Unformatted text preview: positive. EXAMPLE 7–19 Entropy Generated when a Hot Block Is Dropped in a Lake
A 50-kg block of iron casting at 500 K is thrown into a large lake that is at a
temperature of 285 K. The iron block eventually reaches thermal equilibrium
with the lake water. Assuming an average specific heat of 0.45 kJ/kg · K for the
iron, determine (a) the entropy change of the iron block, (b) the entropy change
of the lake water, and (c) the entropy generated during this process. cen54261_ch07.qxd 11/18/03 9:57 AM Page 327 327
CHAPTER 7 SOLUTION We take the iron casting as the system (Fig. 7–67). This is a closed
system since no mass crosses the system boundary during the process.
To determine the entropy change for the iron block and for the lake, first we
need to know the final equilibrium temperature. Given that the thermal energy
capacity of the lake is very large relative to that of the iron block, the lake will
absorb all the heat rejected by the iron block without experiencing any change
in its temperature. Theref...
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