Heat conduction in an annulus (Fig. 10B.4), 

(a) Heat is flowing through an annular wall of inside radius r₀ and outside radius r₁. The thermal conductivity varies linearly with temperature from k₀ at T₀ to k₁ at T₁. Develop an expression for the heat flow through the wall. 

(b) Show how the expression in (a) can be simplified when (r₁ - r₀)/r₀ is very small. Interpret the result physically.