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 30 - Current and Resistance - Exercises and Problems - Page 889: 55

Answer

$2\;\rm A,\;5\times 10^{-5}\;m/s$

Work Step by Step

The current is the same in both wires according to the current conservation principle. Hence, $$I_1=I_2=I=\color{red}{\bf 2}\;\rm A$$ However, the current density is not the same in both cases. Recalling that the drift speed is given by $$v_d=\dfrac{J}{en_e}$$ where $J=I/A$, $$v_d=\dfrac{I}{Aen_e}$$ So that, $$v_{d1}=\dfrac{I}{A_1en_e}\tag 1$$ and $$v_{d2}=\dfrac{I}{A_2en_e}\tag 2$$ Divide (2) by (1); $$\dfrac{v_{d2}}{v_{d1}}=\dfrac{\dfrac{I}{A_2en_e}}{\dfrac{I}{A_1en_e}}=\dfrac{A_1}{A_2}$$ where $A=\pi r^2=\pi (D/2)^2=\pi D^2/4$ whereas $D$ is the diameter of the wire, $$\dfrac{v_{d2}}{v_{d1}} =\dfrac{\dfrac{\pi D_1^2}{4}}{\dfrac{\pi D_2^2}{4}}=\dfrac{D_1^2}{D_2^2}$$ Thus, $$v_{d2}=\dfrac{D_1^2}{D_2^2}v_{d1}$$ Plug the known; $$v_{d2}=\dfrac{(1)^2}{(2)^2}(2\times 10^{-4})$$ $$v_{d2}=\color{red}{\bf 5\times 10^{-5}}\;\rm m/s$$
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