The magnetic needle of a `V.M.M` completes `10` oscillaitons in `92`seconds. When a small megnet is placed in the magnetic meridian `10cm` due north of needle with north pole towards south completes `15` oscillations in `69`seconds. The magnetic moment of magnet

Options

A. `4.5Am^(2)`
B. `0.45Am^(2)`
C. `0.75 Am^(2)`
D. `0.225Am^(2)`

Answer

C

Explanation

`T_(1)=9.2S, T_(2)=4.6 S` and `(T_(1))/(T_(2))=sqrt((B_(2))/(B_(1))`
`rArr(9.2)/(4.6)=sqrt((B-B_(H))/(B_(H))=B=5B_(H)=1.5xx10^(-4)T`
`rArr(M_(@))/(4pi)(2M)/(d^(3))=1.5xx10^(-5)rArrM=0.75 Am^(2)`