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Magnetism

Magnetism. For next time: Read: Chapter 21, Sections 1 & 2. What do we know?. a magnet is NOT a charged rod a magnet has two poles: north & south like poles repel unlike poles attract Can we ever find an isolated pole?. Magnetic Field.

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Magnetism

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  1. Magnetism For next time: Read: Chapter 21, Sections 1 & 2

  2. What do we know? • a magnet is NOT a charged rod • a magnet has two poles: north & south • like poles repel • unlike poles attract • Can we ever find an isolated pole?

  3. Magnetic Field • Operational Definition: A vector whose direction is indicated by a compass, which is really just a small magnet. The field is in the direction of the “north seeking” pole. • Use symbol B for magnetic field. • Iron filings can act like a compass.

  4. Which way will the magnet rotate? N S B Field

  5. N S Which way will the magnet rotate? B Field

  6. N S Magnetic Field Vectors Due to a Bar Magnet

  7. N S

  8. N S

  9. N S

  10. N S

  11. N S

  12. N S

  13. N S

  14. N S

  15. N S

  16. Canadian Question of the Day • If the north pole of a compass is attracted to a south pole, why does it point toward Canada?

  17. Did you know: • Magnetic north (actually the location of the south pole of the magnet) is in Hudson Bay some 1300 km from geographic north. It has moved about 700 km since 1904!!

  18. Magnetic Field Lines • The direction of the magnetic field at any point is tangent to the magnetic field line at that point.

  19. N S

  20. N S

  21. Cathode Ray Tube

  22. S N B F What direction will the beam be “pushed?” What direction is the magnetic field (at the location of the beam)? What direction is the force on the beam?

  23. Magnetism is Goofy

  24. For a charged particle to experiencea force due to a magnetic field • a) the charge must be moving, and • b) the velocity of the moving charge must have a component that is perpendicular to the direction of the magnetic field. • NOTE: the direction of the force is always perpendicular to both the velocity vector and the magnetic field B.

  25. How Big is the Force? • Force is proportional to: • perpendicular component of the velocity • charge (q) • size of the field (B) F = K q vperp B F = q vperp B

  26. Some typical fields: • surface of the Earth10-4T • small bar magnet 10-2T • MRI 2-3 T • magnet lab 30 T • surface of neutron star 108T

  27. All charges below feel the same force. B

  28. vperp vperp All charges below feel the same force. vperp B

  29. vperp vperp All charges below feel the same force. vperp B vperp But, how do we find the direction?

  30. Right Hand Rule • Fingers in the direction of v • Wave into B (might have to turn wrist) • Thumb in direction of force on positive charge

  31. F B What direction will the beam be “pushed?” S What direction is the magnetic field (at the location of the beam)? N What direction is the force on the beam?

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