Question

An oil drop has 7.99 $\times$ ${10}^{-19}$ C charge. The number of electrons that the oil drop has is .

• A. 2
• B. 5
• C. 3

Answer 1

The correct option is B 5

Just recall Millikan oil drop experiment.

From that, we can say

Charge on the electron = 1.6 $\times$ ${10}^{-19}$C

Total charge on the oil drop = 7.99 $\times$ ${10}^{-19}$C

$\to$ If there are no electrons on oil drop, there should be no charge.

Hence, the number of electrons = $\frac{7.99\times {10}^{-19}}{1.6\times {10}^{-19}}$ = 5

Therefore there are 5 electrons on the oil drop.