question_answer

In Millikans oil drop experiment, a charged oil drop of mass \[3.2\,\times \,{{10}^{-14}}\,kg\] is held stationary between two parallel places 6 mm apart, by applying a potential difference of 1200 V between them. How many electrons does the oil drop carry? \[(g=10m{{s}^{-2}})\]

A)  7                                            

B)  8

C)   9                                           

D)  10

Correct Answer: D

Solution :

  According to figure, it is clear that electrical force is balanced by weight of body, ie,                                 \[qE=mg\] or                            \[ne\frac{V}{l}=mg\] \[\therefore \]  \[n\times 1.6\times {{10}^{-19}}\times \frac{1200}{6\times {{10}^{-3}}}\]                                 \[=3.2\times {{10}^{-14}}\times 10\] or            \[n=\frac{3.2\times {{10}^{-13}}\times 6\times {{10}^{-3}}}{1.6\times {{10}^{-19}}\times 1200}\]                                 \[n=10\]