The cable

The coaxial cable that you are going to model is shown below at equilibrium.

The cable is long and straight, so you can assume that variations in the electric field are in the radial direction only. Insulators A and B have the following properties at room temperature.

Material	$\rho$ (C/m)	$\epsilon$
Insulator A	$6 \times 10^{-3}$	3.2
Insulator B	$2 \times 10^{-3}$	2.2

However, when the cable is heated or cooled, the insulations expand and contract differentially. The entire cable is encased in a conducting shielding that prevents the cable radius from changing, and the conductor does not expand appreciably under the temperature changes under consideration. Thus, the boundary between insulator A and insulator B changes from the room temperature value of 50 mm.