Magnetism

• Textbook Sections 22-1 – 22-3, 22-8

Physics 1161: PreLecture 12

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

Earth’s Inconsistent Magnetic Field

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

ComparisonElectric vs. Magnetic

Electric MagneticSource: Charges Moving ChargesAct on: Charges Moving ChargesMagnitude: F = qE F = q v B sin()Direction: Parallel to E Perpendicular to v,B