Class 12 NEET Physics Dual Nature of Matter and Radiation

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

e

m

=

E2

2VB2

Specific charge of the cathode rays,

e

m

=

E2

2VB2