A bar magnet 20 cm in length is placed with its south pole towards geographical north the neutral points are situated at a distance of 40 cm from the centre of the magent if `H=3.2xx10^(-5)` weber/`metre^(2)` then the pole strength of the magnet is

Options

A. `9000 ab -axxcm`
B. `900 ab-axxcm`
C. `450ab-axxcm`
D. `225 ab-axxcm`

Answer

C

Explanation

Since neutral points are obtained on axial line of bar magnet
`therefore` for axial line B=H=`(mu_(0))/(4pi)(2m)/(d^(3))`
`H=(4ml)/(d^(3))`
`(Hd^(3))/(4l)` =m