A charge q is placed at some distance along the axis of a uniformly charged disc of surface charge density σ. The flux due to the charge q through the disc is ϕ. The electric force on charge q exerted by the disc is

We know that if a point charge subtends a half-angle a on the circular cross-section of a disc, then flux passing through the disc is:ϕ=q2εo(1−cos⁡α)Also, if a point charge q lies on the axis of a charged disc of charge density σ and subtends a half-angle α, then it experiences a forceF=qσ2εo(1−cos⁡α)=σϕ