Find the maxim potential an isolated conductor could be loaded,aiming at it an electrons beam, with the speed v=8.0Mm/s
By the time the electrons are approaching to the loaded conductor (with electrons), they are facing a rejection force bigger and bigger. The rejection force is produced by the electrons from conductor, so the approaching electrons are deccelerated. Their kinetic energy is decreasing for realising mechanical work against rejection force. Their kinetic energy disappears at limit, and just mechanical work is accomplished, against electrical rejection force.
The mechanical work performed by the field forces for moving the load q:
L'=mechanical work accomplishedfrom outside, against the filed forces: