Each atom of an iron bar (`5cmxx 1cm xx 1cm`) has a magnetic moment `1*8xx10^(-23)Am^2`.
(a) What will be the magnetic moment of the bar in the state of magnetic saturation.
(b) What will be the torque required to place this magnetised bar perpendicular to magnetic field of `15000 gauss`?
Density of iron `=7*8xx10^3kg//m^3`, Atomic wt. of iron `=56`,
Avogadro's number `=6*023xx10^(23)//gm` mole.

Explanation

Here, volume of specimen `V=5xx1xx1c.c.=5xx10^-6m^3`
density of iron `rho=7*8xx10^3kg//m^3`
Mass of specimen `=V.rho=(5xx10^-6)xx7*8xx10^3kg=39xx10^-3kg`
As `N=6*026xx10^(26)//kg` mole and `A=56kg`
`:.` Number of atoms in iron bar `=(6*02xx10^(26)xx39xx10^-3)/(56)=4.19xx10^(23)`
`:.` Saturated magnetic moment of the bar, `M=4.19xx10^(23)xx1*8xx10^(-23)=7.54Am^2`
`tau=MB sin theta=7.54xx(15000xx10^-4)sin 90^@=11*31Nm`