At a point on the exial of a magnet,the magnetic at a induction is found to be 80 `muT`.The induction at a point on its equatorial line at twice the distance of the first point will be

Options

A. `10muT`
B. `5muT`
C. `20muT`
D. `40muT`

Answer

B

Explanation

`B("axis")=80muT,B_(eq)=?`
Now `B'_(eq)=? if r'_(eq) =2r_(eq)`
`B("axis")=2B_(eq)BN_(eq)=(B_("axis"))/(2)=(80)/(2)=40muT`
NOw`Boo(1)/(r^(3))`
`(B_(eq))/(B_(eq))=((r_(eq))/(r'_(eq)))^(3)`
`B'_(eq)=((r_(eq))/(2r_(eq)))^(3)xxB_(eq)=(40)/(8)=5muT`