# Why do we have to do work to remove iron nucleus from an inductor?

Why do we have to do work to remove iron nucleus from an inductor?

Since the energy associated with the inductor is $$E=\frac{Li^2}{2}$$, and $$L$$ is proportional to relative magnetic permeability $$\mu_r$$, then $$L=\mu_rL'$$ and $$E=\frac{\mu_r L'i^2}{2}$$. Ok... Let say that $$\mu_r > 1$$ for the iron and $$\mu_r = 1$$ for the air, then the magnetic energy with the iron ($$E_+$$) is greather than the magnetic energy without the iron ($$E_-$$). So, if I am going from a higher energy to a lower energy state, why I should do work on the system to move from one state from another?