Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

Published by Pearson
ISBN 10: 0321740904
ISBN 13: 978-0-32174-090-8

Chapter 17 - Work, Heat, and the First Law of Thermodynamics - Exercises and Problems - Page 500: 68

Answer

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Work Step by Step

First of all, we need to find the number of moles of this gas $$n=\dfrac{m}{M_{N_2}}=\dfrac{m}{2M_N}$$ Plugging the known; $$n =\dfrac{14\;\rm g}{2(14)\;\rm g/mol}=\bf 0.5\;\rm mol$$ Noting that $\rm STP$ means standard temperature and pressure at which the temperature is 0$^\circ$C and the pressure is 1 atm. So we can find the initial volume by applying the ideal gas law; $$PV=nRT$$ Hence, $$V_1=\dfrac{nRT_1}{P_1}=\dfrac{(0.5)(8.31)(0+273)}{(1.013\times 10^5)}$$ $$V_1=\bf 11.2\times 10^{-3}\;\rm m^3$$ a) For an adiabatic process, the final temperature is given by $$T_1V_1^{\gamma-1}=T_2V_2^{\gamma-1}$$ Hence, $$\dfrac{T_2}{T_1}=\left[\dfrac{V_1}{V_2} \right]^{\gamma-1}$$ $$T_2=T_1\left[\dfrac{V_1}{V_2} \right]^{\gamma-1}\tag 1$$ Now we need to find the ratio of volumes in terms of pressure. $$P_1V_1^\gamma=P_2V_2^\gamma$$ Hence, $$\left[\dfrac{V_1}{V_2}\right]^\gamma= \dfrac{P_2}{P_1} $$ $$\dfrac{V_1}{V_2}=\left[ \dfrac{P_2}{P_1}\right]^{\frac{1}{\gamma}}\tag 2 $$ Plugging into (1); $$T_2=T_1\left[ \left[ \dfrac{P_2}{P_1}\right]^{\frac{1}{\gamma}} \right]^{\gamma-1} $$ $$T_2=T_1\left[ \dfrac{P_2}{P_1} \right] ^{\frac{\gamma-1}{\gamma}} $$ Plugging the known; $$T_2=(0+273)\left[ \dfrac{20}{1} \right] ^{\frac{1.4-1}{1.4}} $$ $$T_2=\color{red} {\bf 643}\;\rm K=\color{red} {\bf 370}^\circ \rm C$$ ______________________________________________________________ b) We know that the change in thermal energy of the gas during any process is given by $$\Delta E_{th}=Q+W$$ And we know that the heat exchange $Q$ during an adiabatic process is zero. So, the work done on the gas is given by: $$W=\Delta E_{th}-Q=\Delta E_{th}-0$$ $$W=nC_{\rm v}(T_2-T_1)$$ Plugging the known; $$W=(0.5) (20.8) (643-273)$$ $$W=\color{red}{\bf 3.85\times 10^3}\;\rm J$$ ______________________________________________________________ c) As we mentioned above, in part b, $$Q=\color{red}{\bf 0}\;\rm J$$ during any adiabatic process. ______________________________________________________________ d) We need to find the ratio in (2); $$\dfrac{V_1}{V_2}=\left[ \dfrac{P_2}{P_1}\right]^{\frac{1}{\gamma}} =\left[ \dfrac{20}{1}\right]^{\frac{1}{1.4}}=\bf 8.5 \tag 3 $$ This means that the initial volume is about 9 times the final volume. Thus, the initial volume is the maximum volume. Thus $$\dfrac{V_{\rm max}}{V_{\rm min}}= \color{red}{\bf 8.5 } $$ ______________________________________________________________ e) To draw this process, we need to find the two volumes in L. $$V_1=11.2\times 10^{-3}\times 10^3=\bf 11.2\;\rm L$$ $V_2$ is given from (3) $$V_2=\dfrac{V_1}{8.5}=\dfrac{11.2}{8.5}=\bf 1.32\;\rm L$$ Hence, The initial point is: $(V_1,P_1)=\rm (11.200\;L,1\;atm)$ And the final point is: $(V_2,P_2)=\rm (1.318\;L,20\;atm)$ ---
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