A thick cylindrical shell made of material of thermal conductivity k has inner and outer radii r and R respectively and its length is L. When the curved surface of the cylinder are lagged (i.e., given insulation cover) and one end is maintained at temperature `T_(1)` and the other end is maintained at `T_(2)(lt T_(1))`, the heat current along the length of the cylinder is H. In another experiment the two ends are lagged and the inner wall and outer wall are maintained at `T_(1)` and `T_(2)` respectively. Find the radial heat flow in this case.

Answer

`(2L^(2)H)/(R^(2)-r^(2)ln((R)/(r)))`