where Ax/2 is the volume of the volume element (note that the boundary element
has half thickness), g ·
0 is the rate of heat generation per unit volume (in
W/m3) at x 0, and A is the heat transfer area, which is constant for a plane
wall. Note that we have x in the denominator of the second term instead of
x/2. This is because the ratio in that term involves the temperature difference
between nodes 0 and 1, and thus we must use the distance between those two
nodes, which is x.