(Question source: Halliday Resnick, 9th edition- exercise 23, Q20.)
The above question says that A and B are two non-conducting spheres enclosing a point charge in the centre. The second figure shows a graph of electric flux as a function of radius r. The scale of the vertical axis is set by $\Phi_s = 5.0 \times 10^5$ Nm$^2$/C. (a) What is the charge of the central particle? What are the net charges of (b) shell A and (c) shell B?
So to begin with, I understand that both shells would be uniformly charged since the field is uniform. However, I don't see the point of making them nonconducting shells, if we are treating them as metallic gaussian surfaces anyway unless insulators behave differently where electrostatic flux is concerned.
I don't understand why, for two values of $r$, there is a steep decline (multiple values) of $\Phi$. Could someone please explain?