628. A monoenergetic electron beam illuminates a spot with a radius of r ≈ 10-3 cm in the center of the screen of a cathode-ray tube. Using the uncertainty relation, find how many times the uncertainty Δx: the coordinates of an electron on the screen in the direction perpendicular to the tube axis are less than the spot size r. The length L of the cathode-ray tube is taken equal to 0.50 m, and the voltage U, accelerating the electron, equal to 20 kV.