The gap between the plates of a parallel-plate capacitor is filled up with two dielectric layers I and 2 with thicknesses d1 and d2, permittivities ε1 and ε2, and resistivity Pl and p2. A dc voltage V is applied to the capacitor, with electric field directed from layer 1 to layer 2. Find or, the surface density of extraneous charges at the boundary between the dielectric layers, and the condition under which σ = O.