The Direction of Heat Transfer
A temperature reservoir is defined as a large body whose temperature does not change when heat is removed or added. Figure 4.4 shows heat transfer between two reservoirs of temperatures TH and TL, where TH is the
temperature of the hotter system. The heat is transferred through a heat conductor, which is the thermodynamic system to be evaluated. A pure heat transfer problem is studied, where the conductor receives the heat flows Q˙ H and Q˙ L, and exchanges no work with the surroundings, W˙ second law (4.23, 4.24) applied to the heat conductor read
Clausius’ original derivation of the second law is based on the statement that heat will go from hot to cold by itself, but not vice versa. We shall use this statement to learn more on thermodynamic temperature T . Since we declared TH as the temperature of the hotter reservoir, heat should go from the reservoir at TH to the reservoir at TL. According to Fig. 4.4 the proper direction of heat transfer in accordance to Clausius’ statement is for positive
Figure 4.5 gives an illustration of the allowed process, where heat goes from hot to cold, and the forbidden process, where heat would go from cold to hot by itself.