3)          A single pane of glass 1 cm thick, 50 cm wide and 100 cm long, loses heat from the inside
             of a house at a rate of 1.25 kW, when the inside of the house is maintained at 68F and the
             outside temperature is 23F.

    (a)     Calculate the thermal conductivity of the glass                                                           (5)

   
(b)     The thermal conductivity of air is 40 times smaller than that of glass. The window pane is
              replaced by a double-glazed pane having a 1 mm air gap, indicated below. What are the
              temperatures of the inner faces of the glass (T₁, T₂) ?                                             (10)

              In steady state heat flow through each glass pane and the air gap must be equal, thus,

   
(c)     Determine the rate of heat loss through this, double-glazed, window.                          (6)

   
(d)     In practice a double-glazed window is never able to achieve the reduction in heat loss
             determined from (a) to (c) above. Can you explain why this is the case ?                     (4)

             Air in the gap is not stagnant. Convection currents will be set up which will add to the
             conduction heat transfer.