The walls of a house consists of L=0.02 m thick plywood backed by

insulation with the same thickness. The temperature of the inside surface of the wall (the insulation side) is T2=25 oC, while the temperature at the outside surface (the plywoodside) is T1= -15 oC, both being constant. The thermal conductivities of the plywood and insulation are, respectively k1=0.09 J/(s m oC) and k2=0.03 J/(s m oC).

a) Find the temperature T at the plywood – insulation interface.

b) If the total surface area of the walls is 70 m2, find the amount of heat the house loses in 24 hours. (Assuming all the heat losses are due to the thermal conductivity of the walls).

c) Does the entropy of the house change? If it does, by how much?

d) Does the entropy of the Universe change? If it does, by how much?

e) If the cost of electric power is $0.15 per kilowatt hour, how much one has to pay to keep the house warm for 24 hours?