1. Magnetism
A Whole New Topic
March 8, 2006
Magnetism

2. Introductory Material
Magnetism

3. Dis Here Week
Today we begin magnetism – a topic that will occupy us for most of the remainder of the semester.
I do not yet have the exam papers (Tuesday PM) but I will return them if I get them.
Friday – Quiz on DC Circuits
New material on the web
New WebAssign on board – READ THE CHAPTER!!
Magnetism

4. DEMO ON Magnetism
Magnetism

5. Lodestone (Mineral) Lodestones attracted iron filings.
Lodestones seemed to attract each other.
Used as a compass.
One end always pointed north.
Lodestone is a natural magnet.
Magnetism

6. Magnetism
Refrigerators are attracted to magnets!
Magnetism

7. Applications Motors Navigation – Compass Magnetic Tapes Television
Music, Data
Television
Beam deflection Coil
Magnetic Resonance Imaging
High Energy Physics Research
Magnetism

8. Magnets Like Poles Repel Opposite Poles Attract
Magnetic Poles are only found in pairs.
No magnetic monopoles have ever been observed.
S N
Shaded End is NORTH Pole
Shaded End of a compass points
to the NORTH.
Magnetism

9. Observations
Bring a magnet to an electrically charged object and nothing happens. No forces.
Bring a magnet near some metals (Co, Fe, Ni …) and it will be attracted to the magnet.
The metal will be attracted to both the N and S poles independently.
Some metals are not attracted at all.
Wood is NOT attracted to a magnet.
Neither is water.
A magnet will force a compass needle to align with it. (No big Surprise.)
Magnetism

10. Magnets
Magnetic Field
Cutting a bar magnet in half produces TWO bar magnets, each with N and S poles.
Magnetism

11. Consider a Permanent Magnet
Magnetism

12. Introduce Another Permanent MagnetS
pivot
The bar magnet (a magnetic dipole) wants to align with the B-field.
Magnetism

13. Field of a Permanent MagnetS
The south pole of the small bar magnet is attracted towards the north pole of the big magnet.
Also, the small bar magnet (a magnetic dipole) wants to align with the B-field.
The field attracts and exerts a torque on the small magnet.
Magnetism

14. Field of a Permanent MagnetS
Show disk 19, demo 6 (minimum energy configuration of magnets)
The bar magnet (a magnetic dipole) wants to align with the B-field.
Magnetism

15. The Magnetic Field Similar to Electric Field … exists in space.
Has Magnitude AND Direction.
The “stronger” this field, the greater is the ability of the field to interact with a magnet.
Magnetism

16. Convention For Magnetic FieldsB
X 
Field INTO Paper Field OUT of Paper
Magnetism

17. Experiments with Magnets Show
Current carrying wire produces a circular magnetic field around it.
Force on Compass Needle (or magnet) increases with current.
Magnetism

18. Current Carrying Wire B Right hand Rule-
Current into
the page.
Right hand Rule-
Thumb in direction of the current
Fingers curl in the direction of B
Magnetism

19. Current Carrying Wire
B field is created at ALL POINTS in space surrounding the wire.
The B field had magnitude and direction.
Force on a magnet increases with the current.
Force is found to vary as ~(1/d) from the wire.
Magnetism

20. Compass and B Field Observations
North Pole of magnets tend to move toward the direction of B while S pole goes the other way.
Field exerts a TORQUE on a compass needle.
Compass needle is a magnetic dipole.
North Pole of compass points toward the NORTH.
Magnetism

21. Planet Earth
Magnetism

22. Inside it all.
8000
Miles
Magnetism

23. On the surface it looks like this..
Magnetism

24. Inside: Warmer than Floriduh
Magnetism

25. Much Warmer than Floriduh
Magnetism

26. Finally
Hot Hot Hot
Magnetism

27. In Between
The molten iron core exists in a magnetic field that had been created from other sources (sun…).
The fluid is rotating in this field.
This motion causes a current in the molten metal.
The current causes a magnetic field.
The process is self-sustaining.
The driving force is the heat (energy) that is generated in the core of the planet.
Magnetism

28. After molten lava emerges from a volcano, it solidifies to a rock
After molten lava emerges from a volcano, it solidifies to a rock. In most cases it is a black rock known as basalt, which is faintly magnetic, like iron emerging from a melt. Its magnetization is in the direction of the local magnetic force at the time when it cools down.
Instruments can measure the magnetization of basalt. Therefore, if a volcano has produced many lava flows over a past period, scientists can analyze the magnetizations of the various flows and from them get an idea on how the direction of the local Earth's field varied in the past. Surprisingly, this procedure suggested that times existed when the magnetization had the opposite direction from today's. All sorts of explanation were proposed, but in the end the only one which passed all tests was that in the distant past, indeed, the magnetic polarity of the Earth was sometimes reversed.
Magnetism

