1. The problem statement, all variables and given/known data A point charge Q is placed at a perpendicular distance a from an infinite planar LIH (linear, isotropic, homogeneous) dielectric of relative permittivity ε. By considering the normal component of the electric field at an arbitrary point just inside the dielectric to be the superposition of the normal components of the electric fields produced by the point charge Q and the induced surface charge σ on the dielectric, show that σ=- (ε-1)Qa /2∏(ε+1)r^3 2. Relevant equations Normal component due the point charge will be: E=Qa/(4∏ε0εr^3) Induced surface charge due to the electric field from Q will be: σ=P.n (P is the polarisation) σ=(ε-1)E The induced surface charge produced its own electric field which is obtained from gauss' law and is just the surface charge divided by ε0. 3. The attempt at a solution So the difference between these two fields gives the resultant field at the point just inside the dielectric. But I'm uncertain as to why the questions then wants another value for surface charge density, will this change because of the resultant field. I thought this was induced anyway such that it already accounts for the field it produced and that of the point charge. I'm therefore confused as to how to proceed.