A problem involving E, D & P. I have the answer but not sure why for the P bit.A bit of clarification if someone could, please. A thick sheet of polystyrene, of relative permittivity ε = 2.5, is placed normal to a uniform electric field of magnitude 500Vm−1 in air. Find the magnitudes of the electric field E, the electric displacement D and the polarization P inside the material. OK div D = ρf and D = εε0E Boundary conditions Doutside = Dinside → εoutEout = εε0Ein so Ein = Eout / ε Ein = 500/2.5 Ein = 200 Vm-1 Dout = Din = ε0 Eout Dout = 8.85x10-12 x 200 Dout = 1.77x10-9 Fine so far . . . When I get down to calculating P I have to use Din = ε0Ein + Pin why, when everything else has the suffix "in" do I use ε0? Shouldn't it be just plain ε? Thanks in advance.