A simple pendulum has a bob with mass m and charge q . The pendulum...

The charged bob experiences two forces at equilibrium:
Gravitational force: $F_g = mg$ (downward)
Electric force: $F_E = qE$ (horizontal, in direction of field)
These forces are perpendicular to each other. The string tension must balance their resultant.
At equilibrium, the string makes angle $\theta$ with vertical such that:
Vertical: $T\cos\theta = mg$
Horizontal: $T\sin\theta = qE$
The net force from gravity and electric field is $\sqrt{(mg)^2 + (qE)^2}$
Therefore, the tension in the string is: $T = \sqrt{m^2g^2 + q^2E^2}$