As per Gauss Theorem,

**E = σ/2ɛ.**

where σ is surface charge density and ɛ. is the permittivity of free space.

e is charge on electron and r. is min. distance of approach.

Surface potential energy of charged plate = KE of electon

qe = e x **σ****/2****ɛ. x 5 x 10**^{-3}m

10^{-3} = **σ****/2****ɛ. x 5 x 10**^{-3}

**σ = 2/5 ****ɛ.**