The correct option (A) T = 2π[ℓ/{g – (qE/m)}](1/2)
Explanation:
The sphere of mass m has charge + q.
The electric field E is directed vertically up.
hence sphere will experience force in upward direction.
This forces will be F = qE
Due to this force, acceleration produced in the upward direction in sphere is a = (F/m) = (qE/m)
The net acceleration on sphere is g' = g – a = g – (qE/m)
As = Time period = T = 2π√(ℓ/g')
∴ T = 2π√[ℓ/{g – (qE/m)}]