An externally long wire is uniformly charged an electron is resolving about the wire and making  108revolutions per second in an orbit of radius 2cm .  Linear charge density of wire in nC/mis

The electric force  "eE" will provide necessary centripetal force ∴eE=mrw2 ⇒eλ2πε0r=mr4π2n2 ⇒λ=4πεo2 mr2n2π2e         =2×9×10−31×(4×10−4)×1016×109×109×1.6×10−19              π2≈10         =50×10−9C/m         =50n C/m