Consider a uniformly charged sphere with radius R and charge per unit volume . (a) What is the magnitude of the electric field a distance

Consider a uniformly charged sphere with radius R and charge per unit volume ρ. 

(a) What is the magnitude of the electric field a distance r - R/3 from the center of the sphere? 

(b) Explain in words why the portion of the sphere that is more than r - R/3 from the sphere’s center does not contribute to the net electric field at r - R/3. 

(c) Now suppose a point charge is placed a distance r - 2R from the center of the sphere. Will this point charge change the electric field in part (a)? Explain why or why not.