EFFECT OF MATERIALS ON ELECTROMAGNETIC FIELDS
Magnetic properties are important because of their effect on the behavior of materials under an external field or when the external field is removed. The magnetic properties are often discussed using the magnetic permeability of materials. This important quantity is defined through the relation in Eq.
B =μB
where B is flux density and His magnetic field strength.
Permeability governs an important features of the magnetic field and therefore affects any application that uses the magnetic field. Flux density B is often the quantity of interest and has higher values for high values of the permeability for a given source field strength H.
The magnetic properties of materials are defined through the interaction of external magnetic fields and moving charges in the atoms of the material. Static charges are not influenced by the magnetic field since no magnetic forces are produced. Atomic scale magnetic fields are produced inside the material through orbiting electrons. These orbiting electrons produce an equivalent current
loop that has a magnetic moment.
where a2 is the area of the loop, I is the equivalent current and z is a unit vector normal to the plane of current flow. Many such atomic scale loops or magnetic moments exist and the material volume contains a certain magnetic moment density. If N magnetic moments per unit volume are present, and if these
moments are aligned in the same direction, a total magnetization is generated. Equation is used to determine magnetization M.
Hin = M = Nm
The magnetic flux density of the material is then given by:
Bin = μM
The terms H, m and Mare vectors. This implies that a net magnetic field or flux density can only exist if these vectors are aligned in such a way that a total net vector M exists. If the independent vectors mare randomly oriented, as is often the case, the net magnetization is zero.
Additionally, the passage of magnetic fields through space exiting a ferromagnetic material field will align domains to produce a leakage field. This leakage field is capable of attracting very small magnetizable particles of engineered properties, enabling the detection of discontinuities that have accumulated clusters of these particles.
Units of Measure for Magnetic Flux
A magnetic field is made up of flux lines within and surrounding a magnetized object or a conductor carrying an electric current. The term magnetic flux is used when referring to all of the lines of flux in a given area. Flux per unit area is called magnetic flux density, the number of lines of flux passing transversely through a unit area.