Magnetic dipole torque and orientation energy
A fixed magnetic moment in a spatially uniform external field experiences torque τ=m×B.
Key knowledge
A fixed magnetic moment in a spatially uniform external field experiences torque τ=m×B. Its orientation-dependent energy can be written U=−m·B plus an arbitrary constant, so a freely rotating fixed moment tends toward alignment when dissipation allows relaxation.
Values and equations
τ=m×B; U=−m·B+constant
- τ
- Mechanical torque N m
- m
- Fixed magnetic dipole moment A m²
- B
- External magnetic flux density T
- U
- Orientation energy J
Conditions and scope
- Fixed-magnitude localized dipole, field approximately uniform across it and a mechanical orientation coordinate; the energy convention holds the moment fixed.
- An induced moment, spatially varying field, current-source energy accounting or a dynamical spin state needs its respective model. Torque alone gives precession/dynamics, not automatic dissipative alignment.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Classical Electromagnetism (2014): Magnetostatic exercises: The last two dipole exercises explicitly state the exterior dipole field, τ=m×B and U=−m·B; orientation energy follows by integrating mechanical torque for fixed m. No unchecked spatial-force or induced-moment energy formula is asserted.
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