1.2.4 Relationship to Thermodynamics
At this point it is appropriate to note the fundamental differences between heat
transfer and thermodynamics. Although thermodynamics is concerned with the heat
interaction and the vital role it plays in the first and second laws, it considers neither
the mechanisms that provide for heat exchange nor the methods that exist for computing
the rate of heat exchange. Thermodynamics is concerned with equilibrium
states of matter, where an equilibrium state necessarily precludes the existence of a
temperature gradient. Although thermodynamics may be used to determine the amount
of energy required in the form of heat for a system to pass from one equilibrium
state to another, it does not acknowledge that heat transfer is inherently a nonequilibrium
process. For heat transfer to occur, there must be a temperature gradient
and, hence, thermodynamic nonequilibrium. The discipline of heat transfer therefore
seeks to do what thermodynamics is inherently unable to do, namely, to quantify
the rate at which heat transfer occurs in terms of the degree of thermal nonequilibrium.
This is done through the rate equations for the three modes, expressed, for
example, by Equations 1.2, 1.3, and 1.7.