Let there be a positively charged ball of mass m. It is known that if the ball would move with a speed v in a perpendicular magnetic field of induction B then the ball’s trajectory would be a circle of radius r. How big should be the charge q of another ball with the same mass so that the first ball would move with the same speed on the same trajectory in the electric field of the second ball? Assume that no external forces are applied to this system of two balls. All the movements are observed in a laboratory frame of reference.