A capacitor consists of two concentric long cylindrical conductors of radii a and c, where c > a, and each is of negligible thickness.
The space between the conductors is filled with two layers of dielectric materials. Using cylindrical co-ordinates with z-axis along the axis of the cylindrical conductors, the space is filled with an LIH (linear, isotropic and homogeneous) dielectric of relative permittivity and the space with a second LIH dielectric of relative permittivity . The cylinders are sufficiently long that end effects can be neglected. The charge per unit length of the inner conducting cylinder is , and that on the outer conducting cylinder is .
a) Use the integral version of Gauss's law to find the electric field E and the electric displacement D in the region . Hence calculate the electrostatic field energy stored in a length l of the capacitor.
b) Hence find the capacitance of a length l of the capacitor.