Question

In Thomson mass spectrograph, singly and doubly ionised particles from similar parabola corresponding to magnetic fields of $0.8\, T$ and $1.2\, T$ for a constant electric field. The ratio of masses of ionised particles will be

• A. 3 : 8
• B. 2 : 9
• C. 8 : 3
• D. 9 : 2

Answer 1

Answer: (B)

For similar parabola; $y^{2}=\frac{B^{2} l D}{E} \frac{q}{m} x$, will be same for two particles.
It means $\frac{B^{2} q}{m}=$ a constant for these two particles.
$\therefore \frac{m_{1}}{m_{2}}=\frac{B_{1}^{2} q_{1}}{B_{2}^{2} q_{2}}=\left(\frac{0.8}{1.2}\right)^{2} \times \frac{e}{2 e}=\frac{2}{9}$