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Physics 1161: PreLecture 12
Magnetism
• Textbook Sections 22-1 – 22-3, 22-8
Magnets
• North Pole and South Pole
-
+
– Opposites Attract
– Likes Repel
• Magnetic Field Lines
S
– Arrows give direction
– Density gives strength
– Looks like dipole!
S
N
N
S
N
Field Lines of Bar Magnet
S
N
Magnetic field lines don’t start or stop.
There are no magnetic charges (monopoles)
Comparison:
Electric Field Lines vs. Magnetic Field Lines
• Similarities
– Density gives strength
– Arrow gives direction
• Leave +, North
• Enter -, South
• Differences
– Start/Stop on electric charge
– No Magnetic Charge, lines are continuous!
• FYI
– x x x x x x x INTO Page
– • • • • • • • • • OUT of Page
No Magnetic Charges
• Magnetic Fields are created by
moving electric charge!
• Where is the moving charge?
Orbits of electrons about nuclei
Intrinsic “spin” of
electrons (more
important effect)
Magnetic Field Units
•F=qvxB
• SI units: N-s/C-m = Tesla
• 1 Tesla = 10,000 Gauss
• Earth’s magnetic field is
approximately 0.5 Gauss
• Refrigerator magnets are
about 100 Gauss
• Superconducting
electromagnets can be as
much as 40 Tesla
Nikola Tesla
1856- 1943
Carl Friedrich Gauss
1777-1855
Earth’s Magnetic Field
Earth’s magnetic field is similar to
that of a bar magnet tilted 11o from
Earth’s spin axis
– Earth’s north geographic pole
is actually south magnetic pole
The movement of Earth's north
magnetic pole across the Canadian
arctic, 1831--2001. Credit:
Geological Survey of Canada.
Magnetic Field Reversal
• Evidence for 171 magnetic
field reversals during the
past 71 million years has
been reported.
• Earth’s magnetic field is
weakening
• interval between reversals
of Earth’s magnetic field
can be as short as 5,000 or
as long as 50 million years
• Simulation of reversal
• Geodynamo Site
Earth’s Inconsistent Magnetic Field
Direction of Magnetic Force on
Moving Charges
Velocity
out of page
out of page
out of page
out of page
B
right
left
up
down
Force
up
down
left
right
B
Right Hand Rule
Thumb v, Fingers B, palm F
Negative charge experiences opposite F!
F
v
Magnitude of Magnetic Force on
Moving Charges
• The magnetic force on a charge depends on
the magnitude of the charge, its velocity,
and the magnetic field.
F = q v B sin(q)
– Direction from RHR
• Thumb (v), fingers (B), palm (F)
– Note if v is parallel to B then F = 0
V
q
B
Comparison
Electric vs. Magnetic
Source:
Act on:
Magnitude:
Direction:
Electric
Magnetic
Charges
Charges
F = qE
Parallel to E
Moving Charges
Moving Charges
F = q v B sin(q)
Perpendicular to v,B