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磁场[Magnetic Field] [2005-9-26]
iamet 发表在 Ω〖物理〗
| Magnetic fields, commonly denoted B, are caused by currents, or equivalently by the movement of charges. In particular, the fourth of the Maxwell equations (in cgs) gives
 (1)
where c is the speed of light and J is the current density. The magnetic field due to a current loop of given configuration can be calculated directly from the Biot-Savart law in cgs as
 (2)
where I is the current, a differential length element, and r the position vector, and in MKS as
 (3)
where  is the permeability of free space.
The absence of magnetic monopoles results in
 (4)
which is the third of the Maxwell equations.
The magnetic field can be given in terms of the magnetic vector potential by
Magnetic field strength is measured in Gauss or gamma (cgs) or Tesla (MKS). The magnetic field in a solid is denoted H and is measured in Oersted.
In general, magnetic fields are found together with electric fields, with the combination known as an electromagnetic field. Special relativity says that depending on the reference frame the observer, electric and magnetic fields will appear to have different strengths, but the resulting physical effects will be identical. The electromagnetic field is treated most elegantly using the so-called electromagnetic field tensor. |
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