First slide
The bar magnet
Question

A short bar magnet with its north pole facing north forms a neutral point at P in a horizontal plane. If the magnet is rotated by 90o in the horizontal plane, the net magnetic induction at P is (Horizontal component of earth's magnetic field = BH)

Moderate
Solution

At null point,

BH = Be

When magnet is rotated by 900 then the point will be on the axial line and BBa=2Be=2BH and they will be perpendicular to each other so resultant magnetic field will be 

B=BH2+4BH2=5BH

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