How Do You Calculate Electrostatic Potential Outside a Sphere?

[PedroB]

Homework Statement

I'm currently tackling parts (a) and (b) of section ii) decribed here:http://puu.sh/31KIZ.png

Homework Equations

V=(q/4∏ε)*(1/r)

The Attempt at a Solution

From the first section (i) calculating the potential was relatively straight forward since we were only considering a plane. In part (a) however it just asks me for the potential outside the sphere, and I'm unsure how to calculate this since I can't seem to figure out how to determine the potential at any single point due to either charge (especially since I can't work out the distance between them). Though section (b) also seems puzzling to me, I'm guessing that solving part (a) will aid me in figuring it out.

 

[Simon Bridge]

... you already know an equation for the potential due to a point charge don't you?
The point of the method of images is to replace the distributed charges on the conductor with a single charge and no conductor.

Part b requires you to find what Q has to be - so you only need the potential in general for part a.