2. Magnetism Textbook Sections 22-1 – 22-3, 22-8 Physics 1161: PreLecture 12

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

4. Field Lines of Bar Magnet Magnetic field lines don’t start or stop. There are no magnetic charges (monopoles) S N

5. 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

6. 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)

7. 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 Tesla 1856- 1943 Carl Friedrich Gauss 1777-1855

8. 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.

9. 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

10. Earth’s Inconsistent Magnetic Field

11. Direction of Magnetic Force on Moving Charges VelocityBForce out of page right up out of page left down out of page up out of page down Right Hand Rule Thumb v, Fingers B, palm F right left Negative charge experiences opposite F! v B F

12. 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 B  V Magnitude of Magnetic Force on Moving Charges

13. Comparison Electric vs. Magnetic ElectricMagnetic Source: ChargesMoving Charges Act on: Charges Moving Charges Magnitude: F = qE F = q v B sin(  ) Direction: Parallel to E Perpendicular to v,B