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An oil drop has $-6.39\times10^{-19}$ coulomb charge .The number of electrons in this oil drop is

$(a)\;4\qquad(b)\;3\qquad(c)\;2\qquad(d)\;1$

Charge on one electron = $-1.60\times10^{-19}$
$\therefore$ The number of electron = $\large\frac{-6.39\times10^{19}}{-1.60\times10^{-19}}$
Thus the number of electron = 4

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