User:John R. Brews/Sample: Difference between revisions
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The torque exerted upon a current loop of radius ''a'' carrying a current ''I'', placed in a uniform magnetic flux density '''B''' at an angle to the unit normal '''û<sub>n</sub>''' to the loop is:<ref name=Pramanik> | |||
{{cite book |title=Electro-Magnetism: Theory and Applications |author=A. Pramanik |url=http://books.google.com/books?id=gnEEwy12S5cC&pg=PT240 |pages=pp. 240 ''ff'' |isbn=8120319575 |year=2004 |publisher=PHI Learning Pvt. Ltd.}} | |||
</ref> | |||
:<math>\boldsymbol \tau = \mathit I \mathbf {S \times B } \ , </math> | |||
where the vector '''S''' is: | |||
:<math> \mathbf S = \pi a^2 \ \hat{\mathbf u} _n \ . </math> | |||
Consequently the magnetic moment of this loop is: | |||
:<math> \mathbf m = \mathit I \ \mathbf S\ . </math> | |||
===Microscopic examples=== | ===Microscopic examples=== | ||
Revision as of 18:43, 18 December 2010
Magnetic moment
In physics, the magnetic moment of an object is a vector property, denoted here as m, that determines the torque, denoted here by τ, it experiences in a magnetic flux density B, namely τ = m × B (where × denotes the vector cross product). As such, it also determines the change in potential energy of the object, denoted here by U, when it is introduced to this flux, namely U = −m·B.[1]
Origin
A magnetic moment may have a macroscopic origin in a bar magnet or a current loop, for example, or microscopic origin in the spin of an elementary particle like an electron, or in the angular momentum of an atom.
Macroscopic examples
The electric motor is based upon the torque experienced by a current loop in a magnetic field. The basic idea is that the current in the loop is made up of moving electrons, which are subect to the Lorentz force F in a magnetic field:
where e is the electron charge and v is the electron velocity. This force upon the electrons is communicated to the wire loop because the electrons cannot escape the wire, and so exert a force upon it. The electrons at the top of the loop move oppositely to those at the bottom, so the force at the top is opposite in direction to that at the bottom. If the magnetic field is in the plane of the loop, the forces are normal to this plane, causing a torque upon the loop tending to turn the loop about an axis along the direction of the field.[2]
The torque exerted upon a current loop of radius a carrying a current I, placed in a uniform magnetic flux density B at an angle to the unit normal ûn to the loop is:[3]
where the vector S is:
Consequently the magnetic moment of this loop is:
Microscopic examples
At a fundamental level, magnetic moment is related to the angular momentum of fundamental particles. In this discussion, focus is upon the electron and the atom.
Notes
- ↑ V. P. Bhatnagar (1997). A Complete Course in ISC Physics. Pitambar Publishing, p. 246. ISBN 8120902025.
- ↑ For a discussion of the operation of a motor based upon the Lorentz force, see for example, Kok Kiong Tan, Andi Sudjana Putra (2010). Drives and Control for Industrial Automation. Springer, pp. 48 ff. ISBN 1848824246.
- ↑ A. Pramanik (2004). Electro-Magnetism: Theory and Applications. PHI Learning Pvt. Ltd., pp. 240 ff. ISBN 8120319575.