$10\mathrm{.}5.$ A straight, long copper conductor of radius $a$ is covered

with a layer of iron of thickness $d.$ A current of intensity

I exists in this composite wire. Assuming that the iron

permeability is $,$ determine the magnetic field, the magnetic

flux density, and the magnetization in copper and iron parts of

the wire. Note that the current density in the copper and iron

parts of the wire is not the same. Hint: assume that ${}_{Cu}$ is the

conductivity of copper, and ${}_{Fe}$ that of iron. Note that the

electric field intensities in copper and iron are the same $($due

to continuity of $E_{tang}$ on the boundary.