Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 14 - Section 14.3 - Partial Derivatives - 14.3 Exercise - Page 926: 87

Answer

$$ \left(P+\frac{n^{2} a}{V^{2}}\right)(V-n b)=n R T $$ $\Rightarrow$ $$ \frac{\partial T}{\partial P}=\frac{V-n b}{n R}, $$ and $$ \begin{aligned} \frac{\partial P}{\partial V}=\frac{2 n^{2} a}{V^{3}}-\frac{n R T}{(V-n b)^{2}} \end{aligned} $$

Work Step by Step

$$ \left(P+\frac{n^{2} a}{V^{2}}\right)(V-n b)=n R T \quad\quad\quad\quad (1) $$ $\Rightarrow$ $$ T=\frac{1}{n R}\left(P+\frac{n^{2} a}{V^{2}}\right)(V-n b), $$ To find $\frac{\partial T}{\partial P}$, we differentiate implicitly with respect to $P$, being careful to treat $V$ as a constant: $$ \frac{\partial T}{\partial P}=\frac{1}{n R}(1)(V-n b)=\frac{V-n b}{n R}. $$ Now, we rewrite Eq. (1) as follows $$ P+\frac{n^{2} a}{V^{2}}=\frac{n R T}{V-n b} $$ $\Rightarrow$ $$ P=\frac{n R T}{V-n b}-\frac{n^{2} a}{V^{2}}=n R T(V-n b)^{-1}-n^{2} a V^{-2}. $$ To find $\frac{\partial P}{\partial V}$, we differentiate explicitly with respect to $V$, being careful to treat $T$ as a constant: $$ \begin{aligned} \frac{\partial P}{\partial V}&=-n R T(V-n b)^{-2}(1)+2 n^{2} a V^{-3} \\ &=\frac{2 n^{2} a}{V^{3}}-\frac{n R T}{(V-n b)^{2}}. \end{aligned} $$
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