(a) V = 500 V
e/m = 1.76 × 1011 C kg-1
Due to the applied potential difference between the emitter and the collector, an emitted electron gains kinetic energy, which is given by
Using the same formula as in (a), the speed of electrons at 10 mV comes out to be greater than the speed of light. Since no material particle can move with the speed of light, the result is clearly wrong.
In fact, equation (1) is valid for kinetic energy of the electron, so long as V/C < < 1. At 10 mV, the electrons attain a speed comparable to the speed of light i.e. become relativistic.
In the relativistic domain:- The mass of the electron having rest mass m0 is