In Millikan’s  experiment, static electric charge on the oil drops has been obtained by shining X-rays. If the static electric charge on the oil drop is $–1.282\times {10}^{–18}C$. Calculate the number of electrons present on it.

the static electric charge on the oil drop is, q=1.282×10 ⁻¹⁸ C. 

We know that Charge q=ne 

where n is the no. of electrons and e is the charge of 1 electron,

 i.e. 1.602×10 ⁻¹⁹C 

The number of electrons present in the oil drop is, n= 1.282×10 ⁻¹⁸/1.602×10 ⁻¹⁹=8.