lecture 21—wednesday
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
Lecture 21—Wednesday
March 18Magnetic 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
sinWIREF ILB
Fparallel wires 0LI1I2
2d
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
Bar magnet in uniform B-field
a b c d
25% 25%25%25%
c
d
1. a2. b3. c4. d
Magnetic Fields Exert Torques on Current Loops
Slide 24-42
The Torque on a Dipole in a Magnetic Field
IA B sin
Slide 24-43
The Electric Motor
Slide 24-46
Magnetic Resonance Imaging
Slide 24-44
Magnetic Resonance Imaging
Slide 24-45
Quiz #8
• What magnetic field strength and direction will levitate the 2.0g wire? B-field in shaded region.
1.5A
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).
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