An infinite slab of thickness 'w' is having faces faces T1 and T2, The fourier's equation for heat flow along x-direction is $$\frac{Q}{A} = -k \frac{\delta T}{\delta x}$$ Boundary conditions are x = 0, T = T₁ and x = w, T = T₂. Q not equal to F(x), A not equal to F(x) and k is not equal to F(x). Q is given by :

Hollow sphere temperature varies such that at r = R, T = T_R.Boundary condition at r = r₁ temperature T = T₁ and at r = r₂ temperature T = T₂ and Q is given by $$Q = \frac{4 \pi k (T_1 - T_R)}{\frac{1}{r_1} - \frac{1}{R}}$$ $$\frac{T_1 - T_R}{T_1 - T_2} \ \text{is given by}$$