please answer this :

Question

Positive charge Q is placed on a conducting spherical shell with inner radius R1 and outer

radius R2. A point charge q is placed at the center of the cavity. The magnitude of the electric

field produced by the charge on the inner surface at a point in the interior of the conductor a

distance r from the center, is ?

Answer 1

Gauss’ law says that the electric field for R1 < r < R2 = (1/4 pi epsilon0) (q + Qinner) /r^2

where Qinner is the charge on the inner surface of the shell. However, the electric field inside a conductor must be zero.  Hence Qinner = -q

From there is easy to see that the contribution to the electric field due to Qinner is

 -(1/ 4pi epsilon0) q / r^2. 

The charge on the outer surface of the shell is Q + q and contributes nothing to the electric field

for R1 < r < R2