the right question?

Question

A solid conducting sphere with radius R carries a positive total charge Q The sphere is surrounded by an insulating shell with inner radius R and outer radius 2R The insulating shell has a uniform charge density p (p stands for rho)
Find the value of p (rho) so that the net charge of the entire system is zero

Answer 1

Volume of insulating shell =

4/3pi(2R)^3 - 4/3pi(R)^3

= 4/3pi.7R^3

So p = -Q/V = -3Q/(28piR^3)

Answer 2

well the net charge on the hollow sphere should be -Q to make system having no charge
volume of hollow sphere = volume of solid sphere of radius 2R- vol. of solid sphere of radius R
i.e.
4/3 pi (2R)^3 - 4/3 pi (R)^3= 28/3 pi R^3
so
p=-Q/(vol. of hollow sphere)
or
-3Q/28 pi R^3

Answer 3

Good luck with that and finding the right answer.