The frequency of the precession is linked to the strength of magnetic field, which can be detected as absorption of magnetic field supplied at the exact frequency at which the precession occurs. The torque acting on the spins makes them precess. Magnetic moments of atoms and subatomic particles ( electrons, protons, neutrons) when exposed to magnetic field B have their spins quantised. This approach is extensively used in numerical calculations such as finite-element modelling (FEM), where the direct link between the electric current (expressed by current density J) and H is exploited, through the Ampere's law, both for solutions and formulations of boundary conditions. However, under certain conditions it is also possible to express them with respect to H. The equations are typically given with respect to magnetic flux density B because in that form they are valid under more general conditions. The application of Faraday's law of induction and Ampere's circuit law are fundamental for operation of all electric generators, transformers, as well as many transducers and sensors. They can also be formulated on the basis of more fundamental theory of quantum electrodynamics. differential or integral form) or different units (e.g. The equations can be mathematically written in many ways (e.g. Maxwell's equations fully describe mathematically the interrelation between electric and magnetic fields. We choose to define a new vector field H by $$\mathbf$$ We are saying, in effect, that since there are forces “acting on” the charge, there is still “something” there when the charge is removed. We wish now to speak of electric and magnetic fields at a point even when there is no charge present. We have defined them in terms of the forces that are felt by a charge. Sands The Feynman Lectures on Physics 28)įirst, we must extend, somewhat, our ideas of the electric and magnetic vectors, E and B. There are many other names which are used in the literature, all denoting the same quantity:ĭefinition of magnetic field strength $H$ 6) Therefore, strictly speaking, other names like induction or B field which can be encountered in everyday technical jargon 7) are incorrect if referring to a specific value expressed in the units of T. The name magnetic flux density and the symbol $B$ are defined by International Bureau of Weights and Measures (BIPM) as one of the coherent derived physical units. ![]() This is also reflected in the naming convention. These two viewpoints are equivalent for the concerned numerical quantities, but different emphasis might be put on “importance” of either of them. ![]() The quantity $B$ encompasses all of the magnetic response of the medium (including $M$ or $J$, as well as any non-magnetic contribution arising from the applied $H$ due to external electric current, or even internal eddy currents). 3) 4) 5)įrom engineering viewpoint, magnetic field strength $H$ can be thought of as excitation, and the magnetic flux density $B$ as the response of the magnetised medium. In vacuum, at each point the $H$ and $B$ vectors are oriented along the same direction and are directly proportional through permeability of free space, but in other media they can be misaligned (especially in highly anisotropic materials - see also the illustration in the next section).įrom theoretical physics viewpoint, the very definition of magnetic field involves $B$, which is called “ the magnetic field”. \) can be interpreted as an equivalent surface charge density that would give rise to the observed electric field, and in some cases, this equivalent charge density turns out to be the actual charge density.Magnetic field is a vector field in space, and is a type of energy whose full quantification requires the knowledge of the vector fields of both magnetic field strength $H$ and flux density $B$ (or other values correlated with them, such as magnetisation $M$ or polarisation $J$).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |