مجال مغناطيسي

FIG. M-1. Magnetic quantities and units. See also Appendix A, SI units.

The space through which influence on, or by, a magnet is exerted. The torque per unit dipole moment at any point in space that would tend to orient a current-carrying coil or magnet if located at that point. A vector quantity also called the magnetic flux density or the magnetic induction and symbolized by B. The unit of measure for B in the SI and mks systems is the tesla and in the cgs system the gauss or gamma (1tesla=1weber/m²=1 newton/amp.m=10⁴gauss=10⁴ maxwell/cm²=10⁹ gamma). B is defined via the force F on a charge q moving with velocity v:

$\mathbf {F} =q\mathbf {v} \times \mathbf {B}$

This is often written in terms of the force dF produced on a small element of length dℓ that is carrying a current I:

$d\mathbf {F} ={\mathit {I}}d\ell \times \mathbf {B}$

B is related to the magnetizing force H by a constant of the medium called the permeability μ:

$\mathbf {B} =\mu \mu _{0}\mathbf {H}$

The unit of measure for H in the Sl and mks systems is ampere turns per meter and in the cgs system, the oersted (1 ampere turn per meter=4π10^–3 gilbert per centimeter). μ_o is called the permeability of free space; μ_o=4π10^–7 henry per meter in Sl units and 1 gauss per oersted in cgs units. B can also be expressed as

$\mathbf {B} =\mu _{0}\left(\mathbf {H} +\mathbf {M} \right)\qquad in\;the\;SI\;System,$

$\mathbf {B} =\mu _{0}\left(\mathbf {H} +4\pi \mathbf {I} \right)\qquad in\;the\;cgs\;System,$

where M and I are called the magnetization and intensity of magnetization. A magnetic field whose magnitude is given by the inverse-square law surrounds a magnetic pole, and a magnetic field given by Ampere’s law surrounds an electric current. H is sometimes called the magnetic field. See Figure M-1.

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