Question

The mass of the sphere is $3.2\times {10}^{-14}kg$ and it carries a net charge equal to that of $10$ electrons. If the electronic charge is $-1.6\times {10}^{-19}C$ and the acceleration of free fall is $10m{s}^{-2}$, the field required to keep the sphere stationary is :

• A. $5\times {10}^{-6}V/m$
• B. $5\times {10}^{-5}V/m$
• C. $2\times {10}^{5}V/m$
• D. $2\times {10}^{-4}V/m$

Answer 1

The correct option is C $2\times {10}^{5}V/m$

To keep the sphere stationary $qE=mg$
$\Rightarrow E=\frac{mg}{q}$

$=\frac{3.2\times {10}^{-14}\times 10}{10\times 1.6\times {10}^{-19}}$

$E=2\times {10}^{5}V/m$