Sphere is said to be small therefore we can ignore its moment of inertia.

The net downward force acting on the sphere will be,

$$\begin{gathered} F=mg-Eq \\ \Rightarrow F=m\left(g-\frac{Eq}{m}\right) \end{gathered}$$

From the above expression, we can say that the effective acceleration due to gravity will be $g^{\text{'}}=\left(g-\frac{Eq}{m}\right)$.

The time period of the simple pendulum will be,

$T=2\pi \sqrt{\frac{l}{g^{\text{'}}}}=2\pi \sqrt{\frac{l}{\left(g-{\displaystyle \frac{Eq}{m}}\right)}}$.