A small disk of diameter D₁ = 50 mm and emissivity ?₁ = 0.6 is maintained at a temperature of T₁ = 900 K. The disk is covered with a hemispherical radiation shield of the same diameter and an emissivity of ?₂ = 0.02 (both sides). The disk and cap are located at the bottom of a large evacuated refractory container (?₄ = 0.85), facing another disk of diameter D₃ = D₁, emissivity ?₃ = 0.4, and temperature T₃ = 400 K. The view factor F₂₃ of the shield with respect to the upper disk is 0.3.

(a) Construct an equivalent thermal circuit for the above system. Label all nodes, resistances, and currents.

(b) Find the net rate of heat transfer between the hot disk and the rest of the system.

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