Physics 1161 Lecture 08 Magnetism Textbook Sections 22

Physics 1161: Lecture 08 Magnetism • Textbook Sections 22 -1 – 22 -3, 22 -8

Magnets • North Pole and South Pole – Opposites Attract – Likes Repel • Magnetic Field Lines – Arrows give direction – Density gives strength – Looks like dipole! - +

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 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=qvx. 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 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 11 o 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 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 F = q. E Parallel to E Moving Charges F = q v B sin(q) Perpendicular to v, B