When a beam of cathode rays (or electrons) are subjected to crossed electric (E) and magnetic (B) fields, the beam is not deflected, if Force on electron due to magnetic field = Force on electron due to electric field Beu = eE or u=
E
B
....... (i) If V is the potential difference between the anode and the cathode, then ∴
1
2
mu2=eV
e
m
=
u2
2V
......(ii) Substituting the value of u from equation (i) in equation (ii), we get