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If we have an electric dipole as shown:

enter image description here

Net torque on system = $Fdsin\theta$= Rate of change of angular momentum = $I \ddot \theta $

For small displacements along line of E field, $sin\theta \approx \theta$

So $Fd\theta=I\ddot \theta$.

I want to show this is SHM, where have I dropped my negative sign?

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As drawn you have taken $\theta$ to be positive in the anticlockwise direction?

The torque is in the clockwise (negative) direction so should be written as $-Fd \,\theta$.

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