Question

In Thomson mass spectrograph, singly and doubly ionised particles from similar parabola corresponding to magnetic fields of $0.8T$ and $1.2T$ 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; $y^2=\frac{B^{2}lD}{E}\frac{q}{m}x$, will be same for two particles.
It means $\frac{B^{2}q}{m}=$ a constant for these two particles.
$\therefore \frac{m_1}{m_2}=\frac{B_1^2{q}_1}{B_2^2{q}_2}={\left(\frac{0.8}{1.2}\right)}^2\times \frac{e}{2e}=\frac{2}{9}$