Physics: Principles with Applications (7th Edition)

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

Chapter 21 - Electromagnetic Induction and Faraday's Law - Problems - Page 622: 67

Answer

a) $1.77\times10^{-2}\;\rm A$ b) $-12.6^\circ$ c) $117\;\rm V, \;26.1\;V$

Work Step by Step

a) We know that $$I_{\rm rms}=\dfrac{V_{\rm rms}}{Z}\tag 1$$ we are given the arms voltage $V_{\rm rms}$, so we need to find the impedance $Z$. We know that the impedance is given by $$Z=\sqrt{R^2+(X_L-X_C)^2}$$ And in this case, we have $RC$-circuit without an inductive reactance$X_L=0$. Thus, $$Z=\sqrt{R^2+ X_C^2}=\sqrt{R^2+ \left(\dfrac{1}{2\pi f C}\right)^2}$$ Plugging into (1); $$I_{\rm rms}=\dfrac{V_{\rm rms}}{\sqrt{R^2+ \left(\dfrac{1}{2\pi f C}\right)^2}} $$ Plugging the known; $$I_{\rm rms}=\dfrac{ 120}{\sqrt{(6.6\times10^3)^2+ \left(\dfrac{1}{2\pi\cdot 60\cdot (1.8\times10^{-6})}\right)^2}} $$ $$I_{\rm rms}=\color{red}{\bf1.77\times10^{-2}}\;\rm A$$ -------------------------------------------------------------- b) We know that the phase angle is given by $$\phi=\tan^{-1}\left(\dfrac{\overbrace{X_L}^{0}-X_C}{R}\right)=\tan^{-1}\left(\dfrac{ -X_C}{R}\right)$$ $$\phi =\tan^{-1}\left(\dfrac{ -\dfrac{1}{2\pi f C}}{R}\right)=\tan^{-1}\left(\dfrac{ -1}{2\pi f CR}\right)$$ Plugging the known; $$\phi= \tan^{-1}\left(\dfrac{ -1}{2\pi \cdot 60\cdot 1.8\times10^{-6}\cdot 6.6\times10^3 }\right)$$ $$\phi=\color{red}{\bf -12.6^\circ}$$ -------------------------------------------------------------- c) The voltage reading across the resistance is given by $$V_R=I_{\rm rms}R=1.77\times10^{-2}\cdot 6.6\times10^3$$ $$V_R=\color{red}{\bf117}\;\rm V$$ The voltage reading across the capacitor is given by $$V_C=I_{\rm rms}X_C=I_{\rm rms}\;\dfrac{1}{2\pi f C}$$ $$V_C= \dfrac{I_{\rm rms}}{2\pi f C}=\dfrac{1.77\times10^{-2}}{2\pi \cdot 60\cdot 1.8\times10^{-6}}$$ $$V_C=\color{red}{\bf 26.1}\;\rm V$$
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