Proton magnetic moment flip angle 54.7 in an external magnetic field

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SUMMARY

The angle between the proton magnetic moment (μ) and the external magnetic field is established at 54.7 degrees, derived from theoretical calculations involving angular momentum. When a magnetic field is applied in the z direction, the component of angular momentum (Pz) for a spin-half particle becomes quantized, leading to the calculation of cos⁻¹(1/√3), which equals 54.74°. This relationship is crucial for understanding MRI physics and is supported by resources such as the University of Wisconsin's NMR page.

PREREQUISITES
  • Understanding of angular momentum in quantum mechanics
  • Familiarity with spin-half particles
  • Basic knowledge of magnetic fields and their effects on particles
  • Concepts of quantization in physics
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  • Study the principles of quantum mechanics related to angular momentum
  • Explore the mathematical derivation of spin-half particle properties
  • Research the effects of external magnetic fields on particle behavior
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Physicists, medical imaging professionals, and students studying MRI technology or quantum mechanics will benefit from this discussion.

mkj
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i am reading about MRI. i have seen several in several textbooks that the angle between the direction of the proton magnetic moment μ and the direction of the external magnetic field is 54.7
how did they find that theoretical or experimental
if theoretical how can we calculate that
 
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hi mkj! :smile:

loosely speaking, the angular momentum P is fixed

if there's no magnetic field, there's no problem … the "spin axis" can be in any direction

if there is a magnetic field in the z direction, the component Pz is quantised, and (for a spin-half particle) must be P/√3

so the angle between P and z must be cos-11/√3, = 54.74°

(see eg http://www.chem.wisc.edu/areas/reich/nmr/05-hmr-00-nmr.htm)
 

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