An electron is projected from a distance d and with initial velocity u parallel to a uniformly charged flat conducting plate as shown. It strikes the plate after travelling a distance l along the direction of projection. The surface charge density σ of the conducting plate is.

For the motion of the electron along Y-axisd=12eEmt2………………….(1)where E is the electric field due to the charged plate given by E=σε0………………………….(2)where σ is the surface density of charge of the plate. The time taken by the electron is t=ℓu…………………………..(3)From equations (1), (2) and (3), we obtaind=12eσε0mℓu2⇒ σ=2dε0mu2eℓ2