Note Earth's field goes from South to North: At neutral point horizontal component of earth, is opposed by field of magnet so, at neutral point, BH=Bmagnet ⇒ BH=
µ0
4Ï€
.
M
r3
Where, M= magnetic dipole moment of magnet and r= distance of neutral point from centre of magnet Here, BH=3.2×10−5T r−6cm=6×10−2m Hence, 3.2×10−5=10−7×
M
(6×10−2)3
⇒ M=
3.2×10−5×216×10−6
10−7
=6.×10−2A−m2 If length of magnet is taken 2l, then M=m.2l, (∵ m= pole strength) Here, 2l=16cm=16×10−2m ⇒ 6.9×10−2=m×16×10−2 ⇒ m=