A beam of cathode rays is subjected to crossed Electric(E) and Magnetic fields(B). The fields are adjusted such that the beam is not deflected. The specific charge of the cathode rays is given by: (Where V is the potential difference between cathode and anode).

Options

(a) B² / 2VE²
(b) 2VB² / E²
(c) 2VE² / B²
(d) E² / 2VB²

Correct Answer:

E² / 2VB²

Explanation:

For no deflection of beam, eE = evB ⇒ v = E/B
Also, 1/2 mv² = eV ⇒ v = E/B
e/m = v² / 2V = E² / 2VB²