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Magnetism

Magnetism. Demonstration – Magnetic deflection Flowchart – Magnetism and Induction Force on single charge Examples Force on current-carrying wire Examples Current creating magnetic field Examples. Magnetism and Induction.

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Magnetism

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  1. Magnetism • Demonstration – Magnetic deflection • Flowchart – Magnetism and Induction • Force on single charge • Examples • Force on current-carrying wire • Examples • Current creating magnetic field • Examples

  2. Magnetism and Induction • Magnetism is weird. It’s basically a correction to electrostatics when charges are moving, and corrections are always weird. • Electrostatics affects charges, magnetism affects charges in motion • Electrostatics causes linear accelerations, magnetism causes sideways deflections (RH rule, Lentz’s Law) • Magnetism is very, very (societally) important! • Motors, generators, loudspeakers, credit cards, hard drives, transformers…….

  3. Animations • Saturday night at the movies • Nat & the old TV, loudspeaker • CRT Beam video • MIT TV video

  4. Magnetism and Induction Flowchart • B field and moving charge creates force

  5. Force on Moving Charge Basic Force Law F = q v x B • Cross product vs. dot product • Magnitude qvB sinθ • Maximum when v perpendicular to B • Minimum when v parallel to B • Direction right-hand rule • RHR for positive charge • RHR for negative charge • B units Tesla = 1 N/A-m (show units)

  6. Examples – Force on Moving Charge • Example 20-4 – Simple B calculation • Problem 11 • Explain x and · • Problem 12 • Other problems

  7. More examples – Force on Moving Charge • Example 20-5 - Curved electron path • Problem 18 • r = (1/B) sqrt(2mV/q) • rd = sqrt(2) rp • ra = sqrt(2) rp

  8. Magnetic Deflection

  9. Magnetic Deflection

  10. Magnetism and Induction Flowchart • B field and current creates force

  11. Force on Current carrying wire • For single charge F= q v x B • For many charges (in wire) F = Δq v x B F = Δq (ΔL/Δt) x B F = Il x B

  12. Examples • Example 20-1 • Perpendicular to field • Parallel to field • Example 20-2 • Force on sides • Force on bottom • Problem 7

  13. Magnetism and Induction Flowchart • Current creates B field

  14. Ampere’s Law • Gauss’s Law for closed surface • Ampere’s Law for closed loop • Ampere’s Law around long straight wire • B = μoI/2πr • μo = 4π x 10-7 T-m/A • Direction RHR • Forms “loops” compliments electric “spokes” • Electromagnetic field

  15. Simple B Field calculations • Examples 20-7 • Example 20-8 • Example 20-9 • Spinning electrons create magnetic fields • Aligned vs. random • Magnetic materials

  16. Force between Parallel Wires • Wire 1 sets up field (flowchart row 3) • Wire 2 feels force of field (flowchart row 2) • Vice-versa • Don’t memorize equation 20-7 • Example 20-10 • Example 20-11

  17. More Examples - Force between Wires • Flowchart: row 3, row 2 • Problem 41 • Problem 43 • Problem 45

  18. Motors • Motor • Flowchart: row 3, row 2 • F = IaB • τone side = IaB (b/2) sinθ • Θ angle between Force and Moment arm • Also angle between Field and Loop perpendicular • τboth sides= laB b sinθ = IAB sinθ • End contributions cancel • Multiple turns • Demo

  19. Loudspeakers • Flowchart: row 3, row 2 • Layout • Demo

  20. Electrostatic vs. Magnetic

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