A magnetic needle has a magnetic moment of `5 xx 10^(-2) Am^(2)` and moment of inertia `8xx10^(-6)` kg `m^(2)`. It has a period of oscillation of 2 s in a magnetic field `vecB`. The magnitude of magnetic field is approximately

Options

A. `3.2 xx 10^(-4) T `
B. `1.6 xx 10^(-4) T `
C. `0.8 xx 10^(-4) T `
D. `0.4 xx 10^(-4 )T `

Explanation

`T = 2pi sqrt((1)/(mB)) , B = (4 pi ^2 I)/( mT^2)`
` B=( 4 xx 10 xx 8 xx 10^(-6))/( 5 xx 10 ^(-2) xx 47 ) = 1.6 xx 10^(-3) T `