magnetism textbook sections 22-1 – 22-3, 22-8 physics 1161: prelecture 12
Post on 19-Dec-2015
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Magnets+-• North Pole and South Pole
– Opposites Attract– Likes Repel
• Magnetic Field Lines– Arrows give direction – Density gives strength– Looks like dipole!
NS
NS NS
Field Lines of Bar Magnet
Magnetic field lines don’t start or stop.
There are no magnetic charges (monopoles)
S N
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 = q v x B• 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 Tesla1856- 1943
Carl Friedrich Gauss
1777-1855
Earth’s Magnetic FieldEarth’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
Direction of Magnetic Force on Moving Charges
Velocity B Forceout of page right upout of page left downout of page up out of page down
Right Hand RuleThumb v, Fingers B, palm F
rightleft
Negative charge experiences opposite F!
v
B
F
• The magnetic force on a charge depends on the magnitude of the charge, its velocity, and the magnetic field.
F = q v B sin()
– Direction from RHR• Thumb (v), fingers (B), palm (F)
– Note if v is parallel to B then F = 0
BV
Magnitude of Magnetic Force on Moving Charges