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Consider a dipole oscillating in an electric field (figure III.3). When it is at an angle $$\theta$$ to the field, the magnitude of the restoring torque on it is $$pE \sin \theta$$, and therefore its equation of motion is $$I\ddot \theta = -pE\sin \theta$$ where $$I$$ is its rotational inertia. For small angles, this is approximately $$I\ddot \theta = -pE\theta$$ and so the period of small oscillations is
$\label{3.3.1}P=2\pi\sqrt{\frac{I}{pE}}.$
Would you expect the period to be long if the rotational inertia were large?  Would you expect the vibrations to be rapid if $$p \text{ and }E$$ were large? Is the above expression dimensionally correct?