[quote:b8cf85debc] the electric field were suddenly brought to zero when the particle reached point 2. [/quote:b8cf85debc] There is no way you can change the Electric field to zero at the same instance. You always need some time to do it(it could be very small), unless you do not care about the transit effect. Then, there is no force on the charge particle and the particle will move with constant velocity. Yes. the energy will become thermal energy when the charge particle hit the cathode ray tube.