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 18 - The Micro/Macro Connection - Exercises and Problems - Page 523: 13

Answer

$6.55\times10^{25}\;\rm collision/s$

Work Step by Step

We know that the average force due to molecule collision is given by $$F_{avg}=2\dfrac{ N_{\rm coll}}{\Delta t} mv_x$$ Hence the rate of collisions is given by $$\dfrac{ N_{\rm coll}}{\Delta t} =\dfrac{F_{avg}}{2mv_x}$$ Recalling that $F_{avg}=PA$ where $P$ is the pressure and $A$ is the cross-sectinoal area. $$\dfrac{ N_{\rm coll}}{\Delta t} =\dfrac{PA}{2mv_x}$$ we are given the pressure $P$, the cross-sectional area of the wall $A$, and we can find the mass of the nitrogen molecule. where the mass of the nitrogen molecule is given by $$m_{N_2}=2M_N (1.66\times 10^{-27}) $$ where $M_N$ is the atomic mass of the nitrogen atom. So, $$\dfrac{ N_{\rm coll}}{\Delta t} =\dfrac{PA}{4M_N (1.66\times 10^{-27}) v_x}\tag 1$$ So we have to find $v_x$: Noting that $$v_x=\sqrt{(v_x^2)_{avg}}$$ and since the rms speed is for the 3 dimensions, then $(v_x^2)_{avg}=v_{\rm rms}^2/3$ $$v_x=\sqrt{\dfrac{v_{\rm rms}^2}{3}}=\dfrac{v_{\rm rms}}{\sqrt3}$$ Plugging into (1); $$\dfrac{ N_{\rm coll}}{\Delta t} =\dfrac{\sqrt3PA}{4M_N (1.66\times 10^{-27}) v_{\rm rms}} $$ Plugging the known; $$\dfrac{ N_{\rm coll}}{\Delta t} =\dfrac{\sqrt3(2\times 1.013\times 10^5)(0.1\times 0.1)}{4(14) (1.66\times 10^{-27}) (576)} $$ $$\dfrac{ N_{\rm coll}}{\Delta t} =\color{red}{\bf 6.55\times10^{25}}\;\rm collision/s$$
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