The particles are passing through a fixed distance (between the screens), wherein they experience a fixed force due to the electric field. An electron will sho

The particles are passing through a fixed distance (between the screens), wherein they experience a fixed force due to the electric field. An electron will shoot through that distance in no time flat and accrue very little momentum. A proton will plod through and transfer far more. For a 250V potential difference with 20m between screens, you can check it yourself here: https://www.wolframalpha.com/input/?i=(1+electron+charge+%2F+mass+of+proton+*+250+V+%2F+20+m+)+*+sqrt(2+*+20+m+%2F+(1+electron+charge+%2F+mass+of+proton+*+250+V+%2F+20+m+))+*+mass+of+proton https://www.wolframalpha.com/input/?i=(1+electron+charge+%2F... https://www.wolframalpha.com/input/?i=(1+electron+charge+%2F+mass+of+electron+*+250+V+%2F+20+m+)+*+sqrt(2+*+20+m+%2F+(1+electron+charge+%2F+mass+of+electron+*+250+V+%2F+20+m+))+*+mass+of+electron https://www.wolframalpha.com/input/?i=(1+electron+charge+%2F... The ratio of those two values is 42.8. This also makes it easy to see why heavier ions full of neutrons are _awesome_. They give a ratio of 5343.