In Fig 5.4 (b), the magnetic needle has magnetic moment `6.7 xx 10^(-2) Am^(2)` and moment of inertia `I=7.5 xx 10^(-6) Kg m^(-2)`. It performs 10 complete oscillation in 6.70s. What is the magnitude of the magnetic field?

Explanation

The time period of oscillation is,
`T=(6.70)/(10)=0.67s`
From Eq. (5.5)
`B=(4pi^(2)g)/(m T)`
`=(4 xx (3.14)^(2) xx 7.5 xx 10^(-6))/(6.7 xx 10^(-2) xx (0.67)^(2))`
=0.01T