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S E C T I O N 2 2 . 8 • Entropy on a Microscopic Scale 693 Macrostate Possible Microstates Total Number of Microstates All R RRRR 1 1G, 3R RRRG, RRGR, RGRR, GRRR 4 2G, 2R RRGG, RGRG, GRRG, RGGR, 6 3G, 1R GGGR, GGRG, GRGG, RGGG 4 All G GGGG 1 Possible Results of Drawing Four Marbles from a Bag Table 22.1 Example 22.11 Adiabatic Free Expansion—One Last Time Let us verify that the macroscopic and microscopic ap- (A) Using a macroscopic approach, calculate the entropy change for the gas. (B) Using statistical considerations, calculate the change in entropy for the gas and show that it agrees with the answer Solution (A) Using Equation 22.13, we have (B) The number of microstates available to a single mole- cule in the initial volume V i is . For N molecules, w i ! V i /V m nR
ln
4 % S ! nR
ln " V f V i # ! nR
ln " 4V i V i # ! the number of available microstates is The number of microstates for all N molecules in the final f ! 4V i is Thus, the ratio of the number of final microstates to initial Using Equation 22.18, we obtain The answer is the same as that for part (A), which dealt with What If? In part (A) we used Equation 22.13, which was based on a reversible isothermal process connecting the Answer We must arrive at the same result because entropy is i to 4V i , (A : B) during which the temperature drops from T 1 to T 2 , and a reversible isovolumetric process (B : C) that takes the gas back to the initial temperature T 1 . During the reversible adiabatic process, %S ! 0 because Q r ! 0. During the reversible isovolumetric process (B : C), we have from Equation 22.9, nR
ln
4 ! k B ln(4 N ) ! Nk B
ln
4 ! % S ! k B lnW f " k B lnW i ! k B ln
" W f W i # W f W i ! 4 N W f ! " V f V m # N ! " 4V i V m # N W i ! w i
N ! " V i V m # N V P V i 4V i B C A T 1 T 2 Figure 22.20 (Example 22.11) A gas expands to four times its initial volume and back to the initial temperature by means of a two-step process. Explore the generation of microstates and macrostates at the Interactive Worked Example link at http://www.pse6.com. macrostate—two red marbles and two green marbles—corre- most ordered macrostates—four red marbles or four green |