The distance of two points on the axis of a magnet from its centre is `10 cm` and `20 cm` repectively. The ratio of magnatic intensity at these points is `12.5 : 1`. The length of the megnet will be

Options

A. 5cm
B. 25cm
C. 10cm
D. 20cm

Answer

C

Explanation

`B_(1)/B_(2)=d_(1)/d_(2)((d_(2)^(2)-l^(2))/(d_(2)^(1)-l^(2)))^(2) rArr 12.5/1=10/20((400-l^(2))/(100-l^(2)))^(2)`
`rArr l=5 cm`
Hence, length of magnet `=2l=10 cm`