Q.25) An electric dipole with dipole moment $5 \times 10^{-6} \mathrm{Cm}$ is aligned with the direction of a uniform electric field of magnitude $4 \times 10^5 \mathrm{~N} / \mathrm{C}$. The arpole is then rotated through an angle of $60^{\circ}$ with respect to the electric field. The change inthe potential energy of the dipole is :
A) 1.5J
B) 0.8J
C) 1.0J
D) 1.2J
Given,
$p = 5 \times 10^{-6} \mathrm{Cm}$
$E = 4 \times 10^5 \mathrm{~N} / \mathrm{C}$
Initial Angle, $\theta_1 = 0^\circ$
Final Angle, $\theta_2 = 60^\circ$
Thus the change in energy is,
$ U = p E\left(\cos \theta_1-\cos \theta_2\right) = 5 \times 10^{-6} \times 4 \times 10^5 (1 - 0.5) = 1\mathrm{~J}$
Hence, the answer is the option 3.