A short bar magnet of magnetic moment 1.28×10-2Am2  is arranged in the magnetic meridian  with its south pole pointing geographic north. If BH=0.4 G , the distance between the two null points  is

When a north pole of  a bar magnet is facing south , null points are obtained on axial line     Given : Ba=BH,   BH=0.4G=4×10−5T  ,  M=1.28×10−2  Am2We know, at axis , BH=μ04π2Md3⇒4×10−5=10−7×2×1.28×10−2d3 ⇒d3=2×1.284×10−4⇒d3=64×10−6 ⇒d=4×10−2m=4cmThe distance between two null points          2d=8cm