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 32 - The Magnetic Field - Exercises and Problems - Page 960: 73

Answer

${\bf 1.057} \;\rm mA$, ${\bf 12.5}\;\rm T $

Work Step by Step

Assuming that the electron rotates clockwise, so the direction of the current is counterclockwise since we know that the direction of the current is the opposite direction of the direction of the negative charges. This means that the net magnetic field at the center of the Bohr atom is upward (according to the right-hand rule). The magnetic field strength of a current loop (as the motion of the electron around the center of the atom) is given by $$B_{\rm center}=\dfrac{\mu_0I}{2 R}\tag 1$$ Recalling that $I=dq/dt$ where $dq$ here is the charge of the electron itself. So, $$I_{\rm avg}=\dfrac{dq}{dt}=\dfrac{e}{T}$$ where we know that $v=2\pi R/T$, so $T=2\pi R/v$, and hence, $$I_{\rm avg }=\dfrac{ev}{2\pi R}\tag 2$$ Plug the known; $$I_{\rm avg }=\dfrac{(1.6\times 10^{-19})(2.2\times 10^6)}{2\pi (5.3\times 10^{-11})} $$ $$I_{\rm avg }=\color{red}{\bf 1.057} \;\rm mA$$ Plug $I$ from (2) into (1); $$B_{\rm center}=\dfrac{\mu_0 }{2 R} \cdot\dfrac{ev}{2\pi R}=\dfrac{\mu_0 e v}{4\pi R^2}$$ Plug the known; $$B_{\rm center} =\dfrac{(4\pi \times 10^{-7})(1.6\times 10^{-19})(2.2\times 10^6)}{4\pi (5.3\times 10^{-11})^2}$$ $$B_{\rm center}=\color{red}{\bf 12.5}\;\rm T\tag{Upward}$$
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