A heating element is enclosed within a spherical steel sheath of diameter 0.6 mm, which is in turn enclosed in a spherical copper shell of thickness 0.4 mm. The whole arrangement is placed in an air flow at a temperature of 20C. At steady state, the heating element keeps the outer surface of the steel sheath at 30C. The copper has a conductive heat transfer coefficient of k = 400 W.m-.K-, and the convective heat transfer coefficient at its surface is 20 W.m-.K-. a) Write down an expression for the overall thermal resistance for heat flow from the steel sheath to the ambient air. b) Calculate the temperature of the surface of the copper shell (i.e. the surface exposed to the air)