I am using Guru's Electromagneic Theory Fundamentals: https://www.amazon.ca/Electromagnetic-Field-Theory-Fundamentals-Singh/dp/0521116023

For finding the electric field intensity, I am using the equation:

I am unsure about what the value of r prime should be for a solid.

r is the vector position of the point in question - ie the centre of the sphere.

edited Jan 20 by sammy gerbil