An electric dipole of moment $p = 2 \times 10^{-8}$ C·m is placed in a uniform field $E = 10^5$ N/C. The work required to rotate it from $\theta_0 = 0°$ to $\theta = 180°$ is:
A$8 \times 10^{-3}$ J
B$2 \times 10^{-3}$ J
CZero
D$4 \times 10^{-3}$ J
Answer & Solution
Correct answer: D. $4 \times 10^{-3}$ J
Work done against the field $= pE(\cos\theta_0 - \cos\theta) = pE(\cos 0 - \cos 180°) = 2pE$. $= 2 \times (2\times10^{-8})(10^5) = 4\times10^{-3}$ J. Maximum torque $pE\sin 90° = 2\times10^{-3}$ N·m is a separate quantity.
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