Magnetism

1
Magnetism
2
(No Transcript)
3
(No Transcript)
4
(No Transcript)
5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
What do we know?

• a magnet is NOT a charged rod
• a magnet has two poles north south
• like poles repel
• unlike poles attract
• Can we ever find an isolated pole?

9
(No Transcript)
10
Magnetic Field

• Operational Definition A vector whose direction
  is indicated by a compass, which is really just a
  small magnet. The field is in the direction of
  the north seeking pole of the compass.
• Use symbol B for magnetic field.
• Iron filings can act like a compass.

11
(No Transcript)
12
Which way will the magnet rotate?
N
S
B Field
13
Which way will the magnet rotate?
B Field
14
Magnetic Field Vectors Due to a Bar Magnet
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
N
S
24
Canadian Question of the Day

• If the north pole of a compass is attracted to a
  south pole, why does it point toward Canada?

25
North geographic pole
South magnetic pole
North geographic pole
North geographic pole
Southgeographic pole
North magnetic pole
26
Did you know

• Magnetic north (actually the location of the
  south pole of the magnet) is in Hudson Bay some
  1300 km from geographic north. It has moved
  about 700 km since 1904!!

27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
Magnetic Field Lines

• The direction of the magnetic field at any point
  is tangent to the magnetic field line at that
  point.

31
N
S
32
N
S
33
(No Transcript)
34
(No Transcript)
35
Cathode Ray Tube
36
What direction will the beam be pushed?
What direction is the magnetic field (at the
location of the beam)?
What direction is the force on the beam?
37
Magnetism is Goofy
38
For a charged particle to experiencea force due
to a magnetic field

• a) the charge must be moving, and
• b) the velocity of the moving charge must have a
  component that is perpendicular to the direction
  of the magnetic field.
• NOTE the direction of the force is always
  perpendicular to both the velocity vector and the
  magnetic field B.

39
How Big is the Force?

• Force is proportional to
• perpendicular component of the velocity
• charge (q)
• size of the field (B)

F K q vperp B
F q vperp B
40
Some typical fields

• surface of the Earth 10-4 T
• small bar magnet 10-2 T
• MRI 2-3 T
• magnet lab 30 T
• surface of neutron star 108 T

41
All charges below feel the same force.
B
42
All charges below feel the same force.
vperp
B
43
All charges below feel the same force.
vperp
B
vperp
But, how do we find the direction?
44
Right Hand Rule

• Fingers in the direction of v
• Wave into B (might have to turn wrist)
• Thumb in direction of force on positive charge

45
What direction will the beam be pushed?
S
What direction is the magnetic field (at the
location of the beam)?
N
What direction is the force on the beam?