Magnetic  Flux And Flux Density

A magnetising force produces a certain amount of magnetic flux (F), measured in Webers. The magnetic field is represented by imaginary lines of magnetic flux.  The number of lines of flux passing though a given area is called the 'flux density'. Flux density is denoted by the symbol B and given the unit Tesla.

          Flux density (B)  =   Φ/A Teslas

The unit of flux density is actually Webers per m², so:

          1 Tesla  = 1 weber/m²

 PERMEABILITY

When an mmf produces a magnetising force H, a certain flux density B is established.

          Ratio B/H is termed 'the permeability of the material'.

Permeability is an indication of the ability of the flux to permeate the material.  If the material in which the flux is established is a vacuum, or free space, then the ratio is called  'the permeability of free space' and given the symbol μ_o.  This value is considered to be a constant,  4 * 10^-7 H/M

If a flux is established in any material other than air or free space, then the flux density will increase.  The number of times by which the flux density increases is called the 'relative permeability of the material' denoted  by the symbol m_r.  This is not a constant but varies with different material. i.e. steel  =  800.

The product of m_o and m_r is called the 'absolute permeability' and is denoted by the symbol m.

          For all materials        =  μ  =  μ_o  x  μ_r