\(\frac{\partial\overrightarrow{B}}{\partial t}+\left(\overrightarrow{U}.\nabla\right)\overrightarrow{B}=\frac{1}{\text{μσ}}\nabla^{2}\overrightarrow{B}+\left(\overrightarrow{B}.\nabla\right)\overrightarrow{U}\) (6)