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 34 - Electromagnetic Fields and Waves - Exercises and Problems - Page 1030: 34

Answer

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Work Step by Step

$$\color{blue}{\bf [a]}$$ We need to use Galilean fields transformation equations, $$\left.\begin{matrix} &\vec E_B=\vec E_A+\vec v_{BA}\times \vec B_A \\ & \vec B_B=\vec B_A-\dfrac{\vec v_{BA}\times \vec E_A}{c^2}\\ \end{matrix}\right\}\tag 1$$ Assuming that A is the wire's frame, and B is the loop's frame. Now we need to find $B_A$ and $E_A$, so we can use (1) to find $B_B$ and $E_B$. Recalling that the magnetic field for a very long current-carrying wire is given by $$\boxed{\vec E_A=\dfrac{1}{4\pi\epsilon_0}\dfrac{2\lambda}{ r}\hat r}\tag 2$$ where $\hat r$ means radially away from the wire since it is positively charged. And $$\boxed{\vec B_A=0\;\rm T}\tag 3$$ since the wire is stationary and the charges are stationary too (there are no moving charges here). $$\color{blue}{\bf [b]}$$ Now we can use the first formula from (1) to find $E_B$; $$ \vec E_B=\vec E_A+\vec v_{BA}\times \vec B_A $$ where $\vec B_A=0$, so $$ \vec E_B=\vec E_A+\vec v_{BA}\times 0=\vec E_A $$ Hence, $$\boxed{\vec E_B=\dfrac{1}{4\pi\epsilon_0}\dfrac{2\lambda}{ r} } $$ Now we can use the second formula from (1) to find $B_B$; $$ \vec B_B=\vec B_A-\dfrac{\vec v_{BA}\times \vec E_A}{c^2} =0-\dfrac{\vec v_{BA}\times \vec E_A}{c^2}$$ $$ \vec B_B=-\dfrac{\vec v_{BA}\times \vec E_A}{c^2} $$ Plug the known, where $v_{BA}=v\;\hat i$, and $E_A$ is from (2), $$ \vec B_B=-\dfrac{1}{c^2} \left[v\;\hat i\times \dfrac{1}{4\pi\epsilon_0}\dfrac{2\lambda}{ r}\hat r\right]$$ Let's take a point at the top of the wire, the direction of the electric field is upward. Hence, $\hat i\times \hat j=\hat k$ $$ \vec B_B=-\dfrac{1}{c^2} \left[v \times \dfrac{1}{4\pi\epsilon_0}\dfrac{2\lambda}{ r}\;\hat k \right]$$ $$\boxed{ \vec B_B=- \dfrac{v\lambda}{ 2\pi\epsilon_0c^2r}\;\hat k}$$ This means that the direction of the magnetic field at any point above the wire is into the page. $$\color{blue}{\bf [c]}$$ Since the loop is moving to the right, the man on the loop sees a current that passes through the wire where the current is given by $$I=\dfrac{dQ}{dt}\tag 4$$ And we are given that the linear charge density of the wire is $\lambda$, so $\lambda=\dfrac{dq}{dx}$ where $dx$ is a small segment of the wire. Hence, $dq=\lambda dx$, plug that into (4) $$I=\dfrac{\lambda dx}{dt} $$ where $dx/dt=v$ $$\boxed{I=\lambda v}$$ $$\color{blue}{\bf [d]}$$ $$B_B=\dfrac{\mu_0I}{2\pi r}$$ Plug from the previous boxed formula above, $$B_B=\dfrac{\mu_0\lambda v}{2\pi r}\cdot\dfrac{\epsilon_0}{\epsilon_0}$$ where $\mu_0 \epsilon_0=1/c^2$ $$\boxed{B_B=-\dfrac{ \lambda v}{2 \pi \epsilon_0c^2r}\;\hat k} $$ The current due to the frame of the loop is toward the left. So at any point above the wire, the direction of the magnetic field according to the right-hand rule is into the page. $$\color{blue}{\bf [e]}$$ You can see the two boxed formulas of parts b and d above, they are identical.
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