
τ=PE0sinθ If θ is small $\begin{aligned}
\tau & =-\left(P E_0\right) \theta \
I & =m\left(\frac{l}{2}\right)^2 \cdot 2=\frac{m l^2}{2} \
T & =2 \pi \sqrt{\frac{m l^2}{2 \cdot P E_0}}=2 \pi \sqrt{\frac{m l^2}{2 \cdot q / E_0}} \
T & =2 \pi \sqrt{\frac{m l}{2 q E_0}}
\end{aligned}$