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 956: 7

Answer

$({\bf 6.25\times 10^6}{\;\rm m/s})\;\hat z$

Work Step by Step

We know, from Biot-Savart law, that the magnetic field of a moving charged particle is given by $$B=\dfrac{\mu_0}{4\pi }\dfrac{qv\sin\theta}{r^2}$$ Plug the known for the first point, $$1\times 10^{-13}\;\hat j=\dfrac{(4\pi\times 10^{-7})}{4\pi }\dfrac{(1.6\times 10^{-19})v\sin\theta_1}{0.001^2} $$ This one the magnetic field is in the positive $y$-direction which means that the two vectors of $\vec v$ and $\vec r$ are in the $x$-$z$ plane. Hence, the velocity vector is in the positive $z$-direction. Plug the known for the second point, $$-1\times 10^{-13}\;\hat i=\dfrac{(4\pi\times 10^{-7})}{4\pi }\dfrac{(1.6\times 10^{-19})v\sin\theta_2}{0.001^2} $$ Here the magnetic field is in the negative $x$-direction which means that the two vectors of $\vec v$ and $\vec r$ are in the $y$-$z$ plane. Hence, the velocity vector is in the positive $z$-direction as well. From all the above, $\theta_1=\theta_2=90^\circ$, and hence the velocity is then $$1\times 10^{-13} =\dfrac{(4\pi\times 10^{-7})}{4\pi }\dfrac{(1.6\times 10^{-19})v\sin90^\circ}{0.001^2} $$ $$v=\dfrac{1\times 10^{-13}\times 0.001^2}{10^{-7}(1.6\times 10^{-19})}$$ $$v=(\color{red}{\bf 6.25\times 10^6}{\;\rm m/s})\;\hat z$$
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