A straight infinitely long cylinder of radius $R_0=10cm$  is uniformly charged with a surface charge density  $\sigma =+{10}^{-12}C/m^2.$ The cylinder serves as a source of electrons, with the velocity of the emitted electrons perpendicular to its surface. The electron velocity be to ensure that electrons can move away, from the axis of the cylinder to a distance greater than $r={10}^{3}m$  is $100nkm/s.$ Find $n$.  (using rounding off, do not truncate)