# Magnetomotive force in a specific case (the 2 in the nominator)

I am doing a research concerning some linear generators and therefore I am going trough a paper which describes magnetic circuits etc... We all know that the energy of a magnetic field per volume unit is $$W=\frac{B^2}{\mathbf{2}\mu}$$ ($$\mu$$ is the permeability). However the study that I am going trough writes the following thing (the attachement(comparative study of linear generator - Sung in Jeong)) :

I don't really understand where the equation 3.4 is coming from, because if I do some substitution, i find that the energy is not anymore $$W=\frac{B^2}{\mathbf{2}\mu}$$, but $$W=\frac{B^2}{\mu}$$... So i can't get where this $$2$$ comes from in the eq 3.4... Thank you very much for your response...

my email : leo.kruglikov@gmail.com