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Lecture 21—Wednesday

Lecture 21—Wednesday. March 18 Magnetic Forces on Dipoles Magnetic Resonance Imaging. Quick Review of Monday. Force on a straight wire in a uniform B-field: Direction from right hand rule Force between two straight parallel conductors: Direction from right hand rule . Today.

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Lecture 21—Wednesday

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  1. Lecture 21—Wednesday March 18 Magnetic Forces on Dipoles Magnetic Resonance Imaging

  2. Quick Review of Monday • Force on a straight wire in a uniform B-field: • Direction from right hand rule • Force between two straight parallel conductors: • Direction from right hand rule

  3. Today • Force on a magnetic dipole in a magnetic field • Current loop is a magnetic dipole • Find torque on a planar current loop • Start with rectangular loop • Magnetic Resonance Imaging • Atoms are dipoles • But only two orientations wrt magnetic field, with or against. Difference in energy between the two orientations is in the radio frequency range for B≈1 T

  4. Bar magnet in uniform B-field • a • b • c • d c d

  5. Magnetic Fields Exert Torques on Current Loops Slide 24-42

  6. The Torque on a Dipole in a Magnetic Field Slide 24-43

  7. The Electric Motor Slide 24-46

  8. Magnetic Resonance Imaging Slide 24-44

  9. Magnetic Resonance Imaging Slide 24-45

  10. Quiz #8 • What magnetic field strength and direction will levitate the 2.0g wire? B-field in shaded region. 1.5A

  11. Homework due today • Homework due today • Problems: 24: 32, 34, 37 • 32: field strength and direction to levitate wire given mass and I • 34: F with wire not perpendicular to field • 37: force on current loop with radial field (loud speaker).

  12. Homework due Friday • Read: 23: 1-3 Faraday’s Law and electromagnetic induction • Problems: 24: 38, 39, WB 24:23-30 • 38: torque on current loop • 39: ditto • WB 24: 23-30 forces on straight wires and torques on loops

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