== Electrostatics Analysis==
At very low frequencies, as ω→0 and k<sub>0</sub>→0, one can derive the Poisson Helmholtz equation for the electric scalar potential reduces to the Poisson equation subject to specified boundary conditions:
<math>\Delta\Phi(\mathbf{r}) = \nabla^2 \Phi(\mathbf{r}) = -\frac{\rho(\mathbf{r})}{\epsilon}</math>