Overhead electric transmission lines get heated (due to the electric resistivity of the cable material) and lose heat to the ambient air; this transmission energy loss due to heat transfer is referred to as the Joule effect in the conduction material. Consider a 3-cm diameter horizontal cable that dissipates heat at the rate of 50 W per unit length, and that the heat transfer due to surrounding air is only by natural convection. Determine the surface temperature of the cable when 

(a) In the summer time the air temperature is 35°C 

(b) In the winter time the air temperature is –5°C