3.- (25 pts) Two concentric spheres of diameter D₁ = 0.8m and D₂ = 1.2m are separated by an air space and have surface temperatures of T₁ = 400 K and T₂ = 300 K. a) If the surfaces are black, what is the net rate of radiation exchange between the spheres? Draw an schematic of the corresponding thermal network. b) What is the net rate of radiation exchange between the surfaces if they are diffuse and gray with €1 = 0.5 and 2 = 0.05? Draw an schematic of the corresponding thermal network.