# Why is the assumption of spheres being point charges invalid for small separations?

I have a hollow charged sphere and I am measuring the force between it and a grounded metal plate. How does the assumption that they are point charges (using method of image charges) become invalid for small separations?

So how is the force affected at small distances due to the charge being distributed on the surface close to the plate?

Also sidenote- how does the plate being finite affect the use of the method of image charges for this situation?

• Sidenote answer : Edge effects – Yuzuriha Inori Mar 10 '18 at 16:23
• Could you please explain the effect of the edges? – user187573 Mar 11 '18 at 17:04
• Sure! When we have a finite plate, there will be a sudden ending of the charge surface, and thus there will be accumulation of charges there that will cause bending of the electric field lines than what we would get if we had an infinite plate with no edges and no charge density problems. This change in the electric field is called edge effect, and if you make the edge very very sharp and pointed, the charges accumulate so much, there might be a breakdown near the edge (an electric discharge might be ). You might have seen this infinite plate approximation made in capacitors... – Yuzuriha Inori Mar 11 '18 at 17:08
• ... because if there were edges, the electric field lines won't be straight, and we would have a lot of difficulty modelling a capacitor. – Yuzuriha Inori Mar 11 '18 at 17:09