Let there be three electronvolts of energy between a tube’s cathode and plate with a voltage of one volt. Let thermionic emission release one electron at the cathode. From the definition of electronvolt (the energy a point charge gains when it travels through one volt), that electron gains one electronvolt of kinetic energy on its journey to the plate and two electronvolts of voltage’s energy remain. Release of two more electrons will convert the remainder of the tube’s energy to electron kinetic energy. No voltage’s energy will remain in the tube. Perhaps this shows the difference between voltage's energy and energy due to attraction between masses? If the electron bounces (with perfect elasticity) off the plate instead of landing on the plate, will that electron bounce until it gains all three electronvolts of energy. When the bouncing electron travels between the cathode and the plate, how does that change the point charges that cause the voltage?