# مجال مغناطيسي

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/m^{2}=1 newton/amp.m=10^{4}gauss=10^{4} maxwell/cm^{2}=10^{9} gamma). **B** is defined via the force **F** on a charge *q* moving with velocity **v**:

This is often written in terms of the force d**F** produced on a small element of length d**ℓ** that is carrying a current *I*:

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

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

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.