Question

In a Millikan's oil drop experiment, the charge on an oil drop is calculated to be $-6.4\times {10}^{-19}\text{C}$. The number of excess electrons on the drop is:

• A. $3.9$
• B. $4$
• C. $5$
• D. $6$

Answer 1

The correct option is B $4$

As the number of electrons can not be a non-integer value, option (a) can be eliminated.
Charge on an electron $=-1.6\times {10}^{-19}\text{C}$
Charge on the oil drop $=-6.4\times {10}^{-19}\text{C}$

Number of excess electrons $=\frac{\text{magnitude of charge on oil drop (Q)}}{\text{Magnitude of charge on one elctron (q)}}$

$=\frac{6.4\times {10}^{-19}\text{C}}{1.6\times {10}^{-19}\text{C}}=4$