Question

In Thomson mass spectrograph when only electric field of strength 20 kV/m is applied, then the displacement of the beam due to the electric field is 2 cm. If length of plates = 5 cm, and velocity of ions = ${10}^6$ m/s, then q/m of the ions is

• A. ${10}^{6}C/kg$
• B. $25\times {10}^{7}C/kg$
• C. $8\times {10}^{8}C/kg$
• D. ${10}^{11}C/kg$

Answer 1

The correct option is C $8\times {10}^{8}C/kg$

By using $y=\frac{qE{L}^2}{2m{v}^2}$ ; where y = deflection on screen due to electric field only
$\Rightarrow \frac{q}{m}=\frac{2y{v}^2}{E{L}^2}=\frac{2\times 2\times {10}^{-2}\times ({10}^6{)}^2}{20\times {10}^3\times {0.05}^2}=8\times {10}^{8}C/kg$