1. The problem statement, all variables and given/known data Consider a spherical dielectric shell so that ε = ε_0ε_r for a < r < b and ε = ε_0 for 0 < r < a. If a charge Q is placed at the center of the shell, find a) P for a < r < b b) ρ_pv for a < r b c) ρ_ps at r = a and r = b 2. Relevant equations ρ_pv = -div(P) ρ_ps = P [itex]\cdot[/itex]a_n 3. The attempt at a solution I've managed to solve a) with the answer Q/(4*pi*r^2) * (ε_r-1)/ε_rwhich is correct (checked the answers). I've also solved b) which is ρ_pv = 0 since ρ_pv = -div(P). I have trouble to find ρ_ps at r = a and r = b. The answer states that -Q/(4*pi*a^2) * (ε_r-1)/ε_r and -Q/(4*pi*b^2) * (ε_r-1)/ε_r respectively. Where is that negative sign coming from?