An electron is in a vacuum near earth's surface and located at y = 0 m on a vertical y axis. at what value of y should a group of 28 electrons be placed such that its electrostatic force on the first electron balances the gravitational force on the first electron?

It may seem strange that the selected velocity does not depend on either the mass or the charge of the particle. (for example, would the velocity of a neutral particle be selected by passage through this device? ) the explanation of this is that the mass and the charge control the resolution of the device--particles with the wrong velocity will be accelerated away from the straight line and will not pass through the exit slit. if the acceleration depends strongly on the velocity, then particles with just slightly wrong velocities will feel a substantial transverse acceleration and will not exit the selector. because the acc