A magnet freely suspended in a vibration magnetometer makes 40 osillations per' minute at place A and 20 oscillations per minute at a place B. If the horizonatal component of earth's magnetic field at A is ` 36xx10^(-6)T,` then its value at B is

Options

A. `30xx10^(-6) T`
B. `9xx10^(-6) T`
C. `144xx10^(-6) T`
D. `288xx10^(-6) T`

Answer

B

Explanation

Time period of vibration magnetometer,
`T=2pisqrt((I)/(MB_(H)))`
`n=1/2pisqrt((MB_(H))/(I))`
`npropsqrtB`
`n_(A)/n_(B)=sqrt(B_(A)/B_(B))`
`implies40/20=sqrt((36xx10^(-6))/(B_(B)))``impliesB_(B)=(36xx10^(-6))/((2)^(2))=9xx10^(-6)T.`