A 10-cm-diameter sphere maintained at 30?C is buried in the earth at a place where k =1.2W/m· ?C. The depth to the centerline is 24 cm, and the earth surface temperature is 0?C. Calculate the heat lost by the sphere. 3-12 A 20-cm-diameter sphere is totally enclosed by a large mass of glass wool. A heater inside the sphere maintains its outer surface temperature at 170?C while the temperature at the outer edge of the glass wool is 20?C. How much power must be supplied to the heater to maintain equilibrium conditions?