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

3. 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

4. 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

5. Bar magnet in uniform B-field c d 1.a 2. b 3. c 4. d

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

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

8. The Electric Motor Slide 24-46

9. Magnetic Resonance Imaging Slide 24-44

10. Magnetic Resonance Imaging Slide 24-45

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

12. 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).

13. 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