A magnet 20 cm long with its poles concentrated at its ends is palced vertically with its north pole on the table at a point due 20 cm if horizontal compoent of the earth 's field is H=0.3 gauss then the pole strength of the magnet is nearly

Options

A. 1.85 Am
B. 18.5 ab-A-cm
C. 185 A-m
D. 185 ab-A-cm

Answer

D

Explanation

Here `cos theta =cos 45^(@) =(1)/sqrt(2)`
Magnetic induction at P due to N pole
`=(m)/(20)^(2)` along NO
`therefore` Resultant induction at o at the horizontal direction `=(m)/(20^(2))-(m)/(20sqrt(2)^(2) cos theta`
`=(m)/(400)xx0.6465`

At neutral point `(m)/(400) xx0.6465=H=0.3` then
m=185 ab-A-cm