where Φ(<b>r</b>) is the electric scalar potential, ρ(<b>r</b>) is the volume charge density, and ε is the permittivity of the medium.
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The boundary conditions at the interface between two material media are:
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<math> \mathbf{n} \times ( \mathbf{E_2(r)} - \mathbf{E_1(r)} ) = 0 </math>