Two concentric spheres of diameters D₁ = 15 cm and D₂ = 25 cm are separated by air at 1 atm pressure. The surface temperatures of the two spheres enclosing the air are T₁ = 350 K and T₂ = 275 K, respectively, and their emissivities are 0.75. Determine the rate of heat transfer from the inner sphere to the outer sphere by

(a) Natural convection and

(b) Radiation.