A magnet makes `40` oscillations per minute at a place having magnetic field intensity of `0.1xx10^(-5)T`. At another place, it takes 2.5 sec to complete one vibrating. The value of earth's horizontal field at that place is

Options

A. `0.25xx10^(-6)T`
B. `0.36xx10^(-6)T`
C. `0.66xx10^(-8)T`
D. `1.2xx10^(-6)T`

Answer

B

Explanation

`T=2pisqrt(I/(MxxB_(H)))impliesTprop1/sqrt(B_H)`
`rArr T_(1)/T_(2)=sqrt((B_(H))_(2)/((B_(H))_(1))) implies(60//40)/2.5=sqrt((B_(H))_(2)/(0.1xx10^(-5)))`
`rArr(B_(H))_(2)=0.36xx10^(-6)T`