Physics: Principles with Applications (7th Edition)

Published by Pearson
ISBN 10: 0-32162-592-7
ISBN 13: 978-0-32162-592-2

Chapter 29 - Molecules and Solids - General Problems - Page 855: 44

Answer

$ 8.7\times 10^{-34}$

Work Step by Step

First, we need to find the moment of inertia of the uniform rod (baton) with the end masses. Assuming that the length of the rod is $R$, as you see in the figure below. The net moment of inertia around the center of mass of the system is given by $$I_{net}=\frac{1}{12}m_{rod}R^2+m_{mass}\left(\dfrac{R}{2}\right)^2+m_{mass}\left(\dfrac{R}{2}\right)^2$$ Thus, $$I_{net}=\frac{m_{rod}R^2}{12}+ \dfrac{ m_{mass}R^2}{2} \tag 1$$ We know that the kinetic energy of the baton is given by $$KE=\frac{1}{2}I\omega^2\tag 2$$ We also know that the quantized rotational energy levels are given by $$E=l(l+1)\dfrac{\hslash^2}{2I}$$ and since we assume that the quantum number will be very large, $$E=l(l+1)\dfrac{\hslash^2}{2I} \approx l^2\dfrac{\hslash^2}{2I} $$ Solving for $l$; $$l=\sqrt{\dfrac{2IE}{\hslash^2}}=\sqrt{\dfrac{8\pi^2 IE}{h^2}}$$ Plugging from (2); $$l= \sqrt{\dfrac{8\pi^2 I\;\frac{1}{2}I\omega^2 }{h^2}}$$ $$l= \sqrt{\dfrac{4\pi^2 I^2 \omega^2 }{h^2}}$$ $$l= \dfrac{2\pi I \omega }{h } $$ Plugging from (1) and recall that $\omega=2\pi f$ $$l= \dfrac{4\pi^2f \left[\dfrac{m_{rod}R^2}{12}+ \dfrac{ m_{mass}R^2}{2}\right] }{h } $$ Plugging the known; $$l= \dfrac{4\pi^2(1.8) \left[\dfrac{ (0.230)(0.32)^2}{12}+ \dfrac{(0.380)(0.32)^2}{2}\right] }{6.626\times 10^{-34}} $$ $$l=\bf 2.3\times10^{33}$$ So, the differences between rotational energy levels for a spinning baton compared to the energy of the baton is given by $$\dfrac{\Delta E}{E}=\dfrac{\dfrac{\hslash^2 l}{I}}{ \dfrac{l^2\hslash^2}{2I}}$$ $$\dfrac{\Delta E}{E}= \dfrac{\color{red}{\bf\not}\hslash^2 l}{\color{red}{\bf\not}I}\cdot \dfrac{2\color{red}{\bf\not}I}{l^2\color{red}{\bf\not}\hslash^2} =\dfrac{2}{l}$$ Plugging from above; $$\dfrac{\Delta E}{E} =\dfrac{2}{2.3\times10^{33}}=\color{red}{\bf 8.7\times 10^{-34}}$$ Thus, $$ \Delta E =\dfrac{2}{2.3\times10^{33}}=\color{red}{\bf 8.7\times 10^{-34}} E$$ This is a very small difference which will be not detectable.
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