As the electron beam is not deflected, then or FmBevv=Fe=Ee=BE As the electron moves from cathode to anode, its potential energy at the cathode appears as its kinetic energy at the anode. If V is the potential difference between the anode and cathode, then potential energy of the electron at cathode =eV. Also, kinetic energy of the electron at anode =21mv2. According to law of conservation of energy 21mv2v=eV=m2eV From Eqs. (i) and (ii), we have m2eVme= BE=2VB2E2