5) Two concentric spheres of diameter D1= 70 cm and D2= 120 cm are separated by an air space and have surface temperatures of T1
= 117 C and T2= 27 C. (a) If the surfaces are black, what is the net rate of radiation exchange between the spheres, in W? (b) What is the net rate of radiation exchange between the surfaces if they are diffuse and gray with ε1= 0.5 and ε2= 0.05, in W? (c) What is the net rate of radiation exchange if D2 is increased to 2000 cm, with ε2= 0.05, ε1= 0.5, and D1= 70 cm, in W? (d) What is the net rate of radiation exchange if the larger sphere behaves as a black body (ε2= 1.0) and with ε1= 0.5, D2= 2000 cm, and D1= 70 cm, in W?
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