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 Bev=eE or v=
E
B
..........(i) If V is the potential difference between cathode and anode, then ∴
1
2
mv2=eV
e
m
=
v2
2V
..........(ii) Substituting the value of v from equation (i) in equation (ii), we get