$\begin {array} {1 1} (a)\;0.173 \times 10^8C \ kg & \quad (b)\;1.73 \times 10^{11}C \ kg \\ (c)\;17.3 \times 10^{11}C \ kg & \quad (d)\;17.3 \times 10^8C \ kg \end {array}$

Ans : (b)

So, $ \large\frac{1}{2} $$mv_2 = eV$

$ \Rightarrow v^2= \large\frac{2eV}{m}$………..(i)

So, Centripetal force = magnetic force

$ \Rightarrow \large\frac{mv^2}{r} $$= ev$$B$

$ \Rightarrow eB = \large\frac{mv}{r}$

$ \Rightarrow v = \large\frac{eBr}{m}$………….(ii)

So, putting (ii) in (i),

$\large\frac{2eV}{m} = \large\frac{e^2B^2r^2}{m^2}$

$ \Rightarrow \large\frac{e}{m} = \large\frac{2V}{B^2r^2}$

$= \large\frac{2 \times 100}{[(2.83 \times 10^{-4} )^2 \times (12 \times 10^{-2} )^2 ]} $$= 1.73 \times 10^{11} C / kg$

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