reverse direction
No net force

40. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Faraday and Henry’s independent discovery: electromagnetic induction
Text diagram page 615
Faraday’s device – when switch was closed momentary current in one direction produced
current then dropped to zero
when switch was opened momentary current in opposite direction produced

41. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Their Interpretation: Changing magnetic field across a wire produces current in that wire
Changing magnetic field can be produced by:
• movement of wire with respect to field
• movement of field with respect to wire
• varying size of the magnetic field with respect to wire

42. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
The classic generator generates electric current by the first method

43. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Read pages 614 and 615 – new and old technologies
Read An Accidental Discovery, page 617

44. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Lenz’s Law: The direction of a magnetically induced current is such as to oppose the cause of the current.
Demonstrations

45. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields N S
Current is induced around walls of tube with a magnetic field that opposes the falling magnet
Which way does the current flow?
Hint: 2nd Left Hand Rule
What happens as the magnet continues to fall?

46. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Note that the current above the magnet has reversed to attract it back up while current below is continuing to repel it upwards N S

47. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Eddy currents set up in face of paddle create magnetic field attracting them to poles of permanent magnet S N
Why does it stop?

48. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Check and Reflect, page 620, questions 1 – 7
Discuss questions from SNAP, pages

49. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
What direction will current flow if the conducting rod is dragged through the magnetic field as shown?
According to Lenz’s Law there must be a force to left since rod is being pulled right.
Point palm left (for force), fingers into the page for magnetic field. Your thumb points to bottom of page for current direction in the rod. In completed circuit current flow is clockwise.

50. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
If magnet moves, current will flow in coil to produce magnetic field to oppose its motion
Move to left: right end of coil must become S (seeking) to repel. Use 2nd left hand rule to determine direction of current in coil and through galvanometer
Move to right: right end of coil must become N (seeking) to attract.
Current moves left through galvanometer.
Current moves right through galvanometer

51. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
Lenz’s Law provides another way (I think easier way) to predict the current direction for a generator.
Since the current produced opposes the motion that created it, the force (palm) goes against the wires motion, the magnetic field is fixed (fingers), and your thumb indicates the direction the electron flow current.

52. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields
same
electron flow (thumb)
Lenz force (palm)

53. Magnetic Forces and Fields
12.3 Current Carrying Conductors and Magnetic Fields