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 33 - Electromagnetic Induction - Exercises and Problems - Page 1002: 81

Answer

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Work Step by Step

$$\color{blue}{\bf [a]}$$ The length of one side of the loop is 10 cm. The initial separation distance between the two corners is given by the Pythagorean theorem. $$d=\sqrt{2L^2}=\sqrt{2(10)^2}=\bf10\sqrt2\;\rm cm$$ The final separation between the two corners is given by $$d'=2L=2(10)=\bf 20\;\rm cm$$ So that the total distance traveled by the two corners must be $$\Delta x=d'-d=\bf 20-10\sqrt2\;\rm cm$$ Hence, the distance traveled by one corner is given by $$\Delta x'=\dfrac{d'-d}{2}=\dfrac{ 20-10\sqrt2}{2}=\bf10-5\sqrt2\;\rm cm$$ Thus, the time it takes is given by $$\Delta t=\dfrac{\Delta x'}{v}=\dfrac{(10-5\sqrt2)\times 10^{-2}}{0.293 }$$ $$\Delta t=\color{red}{\bf 0.1}\;\rm s$$ $$\color{blue}{\bf [b]}$$ We know that the induced current is given by $$I_{\rm loop}=\dfrac{\varepsilon_{\rm loop }}{R_{\rm loop}}\tag 1$$ We know that the induced emf $\varepsilon$ is given by $$\varepsilon_{\rm loop}=\left|\dfrac{d\Phi_{\rm }}{dt}\right|\tag 2$$ where $ \Phi =\vec A\cdot \vec B =AB\cos90^\circ=BA $ where $A$ changes while $B$ is constant. Plug into (2), $$\varepsilon_{\rm loop}=B \left|\dfrac{d A}{dt} \right| \tag 3$$ We can divide the loop into 4 identical right-triangles. So, while the two corners are moving horizontally, the total area of the loop is 4 times the area of one right triangle (any one of both). Thus, $$A=4A_{\rm triangle}=4\left(\frac{1}{2}xh\right) =2xh$$ where $h=\sqrt{L^2-x^2}$ $$A =2x\sqrt{L^2-x^2}\tag 4$$ From (3), we now have 2 variables, $x$ and $t$, so we need to use the chain rule, $$\varepsilon_{\rm loop}=B \left|\dfrac{d A }{dx}\dfrac{d x }{dt} \right| $$ where $dx/dt=v$, plug from (4), $$\varepsilon_{\rm loop}=B v\left|\dfrac{d }{dt} 2x\left(L^2-x^2\right)^{1/2}\right| $$ $$\varepsilon_{\rm loop}=2B v\left|\dfrac{d }{dt} x\left(L^2-x^2\right)^{1/2}\right| $$ $$\varepsilon_{\rm loop}=2B v\left| \left(L^2-x^2\right)^{1/2}+(-2x)(x)(\frac{1}{2})\left(L^2-x^2\right)^{-1/2}\right| $$ $$\varepsilon_{\rm loop}=2B v\left| \sqrt{L^2-x^2}-\dfrac{ x^2}{ \sqrt{L^2-x^2}}\right| $$ $$\varepsilon_{\rm loop}=2B v\left| \dfrac{L^2 -2 x^2}{ \sqrt{L^2-x^2}}\right| $$ where $x$ as a function of $t$ is given by $x=x_0+vt$ where $x_0=\frac{1}{2}\sqrt{2L^2}=\frac{\sqrt{2}}{2}L$. so $x=\frac{\sqrt{2}}{2}L+vt$ $$\varepsilon_{\rm loop}=2B v\left| \dfrac{L^2 -2 \left[\frac{\sqrt{2}}{2}L+vt\right]^2}{ \sqrt{L^2- \left[\frac{\sqrt{2}}{2}L+vt\right]^2}}\right| $$ Plug into (1), $${I_{\rm loop}=\dfrac{2B v }{R_{\rm loop}}\left| \dfrac{L^2 -2 \left[\frac{\sqrt{2}}{2}L+vt\right]^2}{ \sqrt{L^2- \left[\frac{\sqrt{2}}{2}L+vt\right]^2}}\right| }$$ Plug the known; $$ I_{\rm loop}=\dfrac{2(0.5)(0.293) }{(0.1)}\left| \dfrac{0.1^2 -2 \left[\frac{\sqrt{2}}{2}(0.1)+(0.293)t\right]^2}{ \sqrt{(0.1)^2- \left[\frac{\sqrt{2}}{2}(0.1)+(0.293)t\right]^2}}\right| $$ $$\boxed{I_{\rm loop}=2.93\left| \dfrac{0.1^2 -2 \left[\frac{\sqrt{2}}{20} + 0.293 t\right]^2}{ \sqrt{(0.1)^2- \left[\frac{\sqrt{2}}{20} +0.293t\right]^2}}\right| }$$ $$\color{blue}{\bf [c]}$$ See the dots and the graph, in the second figure below.
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