## Essential University Physics: Volume 1 (4th Edition)

a) We know that the heat loss through the roof will still be 1839 Btu/h. We can find the new heat loss through the walls by using the change in surface area of the walls: $=5963\times (\frac{1506-125}{1506})=5468 \ Btu/h$ Now, we find the heat loss through the windows: $=(\frac{1}{.9}\times(2.5)(5)(10 \ windows)(50^{\circ}F)=6944 \ Btu/h$ Thus, we find: $=(6944+1839+5468)(24 h/day)(30day/month)=10.25 MBtu$ This corresponds to a cost of 356.43 dollars. However, we must also considered the energy that comes through the south windows, which we are asked to find in part b, anyways. Thus, we obtain: $=(30)(24)(30)(2.5)(5)(4\ windows)=1.08\ MBtu$ This corresponds to a cost of $\fbox{37.58 dollars per month}$ This means that the answer to part a is: $=356.43-37.58\approx\fbox{319 dollars per month}$