Two concentric spheres of diameters d1-0.3 m and d2-0.8 m are maintained at uniform temperatures t1 = 700 k and 72 400 k and have emissivities ? 1 0.5 and .7, respectively. determine the net rate of radiation heat transfer between the two spheres. also, determine the convection heat transfer coefficient at the outer surface if both the surrounding medium and the surrounding surfaces are at 30°c. assume the emissivity of the outer surface is 0.35.