Chips of width \(L=15 \mathrm{~mm}\) on a side are mounted to a substrate that is installed in an enclosure whose walls and air are maintained at a temperature of \(T_{\text {sur }}=25^{\circ} \mathrm{C}\). The chips have an emissivity of \(\varepsilon=0.60\) and a maximum allowable temperature of \(T_{s}=85^{\circ} \mathrm{C}\).

(a) If heat is rejected from the chips by radiation and natural convection, what is the maximum operating power of each chip? The convection coefficient depends on the chip-to-air temperature difference and may be approximated as \(h=C\left(T_{s}-T_{\infty}\right)^{1 / 4}\), where \(C=4.2 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}^{5 / 4}\).

(b) If a fan is used to maintain airflow through the enclosure and heat transfer is by forced convection, with \(h=250 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), what is the maximum operating power?

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