We now give an alternate derivation of (2.1) from the study of heat flow. Let D be a region
in Rn. Let x = [x1; : : : ; xn]T be a vector in Rn. Let u(x; t) be the temperature at point x,
1
time t, and let H(t) be the total amount of heat (in calories) contained in D. Let c be the
specific heat of the material and ½ its density (mass per unit volume). Then