Spherical cavity electric field

[GravityGirl]

A positive charge Q = 2800.00 C is uniformly distributed over the volume of a sphere of radius R = 6.00 m. Suppose a spherical cavity of radius R/2 is cut out of the solid sphere, the center of the cavity being a distance R/2 from the center of the original solid sphere (see figure). The cut-out material and its charge are discarded. What is the magnitude of the electric field produced by this new charge distribution at point P, a distance r = 19.00 m from the center of the original sphere?


so i am going to use the principle of superposition

E=Ebigsphere-Elittlesphere

inorder to find E i must find the charge and i can do so by using porportions

Qb/Vb=Ql/Vl

that is 28000/904.7787=Ql/113.0973 and Ql=3500

so i am going to use the formula E=KQ/R^2 for both big a little spheres

and i have E=(K28000/19^2)-(K3500/16^2)

with this i am not getting the right solution...please help me find where i am going wrong

 

[Doc Al]

Looks OK to me. Where is point P with respect to the two spheres? (Since we don't have the diagram, it's up to you to describe it carefully.)

 

[roanoar]

did you ever figure this out?