Uniform constant magnetic field and traditional attractive force Why can't uniform, constant magnetic fields exert a net force on a piece of iron however strong it might get?
 A: Whenever iron is place in any magnetic field, there is a Magnetization ($\vec M$) produced in it which depends on its susceptibility($\chi_m$) and the external field, and therefore there is a net induced dipole moment which changes the field configuration.Suppose a rod shaped piece of iron is placed in a constant uniform magnetic field ($\vec {B_0}$), the induced dipole moment($\vec {\mu}$) will be along the field. The energy (potential) of this dipole is $-\vec\mu.\vec B$. where $\vec B$ is the net magnetic field.
To calculate the force on it we use $\vec F=-\nabla U$ ($U$ is the potential energy).
Therefore the magnetic force on the dipole of moment $\vec\mu$ is $-\vec\mu.\frac{d\vec B}{dx}$ in the $x$ direction and so on for other directions. But since our magnetic field is uniform and constant, All derivatives with respect to "space" will be zero and consequently the force will be zero. This will be true for all magnetic moments induced in the iron piece and hence the net force is also zero.
