MAGNETIC FLUX DENSITY (B)

A magnetic pole placed in free space, gives out lines of force in all directions.

The force experienced by a north pole of one weber when placed at P, at a distance r meters from a point pole of m webers, or the field intensity at P, is given by

H=m4π⋅μ0⋅μr⋅r2   N/Wb

The magnitude of field intensity at all points at a distance r from the point pole will be the same, but its direction will be different at different points; but in each case it will be right angles to the surface containing the point.

Now, let us consider a piece of soft iron is placed in a magnetic field H. Let B lines of magnetic induction per square meter of a surface be imagined at right angles to the flux direction.

The magnitude of B is known as the flux density. In  other words, flux density per unit area of a surface at right angles to the flux direction.

Now B will be proportional to H

i.e.,     B∝H      or        B=μ0⋅μr⋅H

Substituting the value of H from above, we have

B=μ0⋅μr⋅m4π⋅μ0⋅μr⋅r2=m4πr2Wb/m2

[Since 𝘮 is in webers and 4𝜋r² is the surface area of a sphere of radius r in meter, therefore B, the flux density is in webers per square meter].