AIPMT 2010 Physics and Chemistry Paper

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
‌

e

m

=‌

E2

2VB2

Specific charge of the cathode rays ‌

e

m

=‌

E2

2VB2