$1$ Hot water flows inside a PVC pipe $($conductivity $0\mathrm{.}15W{m}^{-1}{K}^{-1})$ of inner diameter $25$ mm and wall thickness $2\mathrm{.}5$ mm $.$ The inside surface temperature is $60C$ and the outside surface temperature is $55C.$ What is the heat transfer rate through the pipe wall, per unit length of pipe? What error would result from treating the pipe as a plane wall $($neglecting its curvature$)?$

$($ans $25\mathrm{.}8$ W$/$m$,0\mathrm{.}1\%)$