Physics: Principles with Applications (7th Edition)

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

Chapter 16 - Electric Charge and Electric Field - General Problems - Page 472: 65

Answer

$QE \ell$.

Work Step by Step

The electric field applies a force of magnitude QE on each charge. The lever arm for each force is the distance r from the pivot point to each charge, which is $\ell /2$. Because the field is perpendicular to the rod, the torque caused by each force is $\tau=Fr=QE\ell /2$. Both torques rotate the rod in the same way, so the torques add together. The net torque is $\tau_{net}=2Fr=2(QE\ell/2) =QE \ell$.
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