At the end of its manufacturing process, a silicon wafer of diameter D = 150 mm, thickness ð = 1 mm, and emissivity ɛ =0.65 is at an initial temperature of T_i = 325°C and is allowed to cool in quiescent, ambient air and large surroundings for which 7_∞ = T_sur = 25°C.

  (a) What is the initial rate of cooling?

  (b) How long does it take for the wafer to reach a temperature of 50°C? Comment on how the relative effects of convection and radiation vary with time during the cooling process.