Magnetic lines of force through soft ring

[abhishek.93]

why there is no magnetic lines of force through soft ring placed in a magnetic field ??

 

[Bob S]

I assume you are modeling a soft magnetic ring in a uniform magnetic field perpendicular to the axis of the ring (or axis of cylinder). The soft ring shields the inside from the outside magnetic field. Here let's choose B=0.01 Tesla for the uniform field outside the ring, and the outside diameter of the ring is d= 10 cm, then roughly all the field from 20 cm flows through (is concentrated in) the iron of the ring. If the thickness of the ring is t = 0.2 cm, then the maximum field in (not inside) the soft iron of the ring is B_iron = d/t times B = 50 *0.01 = 0.5 tesla.
Now find the point in the ring where the flux is highest (where it is parallel to the outside field). Recall that H parallel is continuous across boundaries. The magnetic intensity inside the ring is the same as the magnetic intensity H in the iron. If the relative permeability of the iron is 5,000, then H in the iron is 0.5 Tesla /5000 = 0.0001 amp-turns per meter. Because the relative permeability of the air inside the ring is 1, the field inside the ring is 0.0001 Tesla and is parallel to the field outside the ring. This is true only if the field concentrated in the iron is less than 1 tesla, otherwise the iron saturates..