Question 243 of 311
An electric dipole of dipole moment is placed in uniform electric field of magnitude . Initially, the dipole moment is parallel to electric field. The work that needs to be done on the dipole to make its dipole moment opposite to the field, will be ____ J.
Show answer and solution
Answer: 12
Work done in rotating a dipole
$\text { or } \begin{aligned}
W & =\left(-P E \cos \theta_f\right)-\left(-P E \cos \theta_i\right) \\
& =2 P E \quad\left(\because \theta_f=180^{\circ} \text { and } \theta_i=0^{\circ}\right) \\
& =\left(2 \times 6 \times 10^{-6} \times 10^6\right) \mathrm{J}=12 \mathrm{~J}
\end{aligned}$
$\text { or } \begin{aligned}
W & =\left(-P E \cos \theta_f\right)-\left(-P E \cos \theta_i\right) \\
& =2 P E \quad\left(\because \theta_f=180^{\circ} \text { and } \theta_i=0^{\circ}\right) \\
& =\left(2 \times 6 \times 10^{-6} \times 10^6\right) \mathrm{J}=12 \mathrm{~J}
\end{aligned}$