A spherical conducting shell of inner radius $r_1$ and outer radius $r_2$ has a charge Q. A charge q is placed at the centre of the shell. The surface charge density on outer surface of the shell is:

$\frac{Q+q}{4πr_2^2}$ $\frac{Q-q}{4πr_2^2}$ $\frac{Q+q}{4π(r_2^2-r_1^2)}$ $\frac{Q-q}{4π(r_2^2-r_1^2)}$

$\frac{Q+q}{4πr_2^2}$

The correct answer is Option (1) → $\frac{Q+q}{4πr_2^2}$