Moving Charges And Magnetism

A rod of circular cross-sectional area A and length $L$ is wound uniformly with $n$ turns of an insulated wire. If current flowing through the windings is I, the total magnetic flux produced inside windings is $\phi$. The relative permeability of the rod is ( $\mathrm{N}=$ number of turns per unit length) $\left(\mu_0=\right.$ permeability of vacuum $)$