29. Ancient Navigation
Magnetism

30. This planet is really screwed up!
NORTH
POLE
SOUTH POLE
Magnetism

31. Repeat And it REVERSES from time to time. Navigation DIRECTION N S
If N direction
is pointed to by
the NORTH pole
of the Compass
Needle, then the
pole at the NORTH
of our planet must
be a SOUTH MAGNETIC
POLE!
Compass
Direction
And it REVERSES from time to time.
Magnetism

32. Rowland’s Experiment xxx xxx B INSULATING Field is created by
any moving charge.
Increases with
charge on the
disk.
angular velocity of
the disk.
Electrical curent is a
moving charge.
Rotating
INSULATING
Disk
which is
CHARGED
+ or –
on exterior.
xxx
xxx B ++
Magnetism

33. So much for the observations.
Let’s do the physics!
Magnetism

34. A Look at the Physics q If the charge is moving, there
There is NO force on
a charge placed into a
magnetic field if the
charge is NOT moving. q
There is no force if the charge
moves parallel to the field. q
If the charge is moving, there
is a force on the charge,
perpendicular to both v and B.
F = q v x B
Magnetism

35. WHAT THE HECK IS THAT??? A WHAT PRODUCT?
A CROSS PRODUCT – Like an angry one??
Alas, yes ….
F=qv X B
Magnetism

36. The Lorentz Force This can be summarized as: F or: v q m B
q is the angle between B and V
Magnetism

37. Nicer Picture
Magnetism

38. VECTOR CALCULATIONS
Magnetism

39. See proof of previous approach on the website.
Note
See proof of previous approach on the website.
Magnetism

40. Practice B and v are parallel. Crossproduct is zero. So is the force.
Which way is the Force???
Magnetism

41. Units
Magnetism

42. teslas are
HUGE!
Magnetism

43. The Magnetic Force is Different From the Electric Force.
Whereas the electric force acts in the same direction as the field:
The magnetic force acts in a direction orthogonal to the field:
(Use “Right-Hand” Rule to
determine direction of F)
And --- the charge must be moving !!
Magnetism

44. So… A moving charge can create a magnetic field.
A moving charge is acted upon by a magnetic field.
In Magnetism, things move.
In the Electric Field, forces and the field can be created by stationary charges.
Magnetism

45. Wires A wire with a current contains moving charges.
A magnetic field will apply a force to those moving charges.
This results in a force on the wire itself.
The electron’s sort of PUSH on the side of the wire. F
Remember: Electrons go the “other way”.
Magnetism

46. The Wire in More Detail Assume all electrons are moving
with the same velocity vd.
B out of plane of the paper
Magnetism

47. Magnetic Levitation Magnetic Force Current = i mg
Where does B point????
Into the paper.
Magnetism

48. MagLev
                                                                                     
Magnetism

49. There was a crooked man who lived in a crooked house that was wired with crooked wires
Magnetism

50. Crooked Wire (in a plane) in a constant B field
Magnetism

51. Case 1
The magnetic force on a curved current carrying conductor in a uniform
magnetic field is the same as that of a straight conductor carrying the
same current between the two points a and b.
Magnetism

52. The net magnetic force on a closed current carrying loop is ZERO!
Case 2
The net magnetic force on a closed current carrying loop is ZERO!
Magnetism

53. Current Loop
What is force
on the ends??
Loop will tend to rotate due to the torque the field applies to the loop.
Magnetism

54. The Loop OBSERVATION Force on Side 2 is out of the paper and that on
the opposite side is into
the paper. No net force
tending to rotate the loop
due to either of these forces.
The net force on the loop is
also zero,
pivot
Magnetism

55. An Application The Galvanometer
Magnetism

56. The other sides t1=F1 (b/2)Sin(q) =(B i a) x (b/2)Sin(q)
total torque on
the loop is: 2t1
Total torque:
t=(iaB) bSin(q)
=iABSin(q)
(A=Area)
Magnetism

57. Don't hurt yourself doing this!
A Coil
Normal to the
coil
RIGHT HAND RULE TO FIND NORMAL TO THE COIL:
“Point or curl you’re the fingers of your right
hand in the direction of the current and your
thumb will point in the direction of the normal
to the coil.
Don't hurt yourself doing this!
Magnetism

58. Dipole Moment Definition
Define the magnetic
dipole moment of
the coil m as:
=NiA
We can convert this
to a vector with A
as defined as being
normal to the area as
in the previous slide.
Magnetism