A long, $4 \mathrm{~cm}$-square bar has opposite faces maintained at $80^{\circ} \mathrm{C}$ and $0^{\circ} \mathrm{C}$, respectively, and the other two lose heat by convection to a fluid at $0^{\circ} \mathrm{C} . \mathrm{An}$ electric current flows in the bar and causes a resistance heating of $3 \times 10^{6} \mathrm{~W} / \mathrm{m}^{3}$. The bar has a thermal conductivity of $15 \mathrm{~W} / \mathrm{m} \mathrm{K}$, and the convective heat transfer coefficient is $1500 \mathrm{~W} / \mathrm{m}^{2} \mathrm{~K}$.
(i) Determine the finite-difference approximation for an interior node, and for a surface node on the convectively cooled faces.
(ii) For the given mesh, use Gauss-Seidel iteration to determine the temperatures $T_{1}$ through $T_{9}$ after the first and second iterations.